A preliminary laboratory experiment on coalbed methane displacement with carbon dioxide injection

Transcription

1 Available online at International Journal of Coal Geology 73 (2008) A preliminary laboratory experiment on coalbed methane displacement with carbon dioxide injection Hongguan Yu, Jian Yuan, Weijia Guo, Jiulong Cheng, Qianting Hu Key Laboratory of Mine Disaster Prevention and Control, Shandong University of Science and Technology, 579 Qianwangang Road, Qingdao Economic & Technical Development Zone, Qingdao, Shandong Province, , PR China Received 14 November 2006; received in revised form 14 April 2007; accepted 23 April 2007 Available online 13 May 2007 Abstract CH 4 /CO 2 adsorption isotherm studies on coals samples give adsorption behavior, adsorption ratio, and other influential factors for enhanced coalbed methane recovery (ECBM) and CO 2 sequestration by CO 2 injection. Yet, they only provide static characteristics, and do not provide information on pressure-driven and concentration-driven during gas recovery and injection processes. The purpose of this study is to construct an experimental apparatus for CH 4 displacement with CO 2 injection and primarily study the basic procedure of coalbed methane (CBM), ECBM, and CO 2 sequestration at driven-conditions. An experimental apparatus was built based on the configuration of one-injection well/one-production well under ideal conditions. The apparatus was used to model CBM with CH 4 adsorption, investigate primary production of CBM and ECBM with gases desorption, and study CO 2 sequestration with its breakthrough. The change of pressure and gases amount during gases injection and desorption, and residual quantity after gases desorption were employed to ECBM and CO 2 sequestration with this equipment. The effect of CO 2 injection on CH 4 production was investigated on a small scale experimentally. The results indicate that the recovery procedure of ECBM and CO 2 sequestration with CO 2 injection can be studied with the apparatus and the gases adsorption/desorption characteristics obtained with the apparatus is obviously differ from that obtained with conventional volumetric measurement Elsevier B.V. All rights reserved. Keywords: Coalbed methane; Enhanced recovery; Carbon dioxide; Sequestration; Adsorption 1. Introduction The injection of CO 2 in coal beds can enhance the recovery of coalbed methane (CO 2 -ECBM) and, at the same time, it is a very attractive option for geologic CO 2 storage as CO 2 is strongly absorbed onto the coal (Gunter et al., 1997). Therefore, synergy exists between Corresponding author. Tel.: ; fax: , address: yuhongguan65@163.com (H. Yu). CO 2 sequestration and production of methane, leading to greater utilization of coalbed resources for both their sequestration ability and energy content. Traditionally, CH 4 production from coalbed methane reservoirs (CBMR) happens as a result of pumping the gas out of the reservoir through the natural or induced fracture system. As CO 2 is injected under high pressure through a series of wells, it flushes the gaseous CH 4 from injection side, creating a near 100% CO 2 saturation. The partial pressure of CH 4 in the gaseous cleat-system phase is reduced to zero, a disequilibrium /$ - see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.coal

2 H. Yu et al. / International Journal of Coal Geology 73 (2008) condition in a system containing both CH 4 and CO 2.As a result, CH 4 is pulled into the gaseous phase to achieve partial-pressure equilibrium. On the other hand, CH 4 is pushed from the coal matrix by the CO 2 as CO 2 becomes preferentially adsorbed onto the coal. Different set-ups are possible for the configuration of injection and production wells, such as the seven-spot set-up (one injection/six production), the four-spot setup, and the five-spot set-up (one injection/four production). (Hamelinck et al., 2002). Although there are different technical approaches being implemented successfully from both a technical and economic viewpoint, the one-injection well/one-production well is the basic configuration of CO 2 -ECBM. It is relatively inexpensive and simply provides more information about the drive process. Adsorption experiments have been used to evaluate the potential economic benefits of CO 2 sequestration in combination with CBM production (Hamelinck et al., 2002), predict the potential of enhanced CBM recovery and CO 2 sequestration (Scott, 2003; Yamazaki et al., 2006), analyze the influencing factors on adsorptions of gaseous mixture (Busch et al., 2003; Ceglarska- Stefańska and Zarębska, 2002, 2005; Clarkson and Bustin, 2000), investigate the gas diffusion behavior (Busch et al., 2004; Shi and Durucan, 2003), study the preferential sorption behavior (Busch et al., 2006), and