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2 21 Esl Geothermal Energy R&D Program WATER ADSORPTION Miroslaw S. Gruszkiewicz, Juske Horita, J. Michael Simonson, and Robert E. Mesmer Oak Ridge National Laboratory KEY WORDS geothermal reservoir, adsorption, The Geysers, rocks, porous materials PROJECT BACKGROUND AND STATUS Any reliable model of the behavior of vapor-dominated geothermal reservoirs must include information about the amount of water retained by the rocks as a function of temperature and pressure. All porous materials, including rocks, can retain water as multi-layer adsorbate on all of their internal surface, as pore-filling adsorbate in the micropores (widths up to about 20 A) and as capillary condensate in the mesopores and macropores (widths up to about 1000 A). This retained water has a density close to the density of liquid water even if the temperature and pressure in the reservoir are in the superheated steam region, indicating that only low-density steam should exist in the absence of surface effects. This means that the term "vapor-dominated reservoir" refers to the equilibrium state of the steam present in large rock fractures and the steam produced in the wells, but does not reflect tile fact that water is also stored in such reservoirs in a high-density condensed state. To estimate the size of the available resource and to predict pressure changes during operation it is important to know what fraction of the water is present as adsorbed layers and capillary condensate with an equilibrium vapor pressure lower than that of bulk water. Operating experience suggests that adsorbed and pore water may act as either a source or a potential sink directly influencing the response to reinjection processes. This response may be influenced by the phenomena that cause adsorption/desorption hysteresis. Since the physicochemical complexity of rocks makes it impossible to predict with a reasonable certainty the water retention at different temperatures and pressures, the most useful data would be those obtained at temperatures in the vicinity of the actual reservoir temperatures. The temperature in most of The Geysers reservoir is approximately 240 C, and a higher temperature reservoir is found in the northwest area of the field. The experimental measurements of water adsorption published in the literature have been limited to about 150 C, with most of the experiments conducted at 120 C or 130 C and below (Shang et al., 1995). This project was undertaken in order to significantly expand the knowledge of the adsorption/ desorption processes useful for operating geothermal reservoirs. The following work was completed in this project: 042 Rock cores were prepared and sent to an external laboratory where BET surface area analyses were made using nitrogen and krypton at 77 K. These analyses yielded surface areas and total pore volumes of the samples. Densities were also determined by the helium displacement method. 042 The Oak Ridge National Laboratory (ORNL) isopiestic apparatus was extensively overhauled and modifications were made to achieve the highest possible weighing accuracy. 042 The densities of the samples were measured using the effect of buoyancy in compressed argon gas. 042 All the samples were prepared and submitted for analysis by X-ray powder diffractometry. 4-51

3 U.S. Department of Energy 042 The 150 C, 200 C and 250 C adsorption and desorption isotherms were determined for 12 rock samples taken from three wells at the Geysers geothermal system, in three grain size fractions. Approximately 20 values of p/po from 0 to 0.98 were investigated. 042 Representative samples were also submitted to Micromeritics laboratories for mercury intrusion porosimetry and low temperature nitrogen adsorption/desorption analyses. The results are expected to provide information about pore size distributions in a wide pore size range. 042 The experimental data were reduced by fitting isotherms to empirical correlations of the amount of water retained as a function of relative pressure. Approximate pore size distributions are being calculated from the adsorption/desorption isotherms. PROJECT OBJECTIVES Technical Objectives 042Experimental determination of water retention per unit of mass by rocks from The Geysers geothermal field as a function of vapor pressure at temperatures from 150 C to 250 C when pressure is increasing (*adsorption' isotherms) and decreasing ('desorption' isotherms). 042Characterization of the rocks from The Geysers using the data obtained by us, X-ray diffraction, and low temperature adsorption. 042Correlation of the water retention phenomena in the reservoirs with rock type and T-p conditions. Expected Outcomes 042Obtain accurate and reliable water retention data for representative samples of The Geysers rocks at the reservoir temperature (for the first time). These data, including hysteresis behavior, will be available for use in reservoir models and in injection/production simulations. 042Resolve the issue of the temperature dependence of the amount of water retained. 042Expand the knowledge about the structure of The Geysers rocks and about rock-water interactions. Gain an insight into the type of rock surface-water interactions that are present., both chemical and physical. APPROACH The ORNL high-temperature isopiestic apparatus is a unique facility capable of precise and accurate measurements of the change of mass of a sample under high temperature-high pressure conditions. This ability makes it suitable for measurements of adsorption by materials characterized by small specific surface areas. The parameter measured is the mass of the solid as a function of vapor pressure. The sorptometers used by other investigators for measuring adsorption on geothermal reservoir rocks rely on measurements of vapor pressure in a chamber of known volume containing the sample. The total volume of the instrument is kept small to increase its sensitivity to relative pressure changes, but the small volume also makes the instrument more sensitive to leaks, temperature variation, and errors in calibration of the internal volumes. In the isopiestic apparatus the mass is measured by comparison with a set of standard weights placed inside the pressure vessel together with the samples. The densities of the samples, which have to be known in order to correct the results for the effect of buoyancy, are measured inside the isopiestic apparatus by weighing the samples in vacuum and then in the atmosphere of a compressed gas (e.g. Ar) of known density. The large volume of the isopiestic vessel (about 28 L) makes small leaks 4-52

