Conducted by the Midwest Regional Carbon Sequestration Partnership (MRCSP)

Transcription

1 FINAL REPORT UNDERSTANDING DEEP COAL SEAMS FOR SEQUESTRATION POTENTIAL Conducted by the Midwest Regional Carbon Sequestration Partnership (MRCSP) DOE-NETL Cooperative Agreement DE-FC26-05NT42589 Prepared by: CONSOL Energy Inc Brownsville Road South Park, PA Submitted by: Battelle 505 King Avenue Columbus, OH MRCSP Program Manager: David Ball The U.S. Department of Energy, National Energy Technology Laboratory 3610 Collins Ferry Road P.O. Box 880 Morgantown, WV Program Manager: Traci Rodosta September 2010

2 DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government, nor any agency thereof, nor any of their employees, nor Battelle, nor any member of the MRCSP makes any warranty, express or implied, or assumes any liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendations, or favoring by Battelle, members of the MRCSP, the United States Government or any agency thereof. The views and the opinions of authors expressed herein do not necessarily state or reflect those of the members of the MRCSP, the United States Government or any agency thereof.

3 Understanding Deep Coal Seams for Sequestration Potential Final Report of Work Performed June 21, 2006 through March 31, 2010 for the Midwest Regional Carbon Sequestration Partnership (MRCSP) Phase II Submitted To Battelle Memorial Institute 505 King Avenue Columbus, OH Battelle Subcontract No By CONSOL Energy Inc. Research & Development 4000 Brownsville Road South Park, PA Deborah A. Kosmack September 2010

4 DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. 1

5 ABSTRACT Geologic sequestration of carbon dioxide (CO 2 ) is a possible way to mitigate greenhouse gas emissions. Of the geologic options, sequestering CO 2 in coal beds has several advantages. Combining enhanced coal bed methane extraction with injection and storage of CO 2 could be economical for unminable coal seams. This project is focused on extending the coal data base in the Northern Appalachian Basin. A single drill core was completed down to the Mississippian Formation in the Fallowfield Coal Reserve located in Washington County near Eighty Four, Pennsylvania. Seven coal seams with a thickness from 0.55 feet to 3.33 feet were defined as the core was drilled from 1130 feet to 1554 feet below the surface. Sampling involved obtaining fresh coal samples from the drill core and placing them in canisters for methane desorption testing. Subsamples approximately 6-8 inches long were selected from single lithotypes and submitted for carbon dioxide adsorption isotherm testing. All segments were crushed and tested in accordance with ASTM standards for as-received moisture, carbon, hydrogen, oxygen, nitrogen, sulfur, and calorific values. Representative splits from the stage crushing were reserved for subsequent detailed analysis of petrographic and metallurgical properties. The methane desorption for the coals ranged from 60 standard cubic feet (scf) per ton of coal to 194 scf/ton on an as-received basis. This represents the actual in-situ methane contents of the coals, and this may differ from the methane adsorption capacity because the coal may be over-saturated or under-saturated. The three coal seams that desorbed the most methane were Middle Kittanning at 194 scf/ton, Brookville at 178 scf/ton, and Tionesta at 166 scf/ton. The carbon dioxide adsorption for the coals ranged from 354 scf/ton to 717 scf/ton on an in-situ basis at the estimated pressure of the coal seam. The three coal seams that adsorbed the most carbon dioxide were Middle Kittanning at 717 scf/ton, Brookville at 667 scf/ton, and Upper Kittanning at 665 scf/ton. Sorption capacity of the coals was related to various chemical parameters of the coal such as ash content, volatile matter, and maceral composition. Although we did not measure methane sorption capacity, we can examine the relationship between the carbon dioxide sorption capacity at estimated reservoir pressure and the in-situ methane content on a dry, ash-free basis. This examination showed that the coals were capable of storing from 2.9 to 5.6 times as much carbon dioxide as the in-situ methane content on a scf/ton basis. This ratio may be inflated over the ratio of storage capacity of the two gases because some of the coal may have been under-saturated with methane in their in-situ state. Since estimation of the reservoir size was not part of this study, a total gas storage capacity of any of the coal seams could not be predicted from the experimental data. 2

6 TABLE OF CONTENTS DISCLAIMER... 1 ABSTRACT EXECUTIVE SUMMARY INTRODUCTION GEOLOGY AND GEOGRAPHY Core Drilling into Coal Seams Geophysical Logs Natural Gamma Log Bulk Density Log (Gamma-Gamma Density) Resistivity Log EXPERIMENTAL Core Sampling Method Methane Desorption Test Method Carbon Dioxide Adsorption Test Method Coal Sample Analysis Petrographic Composition Maceral Analysis Vitrinite Reflectance RESULTS AND DISCUSSION Coal Cores Wireline Log Methane Desorption Carbon Dioxide Isotherms Comparison between Methane and Carbon Dioxide Sorption Chemical Analyses of Coals Correlations Between Gas Sorption and Coal Characteristics Petrographic Analysis Lower Freeport Seam Upper Kittanning Seam Middle Kittanning Seam Clarion Seam Brookville Seam Tionesta Seam Lower Mercer Seam Selection of a Coal Seam for Carbon Dioxide Sequestration

7 6. CONCLUSION...44 REFERENCES...47 LIST OF ACRONYMS AND ABBREVIATIONS...47 APPENDICES Appendix A. Detailed Geophysical Log on Fallowfield Core Hole Appendix B. Methane Desorption Laboratory Data and Summary Appendix C. Carbon Dioxide Adsorption Laboratory Data and Summary Appendix D. Petrographic Laboratory Testing on Coal Samples for Metallurgical Properties 4

8 List of Tables Table 1. Electric Logging Methods for Coal Cores...12 Table 2. ASTM Methods Used For Chemical Analysis of Coals...20 Table 3. ASTM Methods Used for Petrographic Analysis of Coals...22 Table 4. Coal Seams Identified in the Northern Appalachian Coal Basin at Fallowfield Reserve...22 Table 5. Stratigraphic Column of Northern Appalachian Seams...23 Table 6. Summary of Laboratory Results of Methane Desorption of Coal Samples...28 Table 7. Adsorption Isotherm Data for Middle Kittanning Coal Sample...29 Table 8. Adsorption Isotherm Parameters for Middle Kittanning Coal Seam...29 Table 9. Carbon Dioxide Isotherm Data from Weatherford Laboratories on Coal Samples...32 Table 10. Carbon Dioxide Adsorption Capacity at Different Pressures...32 Table 11. Ratio of Carbon Dioxide to Methane Sorption Capacity of Tested Coals...34 Table 12. Results from Proximate Analysis and Calorific Value...36 Table 13. Chemical Composition of Coal Seams...36 Table 14. Elemental Composition in Coal Ash...36 Table 15. Petrographic Analysis of Coal Samples...41 List of Figures Figure 1. Yellow Star Marks the Location of the Core Drilling Hole in the Fallowfield Coal Reserve in Washington County, PA Figure 2. Drilling Rig Used at the Fallowfield Site Showing Drilling Bit and Core Section Being Removed Figure 3. Removal of Drilling Core Samples onto Wooden Racks for Geologist to Evaluate Figure 4. Laboratory Canisters in the Field Containing Coal Sample for Methane Desorption Study Figure 5. Methane Desorption Equipment Showing Gas Displacement Measuring Equipment and Ball Mill...17 Figure 6. Schematic of Laboratory Apparatus for Carbon Dioxide Adsorption...19 Figure 7. Gamma, Density, and Resistivity Logs of Core Hole Showing Coal Seam Locations 24 5

9 Figure 8. Expanded Electric Logs of Core Hole Showing Middle Kittanning Coal Seam...25 Figure 9. Initial Desorption of Middle Kittanning Coal Sample Plotted with Square Root of Time to Estimate Lost Gas...26 Figure 10. Desorption Curve for Middle Kittanning Coal Sample...27 Figure 11. Carbon Dioxide Adsorption Isotherm for Middle Kittanning Coal Sample...30 Figure 12. Carbon Dioxide Adsorption Isotherms for Coal Samples Projected to 2500 psia...30 Figure 13. Carbon Dioxide Adsorption Isotherms on a Dry, Ash-Free Coal Basis for Coal Samples...31 Figure 14. Adjustment of Carbon Dioxide Storage Capacity with Moisture and Ash Content of the Coal...33 Figure 15. Correlation of Gas Storage Capacity to Volatile Matter on an As-Received Coal Basis...37 Figure 16. Correlation of Gas Storage Capacity to Volatile Matter on a Dry, Ash-Free Coal Basis...37 Figure 17. Correlation of Gas Storage Capacity to Ash Content on an As-Received Coal Basis...38 Figure 18. Correlation of Gas Storage Capacity to Ash Content on a Dry, Ash-Free Coal Basis...38 Figure 19. Correlation of Gas Storage Capacity to Maceral Composition on an As-Received Coal Basis...39 Figure 20. Correlation of Gas Storage Capacity to Maceral Composition on a Dry, Ash-Free Coal Basis...39 Figure 21. Correlation of Gas Storage Capacity on an As-Received Coal Basis and Distance to Top of Coal Seam...40 Figure 22. Correlation of Gas Storage Capacity on a Dry, Ash-Free Coal Basis and Distance to Top of Coal Seam...40 Figure 23. Correlation of Vitrinite Reflectance to the Depth of the Top of the Coal Seam...41 Figure 24. Middle Kittanning Coal Core Samples...42 Figure 25. Brookville Coal Core Samples