determinate the CO 2 /CH 4 adsorption ratio (Scott, 2003). However, experiments have been limited to measurement of adsorption isotherms under static conditions and did not provide information of gas pressure-driven and concentration-driven. More work is needed to understand and predict displacement behavior (e.g., CO 2 injection and CH 4 production). Further, characterizing this behavior on injection conditions, such as CO 2 injection rate and pressure, is needed. Adsorption/desorption hysteresis isotherm was observed for pure CH 4 or CO 2 (Heller et al., 1980; Bell and Rakop, 1986; Greaves et al., 1993; Ozdemir et al., 2004). Much recent work has been performed on dual (CH 4 / CO 2 ) gas adsorption, and significant hysteresis has been observed in those experiments (Busch et al., 2003; Tang et al., 2005). For ECBM and CO 2 sequestration, adsorption hysteresis significantly affects displacement behaviour (Tang et al. 2005), prediction of adsorbed phase compositions (Greaves et al., 1993), the reliability of reservoir simulation (Chaback et al., 1996), and recovery time. Limited experiments have been completed and experimental devices have been built to study dynamic behavior. In an investigation of the injection, gases adsorption, CO 2 sequestration in CBM reservoirs, the enhanced CH 4 production, and the main factors that affect the overall operation, Tsotsis et al. (2004) described the various experimental techniques that they utilize and discuss their range of application and the value of the data generated. Wolf et al. (1999) built an experimental setup in which underground circumstances can be simulated, and a core flush test was completed under in-situ conditions. The amount of ECBM and CO 2 sequestration are dependent on several critical factors, such as well spacing (Hernandez et al., 2006), injection rate (Stevens et al., 1999), injection timing (Sams et al., 2002), injection well pressure (Sams et al., 2005), coal permeability, coal depth, and coal rank (White et al., 2005). The overall goal of this study is to build an experimental setup for CBM displacement with CO 2 injection and to primarily study the basic process of CBM production, ECBM and CO 2 sequestration at driven-conditions, such as injection time and injection pressure. 2. Experimental apparatus To study the transport/sorption characteristics and to carry out simulated CO 2 -ECBM experiments, we have constructed a new experimental apparatus. At the heart of the experimental system is a stainless steel column capable of withstanding pressures up to 6 MPa, which is placed inside a temperature-controlled air bath. The coal sample is placed in this column. The gases injection and production with the apparatus are similar to the configuration of one-injection well/one-production well at ideal conditions, such as dry coal, and ignored macropore and natural fracture of coal. CO 2 is injected from inlet side of the column, the desorbed gases is exhausted from outlet side. Regardless of the mechanism by which gases are transported through the organic matrix of the coal, the microporosity of coal is the dominant factor in the gas adsorption process (Clarkson and Bustin, 1999a). The compacted coal packed in stainless steel column is used to simulate a coal bed, characterizing the primary porosity systems (micropore and mesopore) of coal, but not representing secondary (macropore and natural fracture) porosity systems. Adsorbed methane on coal sample is used for CBM in situ, and the conventional primary CBM recovery process is simulated using depressurizing adsorbed gas. The CO 2 -ECBM process is simulated using CO 2 injection into coal, in which ECBM is obtained with desorption CH 4 /CO 2 mixture after CO 2 injection and CO 2 sequestration is obtained using the retained CO 2 amount after mixture desorption. In the experimental system, the discharge of water is

3 158 H. Yu et al. / International Journal of Coal Geology 73 (2008) Fig. 1. Schematic diagram of experimental apparatus used in this study. A1, A2,A3 or A4: CH 4,CO 2,HeorH 2 cylinder; B: manometer and regulator on gas cylinder; C: switch valve; D1, D2: shut-off valve of front-column and back-column; D3, D4: shut-off valve used to detect leak; E1, E2: regulator of front-column and back-column; F1, F2: manometer of front-column and back-column; G: coal sample column; H: thermostatic chamber; I: needle valve; J: flowmeter; K: six-port valve; L: GC; M: vacuum pump; N1,N2: moisture absorbent; O1, O2: CO 2 absorbent; P: protective tube; Q: lower-opening bottle; R: graduated cylinder. used to determine CH 4 production, and penetrable CO 2 amount is determined using the absorption train, which is composed of a tube packed with a CO 2 absorbent. The production gas concentration is determined