4 Geothermal Energy R&D Program of less consequence. These characteristics of the isopiestic apparatus make it relatively easy to evaluate the time needed for reaching equilibrium, as the samples may be left under the same vapor pressure for many days and the change of mass with time can be monitored. The isopiestic apparatus is capable of, and has been previously used for, measurements at 250 C and 40 bar. In conjunction with the water adsorption measurements, BET surface area analyses were performed by Porous Materials Inc. on the same rock samples using nitrogen or krypton adsorption measurements at 77 K in order to obtain specific surfaces and total pore volumes.. These parameters are the most important in estimating water retention capability of a porous material. More extensive and precise low temperature adsorption measurements as well as mercury intrusion porosimetry tests are now being performed by Micromeritics Instruments Inc. on the same samples that were used for high temperature water adsorption measurements. These analyses will yield pore size distributions covering both the mesopore and macropore ranges. Simultaneously, X-ray diffraction data will provide sample compositions which might be helpful in explaining the differences in water retention behavior between various samples. RESEARCH RESULTS The measurements were performed on cores taken from the producing steam reservoir. Well numbers and approximate depths are as follows (per Jeff Hulen of the University of Utah who supplied the samples): NEGU-17, ft PRATI-STATE 12, ft MLM-3, ft. The rocks were crushed and sieved. Three fractions were prepared of each core with the following grain sizes: mm ('coarse'), mm ('medium'), and <0.355 mm ('fine'). The surface areas as determined by PMI ranged from 0.64 to 3.52 mvg. Four samples of each rock were loaded in the titanium cups of the isopiestic apparatus (the medium fraction of each of the rocks was loaded in two cups, so that the repeatability of the measurements could be verified). Four of the remaining cups contained pure minerals (silica gel, chlorite, magnetite, and anatase). The data obtained on these samples, which have surface areas and water retaining capacities significantly higher than The Geysers rocks, can be compared with literature results. It was found that the water retention behavior of the fine fraction samples was quite different from either coarse or medium fractions. The mass of the fine fraction samples in vacuum increased significantly and irreversibly after each p/po cycle (vacuum-saturation with steam-vacuum), while the mass of the coarse and medium fraction samples did not change significantly. For example, the mass in vacuum ofthe fine grain size sample of the MLM-3 rock increased irreversibly by about 21.5 mg/g. The coarse grain size fraction of the same rock can retain only up to 3 mg/g water at p/po=0.8 or up to 7 mg/g at the highest relative pressure reached during the experiments (0.98). This indicates that a part of the retained water was bonded chemically to the fresh surfaces created during grinding. The differences in water retention between the coarse and medium grain sizes were not statistically significant. Adsorption on these samples was nearly completely reversible, indicating that only physical adsorption/capillary condensation were taking place. Most of the samples of the coarse and medium fractions showed 'closed' hysteresis loops (no hysteresis below p/po = 0.55). It seems likely that purely physical adsorption with completely closed hysteresis loops would occur on all intact cores of these rocks, since the available internal surface of the solid has already been saturated with chemically bonded water. However, the bulk of the solid is not in chemical equilibrium with water. The shape of the isotherms and the hysteresis loops (type H3 according to the IUPAC recommended classification) is consistent with a pore system composed of open parallel slits (which produce flat isotherms with hysteresis) and wedges closed on the narrow side (which produce sloping isotherms 4-53

