Geothermal Resources Council TRANSACTIONS, Vol. 15, October 1991

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Geothermal Resources Council TRANSACTONS, Vol. 15, October 1991 THE APPLCATON OF HALOGENATED ALKANES AS VAPOR-PHASE TRACERS: A FELD TEST N THE SOUTHEAST GEYSERS Michael C. Adamsl, Joseph J.

1 Geothermal Resources Council TRANSACTONS, Vol. 15, October 1991 THE APPLCATON OF HALOGENATED ALKANES AS VAPOR-PHASE TRACERS: A FELD TEST N THE SOUTHEAST GEYSERS Michael C. Adamsl, Joseph J. Beall*, Steven L. Enedys, and Paul Hirtz4 1. University of Utah Research nstitute, 391-C Chipeta Way, Salt Lake City, UT Calpine Corporation. P.O. Box 11279, Santa Rosa, CA Northern California Power Agency, P.O. Box 663, Middletown, CA Thermochem, nc Skyline Blvd., Santa Rosa, CA 9543 ABSTRACT A new class of vapor-phase tracers, the halogenated alkanes, was field-tested in the Low Pressure Area of the Southeast Geysers geothermal field. Two tracers, R-12 and R-13, were injected into NCPA Well C-11. Tracer was found in 38 of the 49 production wells monitored during the 5 1 day test. Tracer breakthrough was very rapid, with 13 of the 16 wells monitored on the first day after injection showing breakthrough. Peak transit times occurred within 1 to 11 days. Comparison with stable isotope data for this area demonstrated that the vapor-phase tracers and the bulk injection fluid followed similar paths. Although both tracers proved useful in tracing injection flowpaths, R-12 displayed significant decay during the test. R-13, in contrast, showed little or no decay. The decay rate of R-12 appears to have been reduced by a factor of three to four on day 2 of the test. This change may be related to movement of the tracer plume outside of the oxidized zone surrounding the injection well. low ozone depletion potentials is expected to increase dramatically during the next several years. A tracer test using halogenated alkanes to follow the movement of steam derived from condensate was initiated in the "Low Pressure Area" of the Southeast Geysers in Feb (Fig. 1). This tracer test involved 2 halogenated alkanes, R-12 and R-13,1 injection well, and 49 production wells. n this paper, we first present a brief summary of the tracer test results and then use these results to assess the relative stabilities of the two halogenated alkanes. NTRODUClON Successful tracer tests at The Geysers have used either deuterium or tritium to trace the flow of injected fluids. Although both of these tracers provide useful information, they also have certain limitations. Repeated use of tritium, which is radioactive, can saturate the reservoir after several tests. The stable isotope content of power plant condensate varies somewhat with time due to factors such as the rate of cooling tower evaporation. Furthermore, it is not possible to assess the effects of multiple injection wells during a single tracer test. Consequently, the need for other tracers exists. This need is exacerbated by the fact that efficient injection practices have been shown to lower the continued pressure declines experienced by operators at The Geysers (Enedy et al., 1991). The possibility of using halogenated alkanes as vaporphase tracers was first introduced by Adams et al. (1991). These compounds, which are known for their high stability (Norton, 1957; Downing, 1988) and low toxicity (Downing, 1988), are available in a wide variety of species. Thus, multiple injection wells can be tagged simultaneously. Some of the halogenated alkanes, such as R-12, are being phased out of production because of their detrimental effects on ozone concentrations in the stratosphere. However, others, such as R-13, have a low ozone depletion potential. The number of commercially-available halogenated alkanes with vu km Fig. 1. Location map of The Geysers geothermal field. The tracer test was conducted in the Low Pressure Area. TEST DESCRPTON The Low Pressure Area of The Geysers is a region characterized by low reservoir pressures (Enedy, 1989; Barker et al., 1989) and increasing superheat. This portion of The Geysers was chosen for an initial test of the halogenated alkanes as tracers because stable isotope studies had demonstrated that some of the production fluid in the surrounding Northern California Power Authority (NCPA) and Calpine wells was derived from injectate (Beall et al., 1989; Enedy et al., 1991). Furthermore, studies of wells in the Low Pressure Area indicated that injection had the 457