10 1. EXECUTIVE SUMMARY Geologic sequestration of carbon dioxide (CO 2 ) is a possible way to mitigate greenhouse gas emissions. Of the geologic options, sequestering CO 2 in coal beds has several advantages. Firstly, just as CO 2 has been shown to enhance oil recovery, CO 2 injection can enhance methane production from coal beds. Coal also has the advantage of being able to trap CO 2 for long periods of time. Potential major coal basins that may contain suitable beds for sequestration are near many emitting source of CO 2 and this close proximity can improve the economics of mitigating the CO 2. Therefore, it is of value to expand the existing data base of available coal resources and define potential seams that are unminable yet could be used instead for coal bed methane extraction coupled with injection and storage of CO 2. This project was focused on extending the coal data base in the northern Appalachian Basin. A single drill core was completed down to the Mississippian Formation in the Fallowfield Coal Reserve located in Washington County near Eighty Four, Pennsylvania. Sampling involved obtaining fresh coal samples from the drill core and placing them in canisters for methane desorption testing. Subsamples approximately 6-8 inches long were selected from single lithotypes and submitted for carbon dioxide adsorption isotherm testing. All segments were crushed and tested in accordance with ASTM standards for as-received moisture, carbon, hydrogen, oxygen, nitrogen, sulfur, and calorific values. Representative splits from the stage crushing were reserved for subsequent detailed analysis of petrographic and metallurgical properties. A 2.85 inch core was drilled from ground level to 1554 feet below the surface to reach the Mississippian Formation. The surface elevation of the hole was feet AMSL. At a depth of feet from the surface, the Upper Freeport seam was logged. Below the Upper Freeport seam, from 1130 feet to 1554 feet, seven coal seams were defined with various thicknesses. As the core was extracted from the drilling tube, it was logged by the geologist as to the type of strata. With the aid of gamma and density electric logs, the exact depth and thickness of each coal seam was determined. The defined coals with their respective depth from the top of the seam and thickness are listed: Lower Freeport at feet and 3.2 feet thick, Upper Kittanning at feet and 2.0 feet thick, Middle Kittanning at feet and 3.3 feet thick, Clarion at feet and 0.7 feet thick, Brookville at feet and 2.75 feet thick, Tionesta at feet and 0.75 feet thick, and Lower Mercer at feet and 0.55 feet thick. The coal core samples were placed in airtight canisters in the field and transported to an environmentally controlled laboratory for testing. The methane desorption tests were conducted on the seven core samples using the U.S. Bureau of Mines Direct Method. Gas was released from the canisters and measured directly by water displacement. The observed methane gas released from the canister were categorized as; lost gas, desorbed gas, and residual gas. The lost gas was determined by plotting the amount of gas lost in the first day of testing against the square root of desorption time. The desorbed gas was determined by measuring the accumulated gas released from the 7

11 canister over the test period. The residual gas amount was determined by crushing a portion of the coal core in a ball mill after the desorption was completed and then measuring the gas by water displacement. All gas measurements were corrected to standard pressure and temperature. Test results of gas storage were reported on a volume-to-mass ratio by adding the three gas types for each coal and dividing by the appropriate coal weights. The results could be reported on a dry, ash-free coal basis once the chemical properties of the coal were determined. The methane desorption for the coals ranged from 60 standard cubic feet (scf) per ton of coal to 194 scf per ton on an as-received basis. This represents the actual in-situ methane contents of the coals, and this may differ from the methane adsorption capacity because the coal may be over-saturated or under-saturated. The three coal seams that desorbed the most methane were Middle Kittanning at 194 scf/ton, Brookville at 178 scf/ton, and Tionesta at 166 scf/ton. Reporting the values on a dry, ash-free coal basis resulted in an increase in each of the storage capacities; Middle Kittanning at 255 scf/ton, Brookville at 207 scf/ton, and Tionesta at 250 scf/ton. The desorbed methane was related to various chemical parameters of the coal as ash content, volatile matter, and maceral composition. The coal core samples measuring over nine inches thick were further tested to determine their storage capacity for carbon dioxide. The carbon dioxide adsorption testing was conducted by Weatherford Laboratories in Golden, Colorado. The laboratory controlled test was conducted using a reference cell and a sample cell submerged in a constant temperature bath to expose the coal to carbon dioxide gas. The pressure of the carbon dioxide was incrementally increased and the amount of gas adsorbed in the coal located in the sample cell was measured. A curve was developed showing the gas sorbed per mass of sample as the pressure increased. The Langmuir relationship was then assumed to predict the endpoint storage capacity. The carbon dioxide adsorption for the coals ranged from 354 scf/ton to 717 scf/ton on an as-received basis at the estimated pressure of the coal seam. The pressure of the coal seam was estimated from the depth of the coal seam times 0.43 psi/foot to obtain values from 526 psia to 636 psia. The three coal seams that adsorbed the most carbon dioxide were Middle Kittanning at 717 scf/ton, Brookville at 667 scf/ton, and Upper Kittanning at 665 scf/ton. Reporting the values on a dry, ash-free coal basis, the storage capacities increased; Middle Kittanning at 824 scf/ton, Brookville at 829 scf/ton, and Upper Kittanning at 753 scf/ton. Although we did not measure methane sorption capacity, we can examine the relationship between the carbon dioxide sorption capacity at estimated reservoir pressure and the in-situ methane content on a dry, ash-free basis. This examination showed that the coals were capable of storing from 2.9 to 5.6 times as much carbon dioxide as the in-situ methane content on a scf/ton basis. This is slightly higher than the published data referenced here that show bituminous coals have a capacity to store around two times (from 1.82 to 2.25) as much carbon dioxide as methane when comparing strictly adsorption capacities. Some of the coal may have been under- 8

12 saturated with methane in the in-situ state and the use of different laboratory test methods in this study, i.e., desorption for methane and adsorption for carbon dioxide, may have resulted in the measurement of lower values for methane storage. This would lead to higher values of the CO 2 :CH 4 ratio on a scf/ton basis. Since estimation of the reservoir size was not part of this study, a total gas storage capacity of any of the coal seams cannot be predicted from the experimental data. The coal had a wide range of ash content on a dry basis, ranging from 11% for the Upper Kittanning coal to 39% for the Lower Freeport coal. The moisture content was around 1% for most coals except for the Middle Kittanning coal which was around 7%. The lowest sulfur content on a dry basis was in the Lower Mercer coal at 1% and the highest sulfur content was above 6% in the Clarion coal. The volatile matter ranged from 24% on a dry basis at the Lower Freeport location and as high as 34% from the Middle Kittanning coal. The vitrinoid percent was lowest with the Lower Mercer coal at 28% and highest with the Upper Kittanning coal at 79%. The coals were mostly ranked as high-volatile A bituminous coals. 2. INTRODUCTION Geologic sequestration of carbon dioxide (CO 2 ) is a possible way to mitigate greenhouse gas emissions. Of the geologic options, sequestering CO 2 in coal beds has several advantages. Firstly, just as CO 2 has been shown to enhance oil recovery, CO 2 injection can enhance methane production from coal beds. Coal also has the advantage of being able to trap CO 2 for long periods of time. Potential major coal basins that may contain suitable beds for sequestration are near many emitting sources of CO 2 and this close proximity can improve the economics of mitigating the CO 2. Therefore, it is of value to expand the existing data of available coal resources and define potential seams that could be used for CO 2 enhanced coal bed methane extraction, or for sequestration of CO 2. Coal bed methane production combined with CO 2 injection and storage expands the use of a coal resource even if the coal cannot be mined. This project was focused on extending the coal data base in the Northern Appalachian Basin. A single drill core was completed down to the Mississippian formation in the Fallowfield Coal Reserve located in Eighty Four, Pennsylvania. Sampling involved obtaining fresh coal samples from the drill core and placing them in canisters for methane desorption testing. Subsamples approximately 6-8 inches long were selected from single lithotypes and submitted for carbon dioxide adsorption isotherm testing. All segments were crushed and tested in accordance with ASTM standards for as-received moisture, carbon, hydrogen, oxygen, nitrogen, sulfur, and calorific values. Representative splits from the stage crushing were reserved for subsequent detailed analysis of petrographic and metallurgical properties. The report summarizes the results of the core drilling and coal testing. The data collected were then used to do a coal bed evaluation that qualitatively and quantitatively assesses the potential of the coal bed to sequester CO 2. The coal beds 9