using gas chromatography (GC). The experimental device used to simulate CBM and CO 2 -ECBM is shown in Fig. 1. The essential parts of experimental device mainly are gas system including flow and pressure regulator, coal sample column, sampling device and detection system Gas system The gas system indicates flow path of gas. The gas system should be controlled and read based on experimental conditions, including pressure and flow. The regulator (E1, E2) and manometer (F1, F2) were used to regulate and show the inlet and outlet pressure of coal sample column (G). The needle valve (I) and flowmeter (J) were used to adjust and read the gas flow rate. In addition, two shut-off valves (D1, D2) were selected to shut off or turn on the CO 2 or CH 4 gas supply, and a switch valve (C) was used to switch from one gas supply to another gas supply. CH 4, the major CBM component, is supplied with high pressure cylinder (A1). CO 2,injectedgas,issuppliedwith CO 2 high pressure cylinder (A2). Pressure regulators (B) on the cylinders are used to reduce the pressure from a high pressure cylinder, to a suitable pressure value (5 MPa). The switch valve (C) is used for choice between CH 4 and CO 2 supply. The reduced-pressure gas passes through the coal column from the regulator (E1). The pass-through gas volume and composition are determined with adsorbed system and gas chromatography (L) using the six-way valve (K) choice. To ensure the accuracy of the experimental data, the gas system is tested for leak-tightness. The high pressure He supplied with He cylinder (A3), not adsorbed by coal is used leak detection before CH 4 injection, and the He should be exhausted with vacuum pump (M) under controllable condition using shut-off valve (D3, D4) Coal sample column The packed column is 100-cm in length and has an internal diameter of 14 mm and external diameter of 18 mm. The expected coal column employed a packing Fig. 2. Schematic diagram of coal sample column filling.

4 H. Yu et al. / International Journal of Coal Geology 73 (2008) of empty column and quartz wool from the packed coal column. In this procedure, about 100 g of dried coal passing a number 60 mesh sieve is placed in the column. Column temperature affects adsorption and desorption of CH 4 and CO 2, therefore, the column oven should be controlled strictly. For this reason the coal column is located inside a thermostat oven. The oven always has air circulation driven by a powerful fan to ensure an even temperature throughout the oven. The temperature in any part of the oven should be stable to 301±0.2 K in this study. The column oven is shown in Fig. 3. Fig. 3. Schematic diagram of coal sample column and its constant temperature system. procedure that utilized a packing apparatus shown in Fig. 2. One end of the column is connected to a funnel for loading powered coal sample, and the other end is connected to a filler flask and vacuum pump prior to packing. The funnel is used to load. A wad of quartz wool at the end of the column and the filler flask being connected to vacuum pump are employed to protect the coal dust from being sucked into vacuum pump. The pump makes the packing to be swept rapidly through the column and causes the coal to be slightly compacted along the total length of the column. As the column is filled up, a weight (10 kg) is applied for 15 min to compress coal in the column. The packing and compressing process repeat until the column is filled up with compressed-coal. The coal column should be curved U-type for space saving, and plugged with quartz wool at each end. The coal mass is obtained by subtracting the sum of the mass 2.3. Sampling system The experimental apparatus uses a two-position, sixport valve (K in Fig. 1) to switch between routine analysis of gases volume and sampling mode for determination of gases composition (Fig. 4). The gas sampling valve serves as the focus of the sampling procedures, as it allowed the choice of measurement of the desorbed gas (CH 4 or CO 2 ) volume and their composition with changing the position of valve pole. The metering loop is an important device, which is employed for quantitative sampling and is the gas manifold for gas volume measurement. Sketch A of Fig. 4 shows the desorbed gas collection device isolated from the flow of hydrogen that is constantly required for the gas chromatography (GC) to function properly. In this bypass arrangement, the desorbed gas is introduced into the gas volume measuring system and collected for injection into the GC. A simple pull of the sampling valve rod gives the flow arrangement B (Fig. 4) where the collected gas from the desorbed mixture is rapidly Fig. 4. Schematic diagram of tow position, six port valve for sampling. A: the routine position; B: the sampling position.