2 Adams et 31. Unocal Low Pressure Area \ njection Wells Calpine 4 N krn Fig. 2. Location map of the production wells sampled during the test. The tracer injection well, C-11, and other injection wells in the area are also shown. beneficial effect of increasing reservoir pressures and steam production (Enedy et al., 1991). The tracers were injected into NCPA Well C-1 1, which receives condensate from both NCPA and PG&E/Calpine power plants (Fig. 2). This well was first brought on line as an injection well in October 1989, and, with the exception of a five month shutdown for casing repair, has been in continuous service. The primary injection zone of C-1 1 is not known precisely, but is thought to be located in greywacke at a depth of 1159 m (-223 m MSL). The depth to felsite in this region ranges from sea level in the middle of unit 18 to m below sea level near Well C-11 (Enedy et al., 1991). Two tracers were used in this field test, R-12 (CC12F2) and R-13 (CClF3). These tracers were chosen because their chemical and physical properties are among the best known of the halogenated alkanes, and because they were relatively inexpensive and available in large quantities.. Approximately 324 kg of R-12 and 1 kg of R-13 were injected simultaneously into Well C-11. These quantities were based on the relative solubilities of the two chemicals and the results of a tritium tracer test pexformed by Unocal (unpublished data), which provided an indication of the amount of dilution that could be expected. njection of the tracers took 4.5 hours. Forty-nine production wells were monitored during the tracer test, which lasted 51 days. Although the original design of the tracer test called for both tracers to be analyzed in all samples taken, last-minute changes in the analytic equipment dramatically increased the detectability of R-12. The extreme sensitivity of the detector to R-12 required that the tracers be analyzed separately to avoid swamping the R- 13 signd. Rather than double the number of analyses and reduce the wells monitored, it was decided to monitor most of the wells for R-12, which is the most detectable of the two tracers. Samples from three wells were analyzed for both R-13 and R-12 to evaluate their relative behavior and 458

3 AdwS et al. Cal p i ne NCPA Unocal.. Days Since njection Fig. 3. Sampling and analysis schedule for the tracer test. ndividual wells are represented by horizontal lines. Solid mangles denote samples analyzed for R-12, samples shown by open squares were analyzed for both R-12 and R-13. Wells in which the tracer concentrations were extremely low or below detection were not monitored through the entire test period. keep within the original guidelines of the test. Check analyses were also conducted on several other wells for R- 13 during the second half of the test. The sampling and analysis schedule for the wells is shown in Fig. 3. The steam samples were collected in standard evacuated glass cylinders containing cadmium chloride to remove hydrogen sulfide, which would have interfered with the analysis. The tracers were then analyzed on a gas chromatograph equipped with an electron capture detector. This method yields detection limits for R-12 and R-13 of approximately 1-6 and 1-2 pprn, respectively. RESULTS The tracer R-12 was detected in 13 out of the 16 wells sampled one day after injection (day 1). By the end of the test at 51 days after injection (day 51), R-12 had been detected in 38 of the 49 wells sampled. Peak concentrations ranged from 3.2 parts per million-million (ppt) to parts per million (ppm) for R-12, and from 29 ppt to 1.7 ppm for R-13. Transit times were very rapid for some wells. Tracer was detected in fluid from wells as far away as.8 km from the injection well on day 1. By day 5, tracer was detected 1.6 km from the injection well. Contour maps of the peak arrival times and mass recoveries are shown in Figs. 4 and 5, respectively. NCPA Wells C-1 and F-6 and Calpine Well were monitored for both R-12 and R-13. Profiles of these concentrations are plotted versus time in Fig. 6. Profiles of R-12 concentrations corrected for decay are also shown on this figure. As discussed below, the restored concentrations are only integrated up to day 2 of the test. ntegration of the corrected concentrations to day 2 yielded a minimum tracer recovery of 22%. 459