13 were characterized as to their specific adsorption capacity (volume of gas absorbed per ton of reservoir). These data could be used to model the total capacity of the coal bed repository relative to potential sites of CO 2 generation; and thus provide data to evaluate the potential economic benefits of CO 2 sequestration in combination with coal bed methane production Core Drilling into Coal Seams 3. GEOLOGY AND GEOGRAPHY Core drilling into coal seams is conducted by mining companies for commercial reasons. The ultimate objective of a coring program is to collect a group of representative samples that define both the vertical and lateral variability of a mineable Washington County, PA coal seam in a given resource area. It provides information to the mining company on the thickness of the coal seam, the quality of the coal seam, the integrity of the roof rock, and the integrity of the floor rock. When the core is drilled beyond the desired minable coal seams, information is gained on additional coal reserves in the basin. CONSOL Energy Inc. is developing pre-mining core drilling plans that focus on acquiring geological information on the areas that are proposed for mining in Washington Figure 1. Yellow Star Marks the Location of the Core Drilling Hole in the Fallowfield Coal Reserve in Washington County, PA ( County, Figure 1. CONSOL was drilling cores in the area called the Fallowfield Reserve in the county near Eighty Four, PA, which is in the southwestern part of the state. The location of the drill core is marked with the yellow star on map. CONSOL was exploring the Upper Freeport coal seam in this area since the Pittsburgh coal seam was already mined. This project extended the depth of drilling to reach the Mississippian Formation. The objective was to locate, identify, and test any coal seams below the Upper Freeport seam. 10

14 3.2. Geophysical Logs Close observation of the removed core by the geologist provides geologic characteristics of the underlying strata. Besides the actual core, the geophysical logs are commonly the most important set of field data. These logs provide precise data from which the thickness and depth of coal beds can be determined and the lithology of the associated strata can be inferred. A number of geophysical instruments have been developed to reliably scan, analyze, and log boreholes. These electric logging tools are lowered into the hole on a wireline, a thick, flexible sheathed cable that conducts electricity down to the tool, and transmits the tool readings back up. A truck carries enough wireline on a large spool to lower the tool many thousands of feet. Measurements are taken as the tool is raised through the borehole at a constant speed. Information is electrically relayed to the truck where it is displayed on auxiliary instruments, transferred to a chart recorder as a permanent log, or converted to a digital signal that is compiled by a computer and recorded on a tape or disk. The truck is an advanced portable scientific and measurement laboratory on wheels. The electric log tools produce a paper geophysical or electric log (E-logs). A combination or suite of down-hole instruments is employed to adequately geophysically log a borehole. The basis suit of four geophysical logs most commonly used in coal logging operations is gamma ray (neutral), bulk density (Gamma-Gamma Density), resistivity, and caliper. Table 1 summarizes (Luppens and others, 1992) some of the more important characteristics of these borehole logging methods that are used when describing coal beds. 11

15 Method Gamma Ray Bulk density (gamma-gamma) Table 1. Electric Logging Methods for Coal Cores Response to Conditions that Invalidate Log or Units Coal Make Interpretation More Difficult Clean sand adjacent to coal bed. Low natural CPS or API Coal bed containing uranium-bearing gamma minerals. Irregular hole diameter (washout). Low density g/cm 3 Caved shale adjacent to coal bed. Fractured strata surround coal. Highly resistant strata next to coal Resistivity High resistivity ohm-m Caliper log Measure borehole diameter inches or cm bed. Extremely thick mudcakes or drill hole fluid-fractures (washout) or borehole diameter deviations larger than caliper arms Natural Gamma Log The natural gamma log measures the amount of radioactivity emitted by the various stratigraphic lithotypes encountered as a detector is moved up a borehole. Certain minerals found in clays, shales, and sandstones, contain measureable quantities of naturally occurring radioactive isotopes of potassium, uranium, and thorium which emit detectable amounts of gamma radiation. By measuring the amount of this radiation, lithologic interpretations of the strata can be made. Generally, coal and clean sands are low in natural radioactivity. The rock that surrounds a coal bed often contains potassium-rich clay and sometimes uranium-based materials. This difference in radioactive characteristics provides the basis for differentiating coal from rock on a recorded natural-gamma log. During logging, the amount of natural radiation detected by the down-hole probe is recorded and expressed in either CPS (counts-per-second) or API (American Petroleum Institute) units Bulk Density Log (Gamma-Gamma Density) The bulk density log (gamma-gamma density) is used to determine the bulk densities of subsurface formations. Coal has low apparent densities in the range of 1.20 to 1.60 g/cm 3. Less porous and more consolidated strata such as shales and sandstones have higher apparent densities, generally greater than 2.00 g/cm 3. Because coal has such low densities compared with other rocks, the recorded density on the charted log is distinctly lower. This characteristic makes this log the singularly most important geophysical log for coal exploration. As the logging probe is drawn up a borehole, a radioactive source contained within the tool bombards the adjacent strata with gamma-rays. A detector on the probe then measures the amount of gamma radiation back-scattered from the formation. The 12

16 intensity of radiation (in CPS) back-scattered to the detector is inversely related to the density of the formation (g/cm 3 ). Under normal drilling conditions (nearly flat-lying strata) the most reliable thickness determination can be made from the density log and the caliper log. If the roof and floor lithologies are not sand, the resistivity and natural gamma can also be useful, especially if washouts are indicated from the caliper log. Regardless of the method used, the estimated thickness from the geophysical logs must be checked against measured coal core sections for final determination. With experience, thicknesses can be determined from geophysical logs precisely to within about ±3 cm (0.1 ft) or less depending on the type of tool used. Besides depth and thickness of coal and rocks, the geophysical logs can be used to infer analytical and engineering data. For example, the bulk density logs can be used to estimate strength indices, which can give valuable roof support information. Lithology can be determined from density and natural-gamma logs coupled with resistivity logs. The caliper log can give an indication of strata that are soft, highly fractured, or that can readily decompose during drilling. The bulk density log commonly correlates well with the laboratory determined apparent density of the coal core intervals, which can be used to estimate the ahs and calorific content Resistivity Log The resistivity log measures the degree of resistance of a lithologic formation (and the fluids contained in the formation) to the flow of an electric current that is passed through it. The capacity of formations to conduct electricity is not dependent on the minerals that comprise those strata, but rather on their respective physical characteristics (pore geometry) and the amount of mineralized water they contain. On the basis of those factors, the resistivity of a formation is directly dependent upon the resistivity of the water in its pores and its clay mineral content. A probe containing a pair of electrodes at a set spacing is raised through the borehole at a constant speed. An electrical current is passed into the strata encountered along the walls of the borehole and the amount of voltage passed between the electrodes is measured. The voltage differential is then detected and transferred to a chart or digital recorder. The measured voltage difference provides the basis for the resistivity determination. Coal is a poor conductor of electric current and consequently exhibits high resistivity readings. Tight sandstone or limestone beds will also exhibit similar resistivities as coal. Thus, as with the natural gamma log, the resistivity cannot be used alone in identifying and describing lithologic strata. 13

17 4. EXPERIMENTAL 4.1. Core Sampling Method Photographs of the equipment used at the drilling site are shown in Figure 2. A wire line drill-core system was used to drill the core hole. The double tube, core-barrel assembly allows the inner barrel containing the core sample to be hoisted through the drill pipe string without removing any of the rods or the outer core barrel. The size of the outer tube used was 2.85 inches in diameter and the retrieved core sample measured approximately 2 inches. A cored section sample in a single run was 20 feet long. Water was constantly circulated to assist in the drilling. A diamond coring bit was used to allow for coring hard or dense Figure 2. Drilling Rig Used at the Fallowfield Site Showing Coring Bit and Core Section Being Removed lithologies. This bit is comprised of diamonds which have been placed in a mold with a powdered metal and formed into a solid mass on the end of the steel drill bit. The diamond end is referred to as the crown of the bit. The cores were removed from the inner barrel by the field crew with a rod, being careful not to break the coal core sections into pieces. The cores were placed on wooden racks where the geologist measured and defined the different strata layers. A photograph of the core being removed and examined at the site is provided in Figure 3. The measured length of the coal section was compared with the measurement of the cored coal seam interval reported by the driller. The following physical description of each coal core was noted: measured length of described interval, color, coal type, Figure 3. Removal of Drilling Core Samples onto Wooden Racks thickness and for Geologist to Evaluate abundance of 14