5 160 H. Yu et al. / International Journal of Coal Geology 73 (2008) Fig. 5. Schematic diagram for CH 4 measurement using water collection with gas displacement. transferred to the GC for analysis and the gas volume determining system is temporarily cut off. In the routine position, the gas from flowmeter (J in Fig. 1) is first passed through valve space and then through the sampling loop to another valve space and then to a tube packed with moisture absorbent (N in Fig. 1). At the same time, hydrogen carrier gas passed through valve space to the gas chromatography (GC). When the valve is switched into the sampling position, the flow of hydrogen carries gas is reversed through sampling loop, and the adsorbed gas in the loop is rapidly carried to GC Measurement system The measurement system consists of measurement of gas mixture volume and composition. The GC is used to determine the gas composition. The gas collection system, using displacement of water and the absorption train, is used to precisely determine volume of the desorbed CH 4 and breakthrough CO 2. The adsorbed system consists of five U-shaped tubes packed adsorbent as shown in Fig. 1. A first absorption train (N1) for removal of moisture from gas is composed of a tube packed with a water absorbent, second and third tube (O1 and O2) packed with CO 2 absorbents are used to determine the CO 2 volume from desorbed gas. Anhydrous calcium chloride is commonly used for the water absorbent, while sodium hydroxide impregnated in an inert carrier (asbestos) is used as the CO 2 absorber. Another U-shaped tube (N2) packed water absorbent is used to avoid the effect of moisture from bottle water (Q), and a protective U-shaped tube (P), called the protective tube is used to avoid counter flow of water. CO 2 is absorbed by the preweighted absorbers in the sorption train, the breakthrough CO 2 volume at STP is obtained from the gain in weight of absorbers. The desorbed-ch 4 volume is measured using water collection with gas displacement shown as Fig. 5. The gas from adsorbed system passes through protective tube into gas-collecting bottle (Q in Fig. 1), which hold saturated sodium chloride solution. The desorbed CH 4 from coal flows out as small bubbles, separated gas bubbles are accumulated at the top of the bottle. The bottle has a water drainage pipe at the bottom. Exceeded water by the produced gas could be drained through this pipe. Drained out water is measured by using graduated cylinder, and CH 4 volume is expressed at STP. The stoichiometric relationship between the CO 2 volume at STP and the trapped increase of mass is used to the breakthrough volume of CO 2. A Shanghai 1102G gas chromatograph (GC) equipped with a micro thermal conductivity detector (TCD) is used for quantitative analysis of CO 2 and CH 4 concentration. The CO 2 and CH 4 composition determined using GC should be identical with the ratio of CO 2 and CH 4 volume to the sum of the two gas volume, obtained with adsorbed train increase of mass and gas displacement volume of water. 3. Experimental 3.1. Coal sample The coals used in this experiment originate from the Jincheng and Luan mines, Qinshui basin, North China, which was selected for small-sized pilot, the Sino-Canada cooperative project on CBM technology development/ CO 2 sequestration in China and is the first basin to be developed commercially in China (Su et al., 2005). Proximate analysis, and petrographic analysis, including random reflectance measurements, of the coals were carried out. Detailed characteristics of both coals, Table 1 Coal samples used for adsorption experiments Sample Proximate analysis and total sulfur (%) Petrographic analysis (vol.%) Ash VM FC S Vitrinite Inertinite Liptinite MM VR r Jincheng Luan Ash and total sulfur (S) were calculated on a dry basis. Volatile matter (VM) and fixed carbon (FC) were calculated on a dried, ash-free basis.

6 H. Yu et al. / International Journal of Coal Geology 73 (2008) Table 2 The Langmuir constants of gases adsorption on coals for isotherms fit Gas name Jincheng coal Luan coal V L (cm 3 /g) P L (MPa) V L (cm 3 /g) P L (MPa) CH CO including proximate analysis and petrographic analysis for the coals are given in Table 1. The adsorption isotherms of pure CO 2 and CH 4 on dried Jincheng and Luan coal were also obtained using volumetric method at 301 K. The adsorbed volumes of CO 2 and CH 4 on two coals were fitted with Langmuir equation. The Langmuir constants are given in Table 2, and the adsorption isotherms of pure CO 2 and CH 4 is shown in Fig Experimentation The tests started with a complex procedure of filling a coal sample in the high-pressure column, followed by testing of the entire system for leaks. Then the main experimental procedures, including CH 4 injection and desorption, CO 2 injection, and CH 4 displacement, were