4 Adam et al. Unocal 1 i Production wells,# njection Welts 4 N.5 L km Fig. 4. Contour map of the arrival times of R-12 peaks, corrected for decay. A decay constant of.3 per day was used, as discussed in the text. The data are contoured in days. For reference, the property boundries and outline of the Low Pressure Area are shown by the shaded lines. Unocal t d o ~cll( { Production wells njection Wells Calpine J 4 N km Fig. 5. Contour map of the calculated mass recoveries of R-12 from the wells sampled during the test. The mass recoveries shown are corrected for decay using a decay constant of.3 per day. The data were integrated from the first sample to day 2. The mass recovered beyond day 2 was minimal, and is not included because of the uncertainty in the decay constant. The data are contoured in kg. See Fig. 4 for explanations. 46

5 The tracer recovery calculated from the return curves may also be low due to breakthrough prior to the initiation of sampling. Well C-6, which had the highest concentration of tracer by a factor of 3, was first sampled after the tracer peak had passed. n addition, a "gas kick" was recorded at the power plant fed by C-6 a few hours after injection. Thus, some of the tracer return could have gone undetected. DSCUSSON The tracer test had Objectives. These Adams et al. northwest, southeast, and southwest of injection well C-11. These trends are also displayed in the halogenated alkane tracer recovery contours (Fig. 5). Wells that show little change in their stable isotope signature, namely Calpine Wells and 956-4, and NCPA Wells C-4 and C-9, also show low halogenated alkane tracer recovery. These similarities imply that the vapor-phase tracers followed the same path as the bulk injection fluid in this test. The low uressures and superheat found in the Low Pressure Area ari believed to promote very efficient boiling of injectate. This makes extrapolation of the results of this were to evaluate the Of the tracers to the test to other of The Geysers problematic. VapormphaSe phase and the stability of the tracers under geothermal conditions. tracers should also be tested in regions of The Geysers where the reservoir is more saturated' with liquid in the vicinity of the injection wells in order to more completely describe their behavior in vapor-dominated systems. At the inceution of the test, there was a question of whether the vapor-phase tracers would follow the bath of the injected liquid or whether they would fractionate to the earliest-boiling steam and follow a path separate from the bulk of the injectate. One means of evaluating this question is to compare the stable isotope patterns resulting from the injection of isotopically heavy condensate with the results of. the data from the halogenated alkane tracer test. The stable isotope data show high injection recovery trends to the - 1.w OOO c-1 b loo. F-6. Days since njection -...a... R-12 Measured -m- R-12 Calculated (k.3) R-13 Measured - 4- R-13 Calculated (ko.8) Fig. 6. R-12 and R-13 tracer return profiles for Calpine Well and NCPA Wells C-1 and F-6. Calculated return profiles for R-12 are also shown for a decay constant of.3 for the first 2 days, and.8 per day for the remainder of the test. Calculated concentrations using a decay constant of.3 are also shown for the latter part of the test for comparison. For clarity, data points for the first few days of sampling are not shown. The chemical stability of the two tracers can be examined by comparing the shape of their return curves. t is apparent from these curves (Fig. 6) that the two tracers reacted differently to conditions within the reservoir. The measured concentrations of R-12 decreased much more rapidly with time than those of R-13. However, the peaks of the R-12 and R-13 return curves were concordant, indicating that the effect was not due to absorption or the differing solubilities of the tracers in the liquid and the steam. These relationships suggest that either the two tracers decayed at different rates, or that one of the tracers decayed and the other did not. Because both R-12 and R-13 could have decayed, it is not possible to uniquely determine the actual decay rate for either compound. However, an estimate of the decay rate for R-12 can be made if it is assumed that there was no decay of R-13, and that the concentration decreases followed a first-order rate law. The first assumption is reasonablo, in light of the much more rapid decreases in R-12 compared to R-13, as shown in Fig. 6. The second assumption of firstorder decay is supported by the laboratory studies of Norton (1957). For a reaction that follows first-order kinetics, a plot of the logarithm of the ratio of the two tracers vs. time will be linear, with the slope being equal to the decay constant and the intercept equal to the log of the initial tracer ratio. f some decay of R-13 occurred, then the true decay constant of R-12 would be equal to that inferred from Fig. 7 plus the decay constant of R-13. t can be seen in Fig. 7 that the logarithm of the tracer ratios did vary linearly with time. However, in the three wells monitored for both tracers, two distinct linear segments can be discerned from the data. These segments correspond to a decay constant of approximately.3 per day for the first 2 days of the test, and.8 per day for the remainder of the test. The check analyses of R-13, also shown in Fig. 7, show that other wells in the field displayed similar behavior. Although these wells were only sampled during the latter 31 days of the test, their tracer ratios at day 2 are consistent with more rapid decay earlier in the test. The existence of a change in the decay rates of R-12 can also be derived from its measured concentrations in wells for which there are no R-13 data. f the decay rate of.3 is used to restore the R-12 concentrations to a return curve "corrected" for thermal decay, then the concentrations decrease to day 2 but increase thereafter. ntegration of the "corrected" concentrations beyond day 2 yields a recovered mass that is greater than four times the mass actually injected. Thus, the decay constant appears to have changed for all of the wells., 461