18 banding, layering of vitrain and attritus, natural fracture (cleats, slips, joints, etc.), mineralization, hardness, distinctive features, and the nature of roof and floor contacts (sharp, gradational, rooted). The rock core or noncoal core was also described in detail in the field. Figure 4. Laboratory Canisters in the Field Containing Coal Sample for Methane Desorption Study The time that the coal core was exposed to the weather conditions was minimized. This was to curtail the amount of moisture and gas lost during sample retrieval. Since it is not uncommon for gas contents to vary over the length of a coal core due to coal compositional changes as the mineral matter varies, the entire coal core was placed in a desorption canister for testing. If the core was longer than the eighteen inch canister, the separated sections were selected relative to the naturally occurring breaks in the coal seam, such as obvious shale partings. A photograph of the canisters containing recently extracted coal samples from the drilling core in the field can be seen in Figure 4. The total core sample in the canister was immediately transported to a lab with controlled ambient temperature where the methane desorption study was completed. After the desorption study was complete, a sample section measuring between six and eight inches long was sent to Weatherford Laboratories in Golden, Colorado, to conduct the CO 2 desorption study (Subsample 1). The remaining sample was sent to the CONSOL R&D analytical lab for analysis (Subsample 2). To assure that a representative sample of the entire coal core was used for the analytical testing, Subsample 1 was crushed to 8-mesh and homogenized by Weatherford. A quarter of this crushed sample (Subsample 1A) was then returned to CONSOL R&D and the retained piece of the sample was used for the CO 2 adsorption study (Subsample 1B). Subsample 1A was properly mixed with a portion of Subsample 2 to assure a representative sample of the entire coal core was analyzed. A portion of this composite sample was used for petrographic analysis. The remaining portion was crushed to 60 mesh for the chemical analysis Methane Desorption Test Method Gas content in coalbeds can be determined by several different methods. Each method requires a different level of equipment sophistication, auxiliary test data, and data reduction complexity to arrive at a final gas content value. Selection of a gas content testing method is usually determined by the particular application of the gas content data, the degree of accuracy required, and the cost of equipment and testing. 15

19 Regardless of which method is selected, applying good experimental method and laboratory practices is essential to maximize the accuracy of any test procedure. In general, there are two types of methods used to determine the gas content of a coal sample: a direct method and an indirect method. The type of direct method varies, including the original U.S. Bureau of Mines (USBM) direct method (Kissell and others, 1973), National Institute for Occupational Safety and Health (NIOSH) modified direct method (Diamond and others, 2001), quick-crushing gas content test method, and pressure coring. The type of Indirect Method for determining gas content is based on sorption isotherm data or the estimations from wire line geophysical logging tools. We used the U.S. Bureau of Mines (USBM) Direct Method. This method uses standard desorption canisters constructed from an eighteen inch long section of a four inch diameter aluminum pipe. The gas content in the coal is divided into three parts: lost, desorbed, and residual gas. Each of these parts is measured or estimated by a different procedure, and then combined to yield the total gas content in the sample. The lost gas is that portion of the total gas that escapes from the sample during its collection and retrieval prior to being sealed into an airtight desorption canister. Lost gas volumes cannot be directly measured and therefore must be estimated from the subsequently measured desorbed gas volume data. Since the lost gas is an estimated quantity, it is generally considered to be the least reliable component of the total gas content. The volume of gas lost prior to sealing a coal sample into a desorption canister is influenced by the following four variables: sample retrieval time, the physical characteristics of the sample, the type of drilling fluid, and the water saturation per relative amount of free gas. The sample retrieval time includes the time to pull the core barrel from over 1000 feet below the surface, remove the sample from the core barrel, determine the depth interval of the coal seam, inspect and geologically describe the coal sample, and seal the sample in the canister as was discussed in Core Sampling Method, Section 4.1. This process takes about one hour. In terms of desorption, this represents a relatively short lost gas time and reduces the potential error inherent in the estimation technique. The physical characteristics of the retrieved coal sample can influence the desorption rate and hence the volume of lost gas that must be estimated. Blocky coals which remain intact during coring and subsequent retrieval emit their adsorbed gas at a relatively slow rate. However, friable coals which tend to break apart into smaller fragments, release their adsorbed gas faster because of the shorter diffusion distances. 16

20 Once the coal sample is sealed in the desorption canister, the desorbing gas accumulates and can be measured directly by the variation of the water displacement. The volume of gas desorbing from a coal sample gradually declines with time. Desorption measurements are terminated at some point when a low desorption rate is reached. Friable coal samples generally desorb gas very quickly (within days or weeks), while blocky coals may desorb gas for several months. Generally, at the point when the desorption rate reaches the established termination point, some volume of gas still remains in the sample. This residual gas volume can be determined by crushing the sample in an airtight container and measuring the volume of gas released by using the same method used for determining the desorbed gas. Residual gas can comprise 40 to 50 percent of the total gas content, in particular for the high-volatile A bituminous blocky coal seams. In contrast, friable, higher rank (medium to low volatile) bituminous coal seams typically have less than 10 percent residual gas. A photograph of the laboratory setup used for the desorption of the coal cores is provided in Figure 5. The canisters were kept at the controlled ambient temperature of the laboratory. Gas volume measurements were made by periodically releasing the accumulated gas into a waterfilled inverted graduated cylinder. All volumetric readings were corrected to standard temperature and pressure (STP) conditions. The gas industry convention for STP conditions, at 60 F and 14.7 psia, was used to report the coalbed gas content. The reported data was also adjusted for the head space left in the canister after the sample was inserted. Figure 5. Methane Desorption Equipment Showing Gas Displacement Measuring Equipment and Ball Mill As changes in ambient pressure and temperature on the head space volume between subsequent desorption readings could influence the measured change in gas volume, the canisters containing the coal core were filled as full as possible to minimize error. Initially, for the first several hours, the measurements of the desorbed gas volume were made every 15 to 20 minutes. These data were then plotted on a graph of cumulative desorbed gas as it relates to the square root of desorption time. The initial linear portion of the desorption curve was extrapolated through the point on the x-axis representing the lost gas time to estimate the lost gas volume. The time zero was defined as the time when desorption started. Conventionally, when drilling mud is used, time zero is taken as the estimated time at which the sample is halfway out of the well bore. 17

21 When the periodic measurement of desorbed gas was discontinued after several weeks, a portion of the desorption sample was crushed to a powder (-200 mesh) in a ball mill to release the remaining in situ gas. The volume of this residual gas was measured by water displacement method. The gas volume released by crushing is attributed only to the portion of the sample crushed to a powder. The total gas content of the sample was the sum of the lost gas, desorbed gas, and residual gas. The gas volume was divided by the respective weight of the air-dried coal sample. The calculation of gas content on a volume-to-mass (weight) ratio, cm 3 /g or ft 3 /ton, is used by the mining industry to calculate coal reserves on a weight basis. In the event that any part of the core sample from a single coal seam was split into multiple canisters, the data from multiple samples was combined for a single gas content value. The value was normalized by mass rather than calculating a simple arithmetic average. The coal compositional basis for the gas content values was an important data-reporting issue. Raw (as-received) gas content results were corrected to an ash-free value to aid in the comparison of results on a normalized basis. While an ash-free basis for reporting gas content values is desirable from a standpoint of comparing results, it can lead to confusion when calculating gas resource estimates. Hence, gas content was measured and compared using both the as-received basis and the dry, ash-free basis Carbon Dioxide Adsorption Test Method As mentioned in Methane Desorption Test Method, Section 4.2, the gas content in coal seams can be estimated indirectly based on sorption isotherm data or empirical estimation curves of measured gas content plotted against other measurable variables such as coalbed depth and coal rank. Laboratory derived sorption isotherms describe the quantitative relationship between adsorbed methane at varying pressures and a constant temperature, and provide a measurement of the maximum methane sorption or storage capacity of the coal sample. Deriving isotherms in the laboratory assumes the Langmuir relationship accurately predicts the endpoint gas storage capacity data. For the pressure and temperature conditions of most interest in coal seams, the accuracy of the Langmuir relationship is generally considered to be adequate and the isotherm data must be measured at a similar moisture content and temperature of interest. The samples were shipped to Weatherford Laboratories in Golden, Colorado, to conduct the carbon dioxide adsorption tests. Among the collection apparatus was a reference cell and a sample cell connected through tubing equipped with a control valve as shown in Figure 6 on the next page. The cell volume was approximately 218 cm 3. The apparatus was submerged in an oil bath maintained within 0.18 F of the desired temperature. Ideally, the desired temperature of the sample is the temperature of the coal seam underground, but the actual temperature was altered by the surface water being circulated during drilling and the exposure to the atmosphere during geological 18