conducted. The experimental procedures involved gas injection and release with valve, pressure change reading, and gases volume and composition measurement. The experiment was carried out at 301 K, which was determined on the basis of coal-bed temperature in the Qinshui basin. Coal column preparation: The coal column was used to simulate coal bed, which express the primary porosity systems of coal and did not characteristics of coalbed in situ being studied in the future. The samples were crushed to pass through a 60-mesh sieve, then, the powered coal was dried for 2 h at 200 C to remove moisture and adsorbed gases in coal. The column was filled with the powered coal being dried, in which the coal sample was compressed, just as described in Section 2.2. The curved U-type coal column plugged with quartz wool at each end was tightly connected with gas system using screw cap as shown in Fig. 1. Leak detection: In order to achieve reliable result, it is necessary that the set-up does not leak at experimental pressure and the sample surface is thoroughly cleaned to remove adsorbed gases. There are two major steps in leak detection: (i) leak detection survey, and (ii) evacuation of helium (He) in the gas system. Leak detection survey involves knowing where the leak is and how to restore leak location. Leaks are located using soapy water as the pressure is maintained at 5.5 MPa. As any bubble is determined, the contact points should be screwed to ensure the connection is tight. In order to eliminate He gas and moisture in coal, the vacuum (M) is used to vent the He for at least 24 h. As the gas in the coal column is exhausted, the pump vacuum is closed to prepare for the following CH 4 adsorption experiment. CH 4 adsorption: The switch valve (C in Fig. 1) is changed from He to CH 4, the regulator on the CH 4 cylinder (A1) is adjusted to ensure that the reading of the monometer (B) on the CH 4 cylinder is at 5 MPa. The regulator (E1) is adjusted to ensure that the reading of the monometer (F1) is at 4.5 MPa. The change of two monometer readings (F1 and F2) is recorded at 1 h interval. As the monometer (F2) reading is stable for 5 h, meaning adsorption equilibrium, the valve (D1) is closed to prepare for the following CH 4 desorption. CH 4 desorption: A desorption step is initiated by reducing the equilibrium pressure of the adsorbed CH 4. This adsorption pressure reduction is accomplished by opening the valve (D2) connecting the coal column (G). The needle valve (I) between the coal column and displacement water system is manually regulated to ensure that the desorbed-gas flow is less than 2 ml/min for simulating a step change in pressure. The coal sample then Fig. 6. Experimental adsorption of CH4( ), CO 2 ( ) and their Langmuir isotherms fits (dotted line for CH4 and solid line for CO 2 ) on Jincheng and Luan coal at 301 K.

7 162 H. Yu et al. / International Journal of Coal Geology 73 (2008) desorbs its gas, and the pressure descends from the initial desorption pressure (P 0 ) to a new final equilibrium pressure (P f). The inlet and outlet pressure of the coal column at any given time is recorded at 1 h interval using two monometer readings (F1 and F2), and the volume of desorbed CH 4 along with time (t) is monitored by the data obtained with drainage system. As the outlet pressure of the coal column decreases to 2 MPa, the valve (D2) is closed to end the CH 4 desorption experiment to prepare for the following CO 2 injection. CO 2 injection: The switch valve (C) is changed from CH 4 to CO 2.CO 2 outlet pressure in the CO 2 cylinder (A2) is adjusted to 5 MPa, and the regulator (E2) is adjusted to ensure that the inlet pressure of the coal column is 4.5 MPa. The change of two monometer (F1 and F2) readings is recorded at 1 h intervals until the monometer (F2) reading is stable for 5 h, indicating the end of CO 2 injection. The valve (D1) is closed to end the CO 2 injection to prepare for the following CH 4 displacement. CH 4 displacement: Like CH 4 desorption, the valve (D2 in Fig. 1) is opened and the regulator (E2) and the need valve (I) are adjusted to ensure that the flowmeter (J) reading is lower than 2 ml/min. In this procedure, CO 2 breakthrough volume is determined using adsorbed train (O1, O2), and replaced CH 4 volume is determined using a drainage water method. The rod of the sampling valve (L) should be pulled out about 2 s, as shown in Fig. 4B, to determine the gas composition with GC (L). The valve rod (L) should be pulled in to measure the volume of gases after sampling for GC. As desorbed gas volume is less than 1 cm 3 per hour from the coal column, the CH 4 displacement ends. Volume measurement of retained gas: In this step, retained gas after CH 4 primary production and ECBM recovery is determined using heating coal at maximum 200 C. The