6 Adams et , - - T Q 4 c-1 laboratory experiment. The decay constant calculated from the latter part of the tracer test is similar to that derived from our laboratory data. Thus, R-12 must have been exposed to a reactive compound (or catalyst) for at least the first 2 days of the tracer test. t is unlikely that the reactive compound occurs along all of the fractures followed by R-12 during the tracer test. f this were the case, then the effect pf the compound would have been exhausted nearly simultaneously on day 2 Temperature ( C) $! a & c 2. F-6 Days Since njection Fig. 7. Variation of n (R-12/ R-13) with time during the tracer test for Calpine Well and NCPA Wells C-1 and F-6. Ratios from wells for which check analyses were performed during the latter part of the test are also shown (MSC.). On each of these plots the slope of the line is equal to the decay constant for a first-order reaction. t was.expected from extrapolation of published laboratory data (Adams et d., 1991) that R-12 would decay at a negligible rate at the temperature found in The Geysers reservoir, which is approximately 24oC. These data were derived from experiments in which R-12 was heated in quartz or iron vessels with small quantities of water or oxygen. n some of the experiments, large proportions of hematite were also included. The data from these experiments are shown in Fig. 8. Also shown in Fig. 8 are the two decay rates derived from the field test, and one derived from our laboratory experiments. n our laboratory experiment, a mixture of 24 ml of 54 ppm R-12 in nitrogen and 16 ml of distilled water was heated at a temperature of 275oC in quartz vials. t can be seen in Fig. 8 that the decay rates of R-12 are increased by the presence of either water or hematite. Thus, the overwhelming abundance of water (as steam) in The Geysers reservoir was a significant factor in the decay of R-12. However, the rate of decay calculated from the first 2 days of the tracer test are even higher than that of our 1.o OOO/r( K) Laboratory Data field Data Rl2+Qtr+H2 nitialrate R-l2+Qk Final rate R12+atr+Hm Fig. 8. Decay constants for the decay of R-12 at various temperatures and conditions. The data for the laboratory experiments that do not include water are from Norton (1957). Other data are from this study. throughout the reservoir. However, if the injected tracer had a significant residence time in the vicinity of the injection well, the effect could have been localized The change at day 2 could then have been due to the sphere of retained tracer (in steam) expanding beyond the area encompassed by the reactive substance. f the reactive substance was located near the injection well, its presence may reflect the effects of injection on the reservoir rocks. Because the injected fluid is cooled by contact with the atmosphere, it contains approximately 7 ppm of molecular oxygen. This oxygen could react readily with magnetite or other oxides and sulfides to produce hematite along the walls sides of the fractures. The reaction would form a halo of hematite around the injection well that is constantly expanding as more molecular oxygen is supplied by the injection fluid. R- 12 could also have reacted with the molecular oxygen, but one would expect the rate to have changed as the oxygen was used up by both R-12 and the reservoir rocks. AS shown in Fig. 7, this was not the case because the rate was constant for the first 2 days. Hematite, in contrast, could have been present in large quantities from months of oxygenated injection prior to the tracer test. Thus, hematite, or some other oxide, is the most likely candidate for reaction with R