22 inspection of the core. In this case, the oil bath returned the coal core closer to its original temperature. The temperatures used for testing are provided in the results under Carbon Dioxide Isotherms, Section 5.4 and reflect the reservoir temperature at the depth of interest. Figure 6. Schematic of Laboratory Apparatus for Carbon Dioxide Adsorption Testing the sample at the in-situ moisture content is important to obtain accurate data. For that reason, the coal samples were humidified to reach equilibrium moisture content before the adsorption test was conducted. Samples were crushed to reduce the equilibrium time of the calibration and isotherm measurement steps. Crushing was performed in repetitive stages to allow the samples to pass through the largest screen size desired without undue creation of fines or small particles. The goal of this careful reduction was to minimize sample loss, to achieve consistent particle sizes, and to preserve the original sample composition. Up to seven stages were used to crush the samples to -8 mesh (235mm) and then -60 mesh (0.25mm). After crushing, the samples were homogenized and then divided following ASTM procedures to obtain a representative coal sample. To begin the test, the coal sample mass of 100 to 130 grams was placed in the sample cell and the reference cell was filled with carbon dioxide to the pressure of interest while isolated from the sample cell. The temperature and pressure of the sample cell were recorded. The control valve was opened long enough for the pressures and temperatures to stabilize while the gas escaped from the reference cell into the void volume within the sample cell. The control valve was then closed. This isotherm measurement step occurred as the gas was absorbed into the coal. The reference cell pressure remained constant and the sample cell pressure decreased until it stabilized. The temperature and pressure for each cell was continuously recorded and the end of the process was defined when stability was reached. Six increasing pressure steps were done on each coal sample. The number of molecules absorbed into a sample during each step is determined by material balance. The number absorbed was equal to the number of molecules that flowed out of the reference cell less the increase in the number of molecules in the void volume of the sample cell. The void volume in the sample cell was reduced by the volume of the absorbed phase. The number of molecules was converted to volume of gas at standard temperature and pressure. The volume was converted to gas storage capacity in units of standard cubic feet (scf) per ton of coal. The detailed laboratory testing is explained in a paper published by TICORA Geosciences, Inc. (Mavor and others, 2004). 19

23 Weatherford Laboratories took great care to maximize measurement accuracy while minimizing systematic and random errors during the testing. This assured that the gas storage capacity data was measured with an uncertainty range on the order of 5% or less. The Langmuir relationship for gas storage capacity at in-situ conditions, which is the principal behind the adsorption testing, is expressed in terms of Langmuir parameters as follows: G s = G L (p/(p+p L )) Where: G s = In-Situ Adsorption Gas Storage Capacity G L = Langmuir Storage Capacity p = Reservoir Pressure p L = Langmuir Pressure The estimated gas storage capacity is provided on an in-situ mass basis as reservoir engineering calculations are done in reservoir volumes converted to standard conditions. The equation provides a relationship between the reservoir pressure and storage capacity Coal Sample Analysis The main purpose of the coal sample analysis was to determine the rank of the coal along with its intrinsic characteristics. The data obtained were used to correlate gas adsorption capabilities. Table 2 below provides the ASTM test methods used to obtain the chemical information on the coal samples. The proximate analysis was conducted to determine the moisture content, volatile matter, carbon content, and ash content of the coal samples. The ultimate analysis and ash analysis provided details of the chemical composition. Table 2. ASTM Methods Used For Chemical Analysis of Coals Chemical Parameters of Coal Samples Total Moisture Moisture as determined, volatile matter, fixed carbon, and ash Carbon, Hydrogen, Nitrogen Oxygen (by difference) Sulfur Heating Value (Btu) Chlorine Mercury Major Ash elemental Coal sample preparation, up to 1000 grams (Required for samples that do not pass a -60 mesh screen) Method ASTM D3302 ASTM D5142 ASTM D5373 ASTM D3176 ASTM D4239 ASTM D5865 ASTM D6721 ASTM D6722 ASTM D6349 ASTM D

24 All parameters were analyzed at CONSOL Energy R&D in South Park, Pennsylvania. Quality control was ensured by analyzing a standard with the coal samples and duplicating the analysis of one in ten samples Petrographic Composition Maceral Analysis The coals were analyzed for type, which is related to their reactive and inert maceral content. The maceral analysis was performed using a microscope to determine the volumetric percentages of the various macerals present in a coal. The maceral composition has a profound effect on coke making and such information is useful when blending coals to produce high-quality coke. These components may also be important for the adsorption and storage of gases. Coals are commonly classified in terms of bright or dull types. Bright coals are generally considered superior to dull coals for coke-making. Microscopically, bright coals contain a predominance of the reactive maceral vitrinoid and may have significant amount of resinoid and exinoid macerals. The vitrinoid macerals soften and resolidify to form the continuous bond phase during carbonization. The exinoids and resinoids produce mostly byproducts but also contribute to the bond phase in coke. Dull-banded bituminous coals microscopically contain a greater abundance of the inert macerals. The inert macerals do not soften during carbonization and act as inert filler in the coke structure. The organic inert macerals are composed of micrinoids, fusinoids, and semifusinoids. The inorganic inerts are the ash-forming materials or mineral matter composed largely of silicon, aluminum, iron, calcium, and alkalis such as sodium and potassium Vitrinite Reflectance The mean maximum reflectance of the vitrinoid maceral, the major reactive component in coal, was determined microscopically to determine the rank of the coal. Since the reflectance of vitrinoids is not influenced by other coal properties as volatile matter is, reflectance is a more accurate measure of bituminous coal rank. Volatile matter can be affected by the presence of carbonates and changes in petrographic composition which can interfere with the determination of rank. Vitrinite reflectance increases as coal rank increases. Vitrinite reflectance is useful when blending coal to produce high-quality coke and are commonly specified in coal contracts. Reflectance measurements on individual vitrinoid macerals in bituminous coal may range from about 0.5 to 2.0% reflectance. Vitrinite reflectance measurements are classified into types, each type representing a reflectance range of 0.1%. For example, V type 8 contains all the vitrinoids with a reflectance of 0.8% through 0.9%. The reflectance range for coking coals, high volatile B to low volatile bituminous coals is %. 21

25 Table 3 below provides the ASTM test methods used to obtain petrographic information on the coal samples. Table 3. ASTM Methods Used for Petrographic Analysis of Coals Petrographic Parameters of Coal Samples Maceral Analysis Vitrinite Reflectance Method ASTM D2797, D2799 ASTM D2797, D Coal Cores 5. RESULTS AND DISCUSSION The hole was drilled and cored from ground level to 1554 feet below the surface to reach the top of the Mississippian Formation. The surface elevation of the hole was feet. At a depth of feet from the surface, the Upper Freeport seam was logged. Below the Upper Freeport seam, from 1130 feet to 1554 feet, seven coal seams were defined with various thicknesses. Table 4 summarizes the seams found by coal seam name, depth, and thickness. Table 5 shows the age, group, and geological formation of all the coal seams in the core hole below the Upper Freeport coal. Coal seams that have been logged in this region that were not present in this single core include the Lower Kittanning coal, the Upper Mercer coal, the Quakertown coal, and the Sharon coal. Many of these coals seams appear as isolated -deposits in the coal basin. Table 4. Coal Seams Identified in the Northern Appalachian Coal Basin at Fallowfield Reserve Coal Name Depth from Surface to Top of Coal Seam Core Length Feet Feet Inches Lower Freeport Upper Kittanning Middle Kittanning Clarion Brookville Tionesta Lower Mercer

26 Table 5. Stratigraphic Column of Northern Appalachian Seams Age Group Formation Code Major Units Pennsylvanian Pottsville Allegheny Clarion Kittanning Freeport UFR Upper Freeport Rider Coal UF Upper Freeport Coal UFS Upper Freeport Coal Expected Horizon (Surface) UFL Upper Freeport Leader Coal LUF Upper Freeport Limestone BUT Butler Sandstone LFR Lower Freeport Rider Coal LF Lower Freeport Coal LFL Lower Freeport Leader Coal LLF Lower Freeport Limestone FSS Freeport Sandstone UKR Upper Kittanning Rider Coal UK Upper Kittanning Coal UKL Upper Kittanning Leader Coal JTL Johnstown Limestone WOU Upper Worthington Sandstone WSM Washingtonville Marine Shale MKR Middle Kittanning Rider Coal MK Middle Kittanning Coal MKL Middle Kittanning Leader Coal WOL Lower Worthington Sandstone CM Columbiana Marine Shale LKR Lower Kittanning Rider Coal LK Lower Kittanning Coal LKL Lower Kittanning Leader Coal KS Kittanning Sandstone VP Vanport Marine Limestone SC Scrubgrass Coal CR Clarion Coal BV Brookville Coal HSS Homewood Sandstone TC Tionesta Coal UML Upper Mercer Marine Limestone UM Upper Mercer Coal LML Lower Mercer Marine Limestone LM Lower Mercer Coal CNU Upper Connoquenessing Sandstone QK Quakertown Coal CNL Lower Connoquenessing Sandstone SH Sharon Coal SHC Sharon Conglomerate Mississippian MS Mauch Chunk 23