valve pole (K in Fig. 1) is pushed in, as shown in Fig. 4A, to determine gas volume. The regulator (I) is adjusted and the temperature of the coal column is controlled to ensure that the desorbed CO 2 is completely adsorbed by CO 2 absorber (O). Temperature of the coal column oven (H) is gradually elevated from experimental temperature (28 C) up to 200 C to remove all CO 2 and CH 4 in coal. The adsorbed-gas volumes are determined using adsorbed train and drainage system. the coal column. A graphic representation of how outlet pressure of the coal column varies with time through CH 4 injection is shown in Fig. 7. A comparison of pressure-change curves reveals that CH 4 penetrates through the coal column in about 6 h for Jincheng coal and 5 h for Luan coal, and equilibration time for Jincheng coal and Luan coal is about 100 h and 95 h, respectively. The equilibration-pressure difference between inlet and outlet of the coal column is 0.9 MPa for Jincheng coal and 0.6 MPa for Luan coal. Therefore, sorption equilibration is faster for Luan coal than that for Jincheng coal, in which the outlet pressure of Jincheng and Luan coal column is 3.9 MPa and 3.6 MPa, respectively. The trends of CH 4 penetration through coals and CH 4 adsorption on coals in the coal-column are consistent with the adsorption isotherms (Fig. 6), in which CH 4 on Jincheng coal with stronger adsorption is more difficult penetrating through coal than that of Luan coal with weaker adsorption. The outlet pressure of Jincheng coal column is lower than that of Luan coal, which is caused mainly the adsorption capacity difference between Jincheng and Luan coal at same conditions. Commonly, gas transport in coal is considered to occur at two scales: (1) laminar flow through the cleat system, and (2) diffusion through the coal matrix. Flow through the cleat system is pressure-driven and may be described using Darcy's law, whereas flow through the matrix is assumed to be concentration-driven and is modeled using Fick's law of diffusion (Busch et al., 2004). Gas transport in coal is dependent on coal composition, pore structure, grain size, moisture in coal, temperature, gas pressure, and sorption isotherm, which were obtained by volumetric experiments (Busch et al., 2004; Clarkson and Bustin, 1999a,b). As the CH 4 is injected into the coal column, adsorption occurs and adsorbed volume can be expressed with a Langmuir isotherm. Adsorption is a 4. Results and discussion 4.1. The pressure change through CH 4 injection The outlet pressure change of the coal column can be used to investigate the CH 4 adsorption characteristics in Fig. 7. Change curve of outlet pressure of coal column during CH 4 injection.

8 H. Yu et al. / International Journal of Coal Geology 73 (2008) dynamic process, in which a period of time is practically needed to reach their adsorption equilibrium for each coal matrix. CH 4 adsorption and desorption on coal matrix are carried out along with the coal column. As a result of CH 4 adsorption/desorption and its concentration gradient, the CH 4 is transported in coal by Fickian diffusion. Actually, equilibrium time for adsorption process depend on coal particle size, adsorption characteristic and compaction degree. As coal does not have capacity to adsorb all the CH 4, additional CH 4 molecular is adsorbed in other active sites. So, the CH 4 will breakthrough the coal column and the outlet pressure of the coal column will rise. As adsorption equilibrium for each coal matrix is built, the outlet pressure of the coal column is reached maximum CH 4 desorption As the inlet pressure of the coal column is 4.5 MPa and the desorption rate of CH 4 is less than 2 cm 3 /min, just as described in Section 3.2, the change curves of the desorbed-ch 4 volume and the inlet and outlet pressure change of the coal column for two coals are shown in Fig. 8. The outlet pressure of the coal column rapidly decreases with time, in which the desorption time is 35 h for Jincheng coal and 32 h for Luan coal from the initial desorption pressure to final equilibrium pressure (2 MPa). The desorption time is shorter than adsorption time from initial pressure (3.6 MPa for Jincheng coal and 3.9 MPa for Luan coal) to final outlet pressure (2 MPa), which is 59 h for Jincheng coal and 57 h for Luan coal (Fig. 7). The shorter desorption-time expresses sorption desorption hysteresis. Volumetric shrinkage associated with gas desorption from the coal matrix (St. George and Barakat, 2001) can cause higher Fig. 8. Change curve of intlet pressure of coal column and desorbed- CH 4 volume during CH 4 desorption. Fig. 9. Change curve of outlet pressure of coal column during CO 2 injection. flow rate and enhanced permeability, and volumetric swelling associated with gas adsorption from the coal matrix can cause lower flow rate and