7 . The literature data summarized by Adams et al. (1991) and shown in Fig. 8 demonstrate that, at least in anhydrous systems, R-12 reacts with metal oxides at a rate far exceeding systems in which the only mineral is quartz. f the hypothesis that the rapid decay of R-12 during the tracer test was due to reaction with hematite is correct, then a change at day 2 implies that steam containing the tracer passed outside the hematite halo on or about that day. Thus, some information about the geochemical effects of injection may be derived from a tracer test. CONCLUSONS The halogenated alkanes R-12 and R-13 were tested as vapor-phase tracers in the Low Pressure Area of the Southeast Geysers. Approximately 324 kg of R-12 and 1 kg of R-13 were injected simultaneously into the NCPA injection Well C-11 over a period of 4.5 hours. A total of 49 wells were monitored for the presence of tracer, which was detected in 38 of these wells. Transit times for the peaks. ranged from 1 to 11 days. R-12 decayed rapidly with respect to R-13 during the test. The decay rate was relatively rapid for the first 2 days of the test, and decreased by a factor of three to four during the remaining 31 days of the test. The decrease may be related to geochemical changes in the reservoir rock surrounding the injection well. No effects of adsorption or fractionation due to solubility differences were detected in this test. The results of this test indicate that the halogenated alkanes can be successfully used as geothermal vapor-phase tracers. ACKNOWLEDGEMENTS This test would not have been possible without the support and cooperation of Calpine Corporation, NCPA, Thermochem ncorporated, and Unocal Geothermal Division. nterpretation of the data and preparation of this paper was funded by the U. S. Department of Energy under contract No. DOE-AC7-85D Such support aoes not constitute an endorsement by the U. S. Department of Energy of the views expressed in this publication. We appreciate the helpful reviews and suggestions of B. Barker and J. Moore. Adams et al. REFERENCES Adams, M. C., Moore, J. N., and Hirtz, P., 1991, Preliminary assessment of halogenated alkanes as vaporphase tracers: 16th Workshop on Geothermal Reservoir Engineering, Stanford Univ., in press. Barker, B. J., Gulati, M. S., Bryan, M. A., and Riedel, K. L., 1989, Geysers reservoir performance: Geothermal Resources Council, Transactions, v. 13, p Beall, J. J., Enedy, S. L., and Box, W. T. Jr., 1989, Recovery of injected condensate as steam in the south Geysers field: Geothermal Resources Council, Transactions, V. 13, p Downing, R. C., 1988, Fluorocarbon refrigerants handbook Prentice-Hall, Englewoad Cliffs, New Jersey. Enedy, K. L., 1989, Downhole enthalpy and superheat evolution in Geysers steam wells: Geothermal Resources Council, Transactions, v. 13, p Enedy, S. L., Enedy, K. L., and Maney, J. J., 1991, Reservoir response to injection in the Southeast Geysers: 16th Workshop on Geothermal Reservoir Engineering, Stanford Univ., in press. Norton, F. J., 1957, Rates of thermal decomposition of CHCLF2 and CF2Cl2: Refiig. Eng., v. 65, pp

VAPOR, LIQUID, AND TWO-PHASE TRACERS FOR GEOTHERMAL SYSTEMS

VAPOR, LIQUID, AND TWO-PHASE TRACERS FOR GEOTHERMAL SYSTEMS Michael C. University of Utah Research Institute, 391 Way, Suite C, Salt Lake City, UT 84108 Key Words: tracers, reactive tracers, partitioning