27 Since this is just a single core sample, there is not enough information to project the volume of each of these seams in the area they were drilled. Thus, we did not predict how much methane or CO 2 can be stored in the reservoir Wireline Log The electric logs obtained in the field were gamma, density, and resistivity logs. The scale of the gamma log is from 0 to 300 CPS, the density log is from 1 to 3 g/cc, and the resistivity log is from 0 to 2000 ohm-m. Logs were provided showing the entire length of the hole at a scale of 1 inch equaling 10 feet. Additional logs were provided around the depths of the coal seams with an expanded scale of 1 inch equaling 1 foot. Figure 7 depicts a section of the log between 1175 feet and 1300 feet that shows three of the coal seams. The figure shows that the density log goes down and the gamma ray goes down when detecting coal. The density of the Lower Freeport coal is from g/cc, the Upper Kittanning is from 1.08 to 1.17 g/cc and the Middle Kittanning is C o a l Top of Lower Freeport Seam at 1180 feet below surface elevation S e a m T h i c k n e s s Top of Upper Kittanning Seam at 1226 feet below surface elevation Top of Middle Kittanning Seam at 1271 feet below surface elevation Figure 7. Locations Gamma, Density, and Resistivity Logs of Core Hole Showing Coal Seam 24

28 from 1.17 to 1.25 g/cc. The values measured in the laboratory showed similar densities for each of the coals at 1.43 g/cc for Lower Freeport, 1.37 g/cc for Upper Kittanning, and 1.34 g/cc for Middle Kittanning. Figure 8 shows the expanded scale for the Middle Kittanning coal seam. The thickness of the coal seam is 3.31 feet or 39.7 inches. C o a l S e a m T h i c k n e s s Thickness of Middle Kittanning Coal Seam at 3.31 feet Figure 8. Expanded Electric Logs of Core Hole Showing Middle Kittanning Coal Seam The complete geological description of the core hole is provided in Appendix A. 25

29 5.3. Methane Desorption The seven coal core samples were desorbed of methane over 12 weeks using the USBM Direct Method. The lost gas volume and residual gas volume were estimated for each core sample and the desorbed gas was measured. Figure 9 shows the plot used to calculate the lost gas for the Middle Kittanning coal seam. The coal seam was split into two canisters because of its length and each canister was measured separately. The straight line drawn through the initial desorbed gas volume shows a lost gas volume of 400 cc of gas at time zero. Figure 9. Initial Desorption of Middle Kittanning Coal Sample Plotted with Square Root of Time to Estimate Lost Gas The desorbed portion of the total gas content was measured periodically from the end of July to the middle of October, 2009 (approximately 12 weeks) by the water displacement technique. The gas desorption slowed to a rate at which essentially little gas was emitted. For the Middle Kittanning coal samples the change was from 0 cc to 5 cc. The range for the other coal samples was from 0 cc for the Tionesta coal to 57 cc for the Brookville coal samples. The cutoff times for the tests ranged from 1820 to 2006 hours (76 to 84 days) of desorbing time. Figure 10 on the following page is a graph of the desorption of one of the Middle Kittanning canisters. The straight line drawn through the first points on the curve shows how the lost gas content was determined. Initially the sampling interval was every 15 to 20 minutes until the core was in the 26

30 canister for six hours. After that time, the frequency of sampling was extended to daily measurements as the desorption rate of the coal slowed. The total desorbed gas measured after 1990 hours and corrected to STP conditions was 5174 cc. The other canister (C-1-5) with the Middle Kittanning coal had a total cumulative desorbed gas of 6686 cc. The difference can be due to the volume of sample in each canister or the non-uniformity of the coal seam. Figure 10. Desorption Curve for Middle Kittanning Coal Sample After the desorption was complete, the residual gas was then determined by crushing a portion of the sample in a ball mill and measuring the gas emitted using water displacement. For the Middle Kittanning coal, the residual gas measured was 1695 cc in one canister and 1688 cc in the other canister. The gas volumes were totaled and calculated on a volume to weight ratio. For the Middle Kittanning sample, the lost gas represented about 5% of the total desorbed gas, the actual desorbed gas was about 74% of the total, and the residual gas was about 21% of the total. The total desorbed gas for Middle Kittanning calculated as a ratio of gas to raw coal was 194 scf/ton and 255 scf/ton of dry, ash-free coal. Table 6 summarizes the results from the desorption of methane for all of the coal samples. The table provides the amount of gas by type, the sample weight, and the gas per ton of coal on a raw basis as well as on a dry, ash-free basis. The values for ash and moisture were obtained from the proximate analysis of 27

31 each coal sample as described in the Coal Sample Analysis, Section 4.4. The raw laboratory data, desorption curves, and summary for all coal samples are provided in Appendix B. Coal Name Lower Freeport Upper Kittanning Table 6. Summary of Laboratory Results of Methane Desorption of Coal Samples Desorbed Residual Raw Lost Gas Total Raw Gas DAF Gas Gas Gas Coal Estimate Measured Estimate Gas Content Content Weight grams scf/ton scf/ton (weighted (weighted avg) avg) Middle (weighted Kittanning avg) Clarion Brookville Tionesta Lower Mercer 255 (weighted avg) The three coal seams that desorbed the most methane per ton of raw coal were Middle Kittanning, Brookville, and Tionesta. The desorbed methane was related to various chemical parameters of the coal as volatile matter, ash content, and vitrinites. It was also of interest to determine if there was a correlation to the depth of the coal seam. Since the coals were similar in rank, there was no reason to plot against the range of coal rank. The correlations between the parameters and methane sorption were all very weak. R- squared (R 2 ) values were used to compare correlations where R 2 is the statistical measure of how well a regression line approximates real data points. An R 2 of 1.0 indicates a perfect fit. The best correlation, at an R 2 of 0.36, related the methane capacity on an as-received basis with the ash content of the coal. It showed that as the ash content increased the sorption decreased. Additional discussion along with the graphs are provided in Correlations Between Gas Sorption and Coal Characteristics, Section Carbon Dioxide Isotherms The coal samples that were sent for carbon dioxide adsorption testing represented coal seams that were thicker or equal to nine inches. This limited the number of coals tested to five with the Clarion and Lower Mercer coal seams eliminated from carbon dioxide adsorption testing. The laboratory results provided by Weatherford Laboratories are provided in Appendix C. The details on the Middle Kittanning coal sample and the highlights from the report are discussed in this section. 28

32 Table 7 summarizes the calculated gas storage capacity at each pressure step taken during the adsorption testing with carbon dioxide for the Middle Kittanning coal sample. The gas storage capacity data were fit with a Langmuir isotherm relationship and resulted in the Langmuir parameters listed in Table 8. The isotherm parameters are determined by the linear regression of p/g s as it relates to p. Confidence intervals were determined to judge the accuracy of the regression. Table 7. Adsorption Isotherm Data for Middle Kittanning Coal Sample Pressure Storage Capacity, in-situ psia scf/ton Table 8. Adsorption Isotherm Parameters for Middle Kittanning Coal Seam Parameter Unit Value Sample Parameters Measurement Temperature F Moisture Content wt fraction Ash Content wt fraction Langmuir Parameters Number of Points 6 Regression Coefficient Langmuir Storage Capacity, Organic Fraction scf/ton Langmuir Storage Capacity, In-Situ scf/ton Langmuir Storage Capacity Range, In-Situ scf/ton 1.78 Langmuir Pressure psia Langmuir Pressure Range psia

33 Figure 11. Carbon Dioxide Adsorption Isotherm for Middle Kittanning Coal Sample Figure 11 illustrates the estimated endpoint storage capacity, the uncertainty, and the 95% confidence intervals for the Middle Kittanning coal sample. The Langmuir regression equations accurately predicted the endpoint storage capacity data and provided a means for interpolation. Middle Kittanning Upper Kittanning Brookville Lower Freeport Tionesta Figure 12. Carbon Dioxide Adsorption Isotherms for Coal Samples Projected to 2500 psia The Langmuir relationships were also used to predict the storage capacity at 2200 psia. Figure 12 shows all five coals graphed together showing the in-situ storage capacity. Middle Kittanning represented the maximum storage capacity at 940 scf/ton. Upper Kittanning and Brookville were very close in behavior at 840 scf/ton. Lower Freeport was at 680 scf/ton and Tionesta was down at 400 scf/ton. 30

34 The presence of moisture and mineral content in the coal interferes with the adsorption of carbon dioxide. Looking at the gas storage on a dry, ash-free coal basis shows the range in adsorbed carbon dioxide is increased from 800 to 1100 scf/ton as seen in Figure 13. The order of maximum to minimum storage capacity changed: Middle Kittanning has maximum capacity at 1080 scf/ton, Brookville is next at 1040 scf/ton, then Upper Kittanning at 950, followed by Tionesta at 860 scf/ton, and dropping to the bottom is Lower Freeport at 830 scf/ton. Middle Kittanning Brookville Upper Kittanning Tionesta Lower Freeport Figure 13. Carbon Dioxide Adsorption Isotherms on a Dry, Ash-Free Coal Basis for Coal Samples The Langmuir relationships were used to predict the storage capacity at the estimated reservoir pressure. Table 9 on the following page shows the results of the storage capacity in-situ ranging from 717 scf/ton with Middle Kittanning coal to 354 scf/ton with the Tionesta coal. The estimated reservoir pressure was calculated from the depth of the coal seam using the conversion factor of 0.43 psi/ft. The estimated reservoir pressures ranged from 526 to 636 psia. On a dry, ash-free coal basis, Brookville had the best storage capacity at 829 scf/ton and Lower Freeport at 681 scf/ton had the worst. 31