lower permeability (St. George and Barakat, 2001; Harpalani and Schraufnagel, 1990). The change in gas flow rate and permeability associated with the shrinkage and swelling can explain the shorter desorption-time or the longer adsorption-time. The dynamics of adsorption and desorption equilibrium on compacted coal sample ceaselessly occurs through the coal column. As the outlet pressure of coals columns decreases to 2 MPa, the inlet pressure of columns decreases to 3.2 MPa and 3.6 MPa from 4.5 MPa for Luan coal and Jincheng coal, respectively. The value of CH 4 -injection pressure difference between inlet and outlet of the coal column is lower than that of CH 4 -desorption pressure, in which the injection pressure difference is 0.9 MPa and 0.6 MPa for Jincheng coal and Luan coal, and the desorption pressure difference is 1.6 MPa and 1.2 MPa for Jincheng and Luan coal. It can be seen that the desorption volume curves consist of two parts: the first part is sharp and the second part is flatter. It is also observed that the initial desorption volume for Jincheng coal is higher before 10 h and lower from 10 h to 26 h than that for Luan coal, and the final desorption volume for Jincheng coal is higher than that for Luan coal. We conducted desorption on Jincheng coal with a 10 cm 3 /min desorption rate. The experimental results showed that the outlet pressure of the coal column provisionally decreased to 2 MPa within 5 h, and the desorption volume was zero within 3 h. As the shut-off valve (D2 in Fig. 1) was closed, the pressure increased to 3 MPa within another 5 h, another desorption volume was obtained as the valve being opened again. We conducted another desorption experiment with Jincheng coal, in which the outlet pressure of the coal column was

9 164 H. Yu et al. / International Journal of Coal Geology 73 (2008) controlled to 2 MPa manually. The desorption volume decreased and sometimes was zero. The results showed that the CH 4 diffusion and desorption rate was slower, and the desorbed-ch 4 pressure in the coal column was zero occasionally CH 4 displacement with CO 2 injection The outlet pressure change of coals columns using CO 2 injection at 4.5 MPa injection-pressure is shown in Fig. 9.AsCO 2 is injected at 4.5 MPa, the outlet pressure of coals columns is 4.16 MPa in 90 h interval and 4.01 MPa in 92 h interval for Luan and Jincheng coal, respectively. The outlet pressure of column is higher and equilibrium time is longer than that as CH 4 injection pressure is 4.5 MPa, in which the 3.9 MPa equilibrium pressure in 57 h interval and 3.6 MPa in 58 h interval is reached for Luan and Jincheng coal, respectively. The higher CO 2 outlet pressure of coals column during CO 2 injection may be caused by replaced-ch 4, and by stronger adsorption of CO 2 on coal than CH 4. The desorbed volumes and volume fraction of CO 2 and CH 4 in desorbed gases are shown in Fig. 10 for Jincheng coal (A) and Luan coal (B). As the outlet pressure of the coal column decreases to 0 MPa from 4.16 MPa for Luan coal and 4.01 MPa for Jincheng coal, the desorbed-ch 4 volume is 2619 cm 3 for Luan coal and 3140 cm 3 for Jincheng coal, respectively, and the desorbed-co 2 volume is 262 cm 3 for Luan coal and 260 cm 3 for Jincheng coal, respectively. As compared with CH 4 desorption, the CO 2 desorption is very small, accounting for 7.60% and 9.08% of the desorbed gas mixture for Jincheng coal and Luan coal, respectively. The initial CH 4 displacement with CO 2 is not associated with CO 2 release, which shows no CO 2 breakthrough in the coal column at beginning of CH 4 desorption. With increase of replaced-ch 4 volume, the discharge capacity of CO 2 increases slowly. With CO 2 breakthrough, the volume fraction of CO 2 increases slowly during CH 4 displacement, which expresses constant CO 2 breakthrough as compared with CH 4 desorption CO 2 sequestration and CH 4 enhancement The desorbed volumes and volume fractions of CH 4 and CO 2 for various desorption and heating separation conditions are shown in Table 3. The primary CH 4 product on, enhanced CH 4 recovery with CO 2 injection, CO 2 breakthrough volume and CO 2 sequestration, could be obtained using Table 3 data analysis. The primary CH 4 production an be calculated using desorbed-ch 4 volume under outlet pressure of the coal Fig. 10. Desorbed volume of CH 4,CO 2 and their volume fraction during gases desorption on Jincheng coal (A) and Loan coal (B). column. The primary CH 4 production is 1042 cm 3 and 976 cm 3, accounting for 22.26% and 24.09% of total CH 4 adsorption for Jincheng coal and Luan coal, respectively, as the outlet pressure of the coal column decreases to 2.0 MPa. The differential recovery ratio between Luan and Jincheng coal is consistent with CH 4 adsorption capacity on two coals. As CO 2 is injected into coal, the displacement CH 4 volume is 3140 cm 3 and 2619 cm 3, accounting for 67.06% and 64.63% of the total adsorbed volume for Jincheng coal and Luan coal, respectively. The breakthrough CO 2 volume is 260 cm 3 and 262 cm 3, accounting for 4.15% and 4.44% of total adsorbed volume of CO 2, respectively. The CO 2 sequestration is about 6000 cm 3 and4618cm 3 for Jincheng coal and Luan coal, respectively. 5. Summary and conclusions As far as the enhanced CBM and CO 2 sequestration are concerned, the challenge one faces is the need to build