5 without hysteresis). U.S. Department of Energy The shapes of the pore size distribution curves suggest that two distinct groups of capillaries may be present: wider mesopores with radii from 40 A up with quite evenly distributed sizes, and narrow pores with radii below 20 A. A relatively large part of the water is retained in the second group of pores. It is tempting to identify the larger pores as slits between constituent grains and crystallites and the smaller pores as cleavage planes or dislocations inside the crystals. They may also represent the structure of two different mineral components of the rocks that may be present in varying proportions in different samples. Finally, it is possible that such pore size distribution curves may be simply the result of the particular mechanism of filling/emptying the pores where instabilities and transitions occur as the saturation of the rock with the condensed phase increases or decreases. The fraction oftotal retention capability attributed to the narrow pores increases in the sequence MLM-3, PRATI-STATE 12, NEGU-17. In all three rocks this fraction appears to be significant (between 30 and 70 per cent). This means that a significant portion of all the water present in the reservoir might require a rather large pressure drop to be produced. Of the three wells investigated the rocks from MLM-3 have the best characteristics from the point of view of geothermal reservoir operation: total capacity is the largest, and most of the water is retained by the pores in the middle of the mesopore range, with high equilibrium vapor pressure. One of the consequences of the presence of hysteresis loops is that the well pressure is not in general a reliable indicator of the amount of the resource available. For example for the NEGU-17 rocks, with the same total amount ofwater retained, the equilibrium vapor pressure (reservoir pressure) may be either 0.7 po or 0.95 po. It is more desirable to remain on the adsorption branch, where at constant water content the equilibrium pressure is higher. If after the injection the system is on the desorption branch, the amount of water retained which is necessary to maintain the same pressure (e.g. p/po = 0.8 ) may significantly exceed the amount required on the adsorption branch. At relative pressures lower than about 0.6, where water is retained mainly by multilayer adsorption and pore filling, there is very little change with temperature of the amount of water retained at a given relative pressure. This change is probably below the limit of accuracy of our experiments, and it may be superposed on mass increases due to the presence of residual powdered rock present on all samples, or mass decreases due to possible traces of volatile and/or soluble materials. However, the effects of increasing temperature should be seen in the high relative pressure range, where capillary condensation occurs, and where the geothermal reservoirs usually operate. Water retention should decrease with increasing temperature, as the capillary condensation region is shrinking closer to p/po = 1. Thus, assuming the validity of the Newton equation and bulk values for surface tension and density of water, pores with a radius of 17 A (corresponding to an arbitrarily set beginning of capillary condensation) would fill at relative pressures (0.54, 0.73, 0.79, and 0.85) at temperatures (25, 150, 200, 250) C, respectively. This is due to the decrease of the surface tension, which is responsible for the capillary condensation, with temperature. As a result, at high temperatures, the relative pressure region where water condenses in wide mesopores would be in a very steep portion of the isotherm, beyond the range of accurate measurements. At 250 C measurements of water retention in pores with radii above about 100 A are not practical. For this reason, there is no plateau on the isotherm as p/po approaches 1, and adsorption measurements can not supply a value for total water retention of the rock. Experimental results for average isotherms of the coarse and medium grain size fractions (see Figure 1 ) show a decrease in water retention with temperature on the desorption branch for all three rocks between 200 C and 250 C. This decrease causes the hysteresis loops to be much flatter at 250 C. The observed changes in water retention are, however, small compared to the prediction of the Newton equation except perhaps at relative pressures very close to saturation. This may indicate that even at high relative pressures, a significant fraction of the water is retained on the surface by multilayer adsorption which is less sensitive to temperature changes. Satik et al. (1995) concluded that the effect of temperature is negligible on the adsorption branch while retention of water increases with temperature on the desorption 4-54

6 Geothermal Energy R&D Program branch. Lack of a significant temperature effect on water retention on the adsorption branch in various geothermal reservoir rocks was noted by Bertani et al. (1996) between 170 C and 200 C. FUTURE PLANS The treatment of the experimental data and description of the results will be continued. This will include the compositions of the samples obtained by X-ray diffraction and the low temperature adsorption and mercury porosimetry results which will be available soon. This information will be analyzed in conjunction with available mineralogic and petrographic data. INDUSTRY INTEREST AND TECHNOLOGY TRANSFER Partial results of this study were presented at the Twenty-First Annual Stanford Geothermal Reservoir Engineering Workshop in Stanford, California, January 22-24, 1996 and published in the workshop Proceedings. The results are expected to be of interest to companies operating geothermal reservoirs where a vapor phase zone is present. The knowledge of the adsorption/desorption isotherms and of the hysteresis characteristics is considered to be essential in modeling the behavior of the reservoirs including the reservoir response to reinjection processes. REFERENCES Gruszkiewicz, M. S., Horita, J., Simmonson, J. M, Mesmer, R. E. (1996) Measurements ofwater vapor adsorption on The Geysers rocks. PROCEEDINGS, Twenty-First Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA, January 22-24, Bertani, R., Parisi, L., Perini, R., Tarquini, B. (1996) High temperature adsorption measurements. PROCEEDINGS, Twenty-First Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA, January 22-24, Satik, C., Horne, R. N. (1995) An experimental study of adsorption in Vapor-dominated geothermal systems. PROCEEDINGS, Twentieth Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA, January 24-26, Shang, S. B., Horne, R. N., Ramey, H. J. (1995) Water vapor adsorption on geothermal reservoir rocks. Geothermics,

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