35 Table 9. Carbon Dioxide Isotherm Data from Weatherford Laboratories on Coal Samples Lower Upper Middle Parameter Unit Brookville Tionesta Freeport Kittanning Kittanning Sample Parameters Measurement Temperature F Coal Moisture Content wt % Coal Ash Content wt % Langmuir Parameters Langmuir Storage Capacity, Dry, scf/ton Ash-Free Langmuir Storage Capacity, In Situ scf/ton Langmuir Pressure psia Adsorbed Gas Storage Capacity Estimated Reservoir Pressure psia Storage Capacity, Dry, Ash-Free * scf/ton Storage Capacity, In-Situ * scf/ton * At estimated reservoir pressure Table 10 shows the best to worst coal seams for storage capacity of CO 2 and how the capacity changed depending on what conditions were evaluated and what basis the evaluation was calculated. Table 10. Carbon Dioxide Adsorption Capacity at Different Pressures At 2000 psia At Estimated Reservoir Pressure In-situ Dry, Ash-Free In-situ Dry, Ash-Free Coal name scf/ton Coal name scf/ton Coal name scf/ton Coal name scf/ton Middle 940 Middle 1080 Middle 717 Brookville 829 Kittanning Kittanning Kittanning Upper 840 Brookville 1040 Brookville 667 Middle 824 Kittanning Kittanning Brookville 840 Upper 950 Upper 665 Upper 752 Kittanning Kittanning Kittanning Lower 680 Tionesta 860 Lower 565 Tionesta 720 Freeport Freeport Tionesta 400 Lower Freeport 830 Tionesta 354 Lower Freeport

36 A regression analysis was done relating the gas storage capacity to the moisture and ash contents of the coals. This was then used to predict different storage capacities knowing the ash and moisture content of the coal. This equation was used to recalculate the storage capacity with the analytical results obtained in the lab. Measurements of moisture and ash were different in the lab samples because it represented the total core obtained compared to a select portion that was used for the carbon dioxide adsorption testing. Figure 14 shows the prediction. Figure 14. Adjustment of Carbon Dioxide Storage Capacity with Moisture and Ash Content of the Coal There are several factors that affect the extent to which each coal can adsorb carbon dioxide. The nature of the coal will determine the maximum adsorption capacity under a given set of conditions, but the sequestration environment will determine the extent to which that ultimate capacity will be realized. The effects of both physical and chemical changes need to be understood. Temperature and pressure might be expected to have a large influence. The effect of different temperatures on adsorption capacity was not evaluated in this study. Coal contains a wide variety of organic and mineral phases in a complex, porous, threedimensional network which varies from one coal deposit to another. The organic portion of the coal is thought to capture CO 2 via surface adsorption, pore filling, and solid solution. Less recognized is the possibility that the mineral phases present in the coal 33

37 may assist via mineral carbonate formation. Thus, the nature of the coal seam itself is an important variable to be considered Comparison between Methane and Carbon Dioxide Sorption The literature compares the amount of carbon dioxide adsorbed to the amount of methane adsorbed for different coals. Although we did not measure methane adsorption capacity, we can examine the relationship between the carbon dioxide adsorption capacity at estimated reservoir pressure and the in-situ methane content that was measured. The samples we tested had a range of carbon dioxide sorption from 354 to 717 scf/ton on an in-situ basis and from 681 to 829 scf/ton on a dry, ash free basis. The range of methane sorption was from 60 to 194 scf/ton on an as-received basis and from 74 to 255 scf/ton on a dry, ash-free basis. This represents the actual insitu methane contents of the coals, which may differ from the methane adsorption capacity because the coal may be over-saturated or under-saturated. The ratio of carbon dioxide to methane sorption capacity in this study ranged from 2.1:1 to 7.5:1 on a scf/ton as-received basis and 2.9:1 to 5.6:1 on a scf/ton dry-ash free basis. The ratios for the specific coal samples tested are provided in Table 11. Table 11. Ratio of Carbon Dioxide to Methane Sorption Capacity of Tested Coals Coal Name In-situ Basis Dry, Ash-Free Basis CH 4 CO 2 CO 2 :CH 4 CH 4 CO 2 CO 2 :CH 4 scf/ton scf/ton scf/ton scf/ton scf/ton scf/ton Lower Freeport Upper Kittanning Middle Kittanning Brookville Tionesta The ratio of 5.6 to 1 for Lower Freeport coal may be high because of the quantity of methane desorbed during the desorption test. It is possible that methane gas was partially released from the coal underground before the core sample was extracted and the laboratory desorption results were not representative of the potential storage capacity of the coal. Since the actual pressure of the coal seam was not measured, the estimated pressure used for determining the storage capacity of carbon dioxide could be higher than the actual coal seam pressure. Testing adsorption for methane and desorption for carbon dioxide in the lab may result in measuring lower values for methane storage, which would lead to higher values of the CO 2 :CH 4 ratio on a scf/ton basis. Reported sorption studies on coals of different ranks have shown that for medium to high volatile bituminous coals the storage ratio is approximately 2:1. The storage capacity increases as the coal rank decreases: for sub-bituminous coal, the ratio has been measured between 7:1 to 10:1, while for lignite coal, the ratio was as high as 13:1. All values are derived from adsorption isotherms done on coal samples. 34

38 Specifically, one study conducted by the U.S. Geological Survey (Stanton and others, 2001) on sub-bituminous and lignite coals in the Powder River Basin, Williston Basin, the Rock Springs area, and the Gulf Coast Region showed coals in the pressure range from 500 to 600 psi having a ratio of CO 2 :CH 4 from 6:1 to 8:1 on a dry, ash-free basis. Another study done by the USGS comparing CO 2 adsorption to coal rank (Burruss, 2002) looked at a bituminous coal sample from West Virginia. They compared it to existing literature on an Appalachian Basin bituminous coal sample and a San Juan Basin bituminous coal sample. In the pressure range from 500 to 600 psi, which is the estimated reservoir pressure of the Northern Appalachian coal seams evaluated here, the Appalachian basin coal had a storage capacity of 300 scf/ton of methane and 600 scf/ton of carbon dioxide which showed a CO 2 :CH 4 ratio of The bituminous coal of San Juan Basis had a sorption capacity of 440 scf/ton of methane and 800 scf/ton of carbon dioxide yielding a CO 2 :CH 4 ratio of 1.82 on a dry, ash-free coal basis. The West Virginia coal tested in the study had a sorption capacity of 250 scf/ton of CH 4 and 575 scf/ton of CO 2 with a CO 2 :CH 4 ratio of In general, the coals had slightly more methane than our Northern Appalachian coals but slightly lower carbon dioxide sorption, which overall decreased the ratios compared to this study. A third study used as a comparison, which was funded by the U.S. Department of Energy, National Energy Technology Laboratory (Pashin and others, 2004), evaluated the Black Warrior coal basin. These coals had higher sorption of both carbon dioxide and methane compared to our Northern Appalachian coals. On a mineral-matter free basis the amount of carbon dioxide absorption for the Black Warrior coals ranged from 600 to 1000 scf/ton and methane adsorbed ranged from 200 to 500 scf/ton. On an asreceived basis, the coals had a carbon dioxide adsorption from 550 to 900 scf/ton and methane adsorption ranging from 150 to 400 scf/ton. The conclusion from the NETL study was that the isotherms from individual coal samples indicated that, at pressures above 500 psi, the coal could hold about twice as much carbon dioxide as methane Chemical Analyses of Coals Table 12 and Table 13 summarize the chemical composition and calorific value of each of the coal samples. Other than the moisture content, the data are provided on a dry basis. Table 14 summarizes the elemental analysis of the ash. Since the coals are raw coal samples (they were not processed through a coal preparation plant to remove some of the non-coal components), the ash content of the coals are higher than what is normally measured on clean coal. 35

39 Table 12. Results from Proximate Analysis and Calorific Value Volatile Heating Moisture Ash Matter Value Heating Value As Determined Dry Dry Dry Coal Seam wt % wt% wt% Btu/lb Btu/lb Lower Freeport Upper Kittanning Middle Kittanning Clarion Brookville Tionesta Lower Mercer Moisture, Ash Free Table 13. Chemical Composition of Coal Seams Carbon Hydrogen Nitrogen Chlorine Sulfur Mercury Coal Seam Dry, wt% Dry, wt% Dry, wt% Dry, wt% Dry, wt% Dry, ppm Lower Freeport Upper Kittanning Middle Kittanning Clarion Brookville Tionesta Lower Mercer Table 14. Elemental Composition in Coal Ash SiO 2 Al 2 O 3 TiO 2 Fe 2 O 3 CaO MgO Na 2 O K 2 O P 2 O 5 SO 3 Coal Name wt% wt% wt% wt% wt% wt% wt% wt% wt% wt% Lower Freeport Upper Kittanning Middle Kittanning Clarion Brookville Tionesta Lower Mercer Correlations Between Gas Sorption and Coal Characteristics The sorption capacities for carbon dioxide and methane were calculated in two way. As shown throughout the Results and Discussion, Section 5, the sorption calculations can be based on the coal as it is received or on a dry, ash-free basis. It was of interest to determine if sorption capacity calculated in both ways could be correlated to different coal characteristics as volatile matter, ash content, and maceral content. The impact of the coal depth on adsorption was also graphed. 36