10 H. Yu et al. / International Journal of Coal Geology 73 (2008) Table 3 Total desorbed amount and its volume fraction of CH 4 and CO 2 during various desorption experiments Coal Desorbed to 2 MPa Desorbed to 0 MPa using CO 2 injection Heating to 200 C Total CH 4 CH 4 CO 2 CH 4 CO 2 CH 4 CO 2 Jincheng coal Adsorbed volume (cm 3 ) Volume fraction (%) Luan coal Adsorbed volume (cm 3 ) Volume fraction (%) experimental apparatus for investigation on behavior of CH 4 and CO 2 in coal bed. Despite its potential importance, the study on ECBM and CO 2 sequestration have, so far, been confined to adsorption/desorption of CO 2 and CH 4 on coals. Furthermore, until recently, the emphasis in these investigations was on preferential sorption behavior of CO 2 and CH 4 on coals at equilibrium pressure rather than on CO 2 displacement. A key element missing in most of these studies is a meaningful characterization of the adsorption/desorption of CO 2 and CH 4 on coals, and difficult to simulate the practical ECBM via CO 2 injection. Our own experimental setup, described above, was built based on the simplest and inexpensive configuration of CO 2 -injection and CBMproduction, i.e. one-injection well/one-production well. The apparatus involves using a number of techniques which aim (i) to model CBM using adsorbed methane, (ii) to simulate conventional primary CBM recovery process using depressurizing adsorbed-gas, and (iii) to study CO 2 - ECBM process with CO 2 injection and gases desorption under simulated CO 2 sequestration conditions. We briefly described the various components of the setup and discussed the experimental data generated with the setup, mean to interpret them, and offer useful information about CO 2 -ECBM. So far CBM displacement experiments with CO 2 injection have been accomplished under ideal conditions, i.e. a constant and slow injection rate of CO 2, dry coal sample, and ignored macropore and natural fracture of coal. In field applications of the CO 2 -ECBM process, the water present in the coal and the cleat fractures may interfere with both the sorption and flow characteristics. The experimental methodology described here can be conveniently adapted to allow one to condition the samples in situ with moisture at various temperature and pressures, to examine the sorption and flow behavior of gases for wet (at various saturation levels) samples, and to measure the absolute and relative permeability. Although the apparatus could not simulate the ECBM and CO 2 sequestration in situ, it is more exact than conventional adsorption and provides the basis to develop in-situ setup. The results of a preliminary experiment show that CH 4 transport rate is dependent on its adsorption on coals. The desorption curve with the apparatus shows similar shape to volumetric measurement. The results of the laboratory experiments were successfully used in CH 4 displacement with CO 2 injection. In addition, a new experimental setup will be developed to simulate ECBM and CO 2 sequestration on field scale based on the ideal apparatus. Acknowledgements Financial support for this study was provided by National Natural Science Foundation of China grant to Qianting Hu and Open Research Fund Program of Key Laboratory of Mine Disaster Prevention and Control (Shandong University of Science and Technology) grant MDPC0607 to Hongguan Yu. The anonymous reviewers are acknowledged for helpful and careful comments and modification of this manuscript that improved the quality of this paper. References Bell, G.J., Rakop, K.C., Hysteresis Of Methane/Coal Sorption Isotherms. Society of Petroleum Engineers of AIME, New Orleans, LA, p. 10. Busch, A., Gensterblum, Y., Krooss, B.M., Methane and CO 2 sorption and desorption measurements on dry Argonne premium coals: pure components and mixtures. International Journal of Coal Geology 55, Busch, A., Gensterblum, Y., Krooss, B.M., Littke, R., Methane and carbon dioxide adsorption-diffusion experiments on coal: upscaling and modeling. Journal of Coal Geology 60, Busch, A., Gensterblum, Y., Krooss, B.M., Siemons, N., Investigation of high-pressure selective adsorption/desorption behaviour of CO 2 and CH 4 on coals: an experimental study. International Journal of Coal Geology 66,

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