40 Figure 15. Correlation of Gas Storage Capacity to Volatile Matter on an As-Received Coal Basis Correlating volatile matter with sorption capacity at the estimated coal reservoir pressure found a weak positive correlation with CO 2 (R 2 = 0.23) and no correlation with CH 4 and volatile matter. Correlations were weaker on a dry, ash-free basis as seen in Figures 15 and 16. Figure 16. Correlation of Gas Storage Capacity to Volatile Matter on a Dry, Ash-Free Coal Basis 37

41 Correlations between sorption capacity and ash content were slightly stronger but negative for both CO 2 and CH 4 sorption and ash content showing an R 2 = 0.43 and an R 2 = 0.36 for each gas respectively. When sorption capacity was calculated on a dry, ash-free basis, the correlation was stronger for carbon dioxide but weaker for methane as seen in Figures 17 and 18. This can be interpreted that mineral matter and moisture have minimal surface area compared to the microporous organic constituents of coal. Accordingly, the adsorption capacity of coal would be expected to decrease proportionally with increasing ash and moisture content. Figure 17. Correlation of Gas Storage Capacity to Ash Content on an As-Received Coal Basis Figure 18. Correlation of Gas Storage Capacity to Ash Content on a Dry, Ash-Free Coal Basis 38

42 A weak positive correlation existed between the vitrinite content and both CO 2 and CH 4 sorption. Figure 19 shows an R 2 = 0.30 for carbon dioxide and an R 2 = 0.25 for methane on an as-received coal basis. The correlation was slightly stronger for carbon dioxide on a dry, ash-free basis but much weaker for methane as seen in Figure 20. Figure 19. Correlation of Gas Storage Capacity to Maceral Composition on an As-Received Coal Basis Figure 20. Correlation of Gas Storage Capacity to Maceral Composition on a Dry, Ash-Free Coal Basis 39

43 The depth of the coal did not show any correlation to methane storage capacity. Carbon dioxide storage capacity showed a very weak negative correlation at an R 2 = 0.20 to coal depth as seen in Figure 21 on an in-situ basis. On a dry, ash-free basis the R 2 was weaker, but positive, which indicates that at greater depths, the carbon dioxide capacity increases as seen in Figure 22. As the depth of the coal increased, the estimated pressure of the reservoir increased could account for the increase in carbon dioxide storage capacity. More data over a wider range of ranks, coal depths, and coal composition could provide better correlations to each of the parameters evaluated. Figure 21. Correlation of Gas Storage Capacity on an As-Received Coal Basis and Distance to Top of Coal Seam Figure 22. Correlation of Gas Storage Capacity on a Dry, Ash-Free Coal Basis and Distance to Top of Coal Seam 40

44 5.7. Petrographic Analysis The results of the maceral analysis and vitrinite reflectance are provided in Table 15. No correlation was found between the vitrinite reflectance and the depth of the coal seam, since the coals had similar reflectance indices as depicted in Figure 23. The following section describes each of the coal seams in reference to the rank, its proximate properties, and its coking capacity. Most of the coals were high-volatile A bituminous coals. The complete report is provided in Appendix D. COAL SEAM Lower Freeport Table 15. Petrographic Analysis of Coal Samples Upper Middle Kittanning Kittanning Clarion Brookville Tionesta Lower Mercer REACTIVES % Vitrinite Liptinite /3 Semi Fus TOTAL REACTIVES INERTS % 2/3 Semi Fus Micrinoids Fusinite Mineral Matter TOTAL INERTS % MEAN-MAX VITRINITE REFLECTANCE CALCULATED COKE STABILITY Figure 23. Correlation of Vitrinite Reflectance to the Depth of the Top of the Coal Seam 41

45 Lower Freeport Seam The sample has dry volatile matter, ash, and sulfur contents of 24.2%, 39.0%, and 2.9%, respectively. It has a mean-maximum vitrinite reflectance of 1.0%. The total reactive maceral content is 66.5%, comprised of 62.9% vitrinite, 2.6% liptinite, and 1.0% semifusinite. The total inert content is 33.5%, comprised of 27.2% inorganic inerts and 6.5% organic inerts. It has a predicted coke stability of 36, which is lower than expected because of the high inert content for this rank coal. It is a high-volatile A bituminous coal Upper Kittanning Seam The sample has dry volatile matter, ash, and sulfur contents of 33.7%, 10.8%, and 1.4%, respectively. It has a mean-maximum vitrinite reflectance of 1.0%. The total reactive maceral content is 84.3%, comprised of 79.3% vitrinite, 3.9% liptinite, and 1.1% semifusinite. The total inert content is 15.7%, comprised of 6.4% inorganic inerts and 9.3% organic inerts. It has a predicted coke stability of 52 which is good. It is a highvolatile A bituminous coal Middle Kittanning Seam The sample has dry volatile matter, ash, and sulfur contents of 34.4%, 23.8%, and 3.8%, respectively. It has a mean-maximum vitrinite reflectance of 0.86%. The total reactive maceral content is 69.1%, comprised of 63.5% vitrinite, 3.2% liptinite, and 2.4% semifusinite. The total inert content is 30.9%, comprised of 15.7% inorganic inerts and 15.2% organic inerts. It has a predicted coke stability of 20 which is lower than expected because of the high inert content for this rank coal. It is a high-volatile A bituminous coal. Figure 24. Middle Kittanning Coal Core Samples A photograph of the core sample is provided in Figure Clarion Seam The sample has dry volatile matter, ash, and sulfur contents of 30.6%, 32.9%, and 6.3%, respectively. It has a mean-maximum vitrinite reflectance of 0.78%. The total reactive maceral content is 68.1%, comprised of 58.7% vitrinite, 8.2% liptinite, and 1.4% semifusinite. The total inert content is 31.7%, comprised of 23.7% inorganic inerts and 8.1% organic inerts. It has a predicted coke stability of 1 which is lower than expected because of the high inert content for this rank coal. It is a borderline high-volatile A /B bituminous coal. 42

46 Brookville Seam The sample has dry volatile matter, ash, and sulfur contents of 32.8%, 14.2%, and 1.7%, respectively. It has a mean-maximum vitrinite reflectance of 1.02%. The total reactive maceral content is 76.7%, comprised of 67.5% vitrinite, 7.2% liptinite, and 2.0% semifusinite. The total inert content is 23.3%, comprised of 8.6% inorganic inerts and 14.7% organic inerts. It has a predicted coke stability of 52 which is good. It is a high-volatile A bituminous coal. Figure 25 shows the blocky characteristic of the coal. Figure 25. Brookville Coal Core Samples Tionesta Seam The sample has dry volatile matter, ash, and sulfur contents of 25.0%, 33.9%, and 2.8%, respectively. It has a mean-maximum vitrinite reflectance of 0.99%. The total reactive maceral content is 51.8%, comprised of 39.4% vitrinite, 8.9% liptinite, and 3.5% semifusinite. The total inert content is 48.2%, comprised of 22.9% inorganic inerts and 25.3% organic inerts. It has a predicted coke stability of 3 which is lower than expected because of the high inert content for this rank coal. It is a high-volatile A bituminous coal Lower Mercer Seam The sample has dry volatile matter, ash, and sulfur contents of 25.4%, 29.9%, and 1.3%, respectively. It has a mean-maximum vitrinite reflectance of 0.98%. The total reactive maceral content is 45.3%, comprised of 28.2% vitrinite, 13.0% liptinite, and 4.0% semifusinite. The total inert content is 54.8%, comprised of 19.2% inorganic inerts and 35.6% organic inerts. It has a predicted coke stability of 0 which is lower than expected because of the high inert content for this rank coal. It is a high-volatile A bituminous coal with a maceral content that would further classify it as cannel coal Selection of a Coal Seam for Carbon Dioxide Sequestration The selection process to determine if a coal seam would be a candidate for carbon dioxide sequestration or instead is mineable would start with developing a sampling plan for core drilling and coal sample collection. Among the initial parameters to then consider would be the depth and thickness of the coal seam and the economic feasibility of mining the coal. Through a chemical analysis of the samples, the amount of ash in the coal will indicate a production yield and the processing requirements in the preparation plant. 43