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2 Geothermal Resources Council Transactions, Vol. 2 6, September 22-25, 2002 Conceptual Models of Karaha-Telaga Bodas and The Geysers Tom Powell, Joe Moore2 and Bill Cumming3 Thermochem Inc., Santa Rosa, CA?EGI University of Utah, Salt Lake City, UT 3Cumming Geoscience, Santa Rosa, CA Keywords Karaha-Telaga Bodas, The Geysers, heat pipe, vapor-dominated systems, conceptual models ABSTRACT Simple heat pipe models don t explain all of the features of vapor-dominated geothermal system at The Geysers. Noncondensible gases do not show a buildup at the top of the reservoir and extensive dissolution that would result from downflowing steam condensate is not observed. In this paper, we compare and contrast the characteristics of the vapor dominated systems at Karaha-Telaga Bodas and The Geysers to develop a conceptual model consistent with downhole measurements and petrographic observations. Karaha-Telaga Bodas (KTB) contains a recently developed vapor-dominated system with temperatures similar to those found in the high temperature portion of The Geysers. Vapor dominated conditions developed quickly, and in parts of the system, the descending condensate boils to dryness at relatively shallow depths. Consequently, liquid downflow is minor. The chemistry of the deep liquid phase at KTB shows a significant contribution by steam condensate, suggesting flow of condensate occurs around the margins of the vapor system. The lack of significant gas buildup at the top of the system suggests that the cap allows the flow of gases through it. A conceptual model for The Geysers with steam condensate flowing around the vapor reservoir rather than through it is proposed. In the model, high temperature geothermal fluid enters at the base of the high temperature reservoir (HTR) and vaporizes to steam. Conductive heat flow in the HTR supplies the enthalpy required for this vaporization, and for the steam to adjust to the pressure of the normal reservoir. The enthalpy requirements of upward flowing steam tends to cool the reservoir above 30 bar pressure and heat the reservoir below this pressure, maintaining a thick normal reservoir. Steam condensate and gas exit the top of the reservoir into a semi-permeable cap and are carried laterally to the margins of the reservoir to continue the convection cycle. Introduction Recent work has characterized the Karaha-Telaga Bodas field in west Java, Indonesia, as a vapor-dominated system with a well-preserved mineralogic record of its evolution and presentday processes (Moore, et. al., 2002). The chemistries of its vapor cap and deep brine are known from well measurements and samples (Powell et. al., 2001), along with the physical structure of the reservoir (Allis, et. al., 2000). These data have allowed the construction of a detailed and internally consistent conceptual model for the hydrothermal system. A number of features at KTB reflect upon aspects predicted by the heat pipe model for the world s largest vapor system at The Geysers, but are not found in the mineral or fluid chemistry there. This paper explores these differences and proposes an alternate model for The Geysers geothermal system. The Karaha-Telaga Bodas System The physical characteristics of the Karaha-Telaga Bodas geothermal system have been detailed by Allis, et. al, (2000); a N-S cross-section from that paper is reproduced in Figures 1. The reservoir is aligned N-S along the axis of a volcanic chain extending from Gunung Cadar at the south to Gunung Putri to the north. At the southern end of this chain is the historically active Gunung Galunggung, which last erupted in Drilling has encountered temperatures up to 350 C at both the northern and southern ends of the system. Drilling success indicates that the exploitable reservoir may be no wider than about 1 km along its 10 km length. The reservoir liquid phase is underpressured by as much as 30 bars relative to regional groundwater, attributed to hydrothermal convection and the development of the steam cap, in response to shallow high (>350 C) temperatures. The liquid reservoir is overlaid by a vapor reservoir with temperatures higher than found in most vapor-dominated systems, and above 300 C 3 69

3 t steam t- water d:zr$ed condensate - 0 km 5 Figure 1. Cross-section of the Karaha-Telaga Bodas field, from Allis, et. a/., (2000). at the southern end. Acid chloride surface waters at the system's southern end indicate conditions allowing transport of chloride in steam within the system. The deep reservoir liquid phase is poorly mixed neutral chloride brine of predominantly meteoric origin (Powell, et. al., 2001). Variations in B/Cl ratios are indicative of steam transport of these elements, suggesting that a major portion of the liquid phase chemistry is derived from steam condensate. Stable isotopes suggest that the reservoir fluid is a mixture of meteoric water with andesitic water in the south and with deep crustal water in the north. The steam phase is better mixed than the liquid phase, but still shows variations in gas chemistry attributed to magmatic and crustal sources. Carbon dioxide and hydrogen are equilibrated at reservoir temperatures (230'-300 C), and hydrogen concentrations are consistent with two-phase fluid at rock-buffered redox conditions. A lack of equilibration in H2S and CH4 suggests a relatively short residence time for reservoir steam. Moore, et. ai., (2002) documents the mineral paragenesis and fluid inclusion data. They show that the system was initially liquid, then boiled to vapor very rapidly, during a major, systemwide depressurizing event. Carbon-1 4 dating suggests that this boiling event occurred years BP in response to the formation of Kawah Galunggung, a major active volcanic crater located approximately 5 km south of Telaga Bodas. As fluids boiled off and silica concentrations increased, chalcedony and then quartz were deposited. Fluid inclusions trapped in the quartz record the conditions within the system at this time. Heating measurements indicate that temperatures in wells T-2 and T-8 at depths e m ranged from >250 to -350 C. The ice-melting temperatures of these inclusions ranged from -0.1"C (0.2 weight percent NaCl equivalent) to -27 C (-24 weight percent NaCI-CaC12 equivalent). However, salinities exceeding 5 weight percent NaCl equivalent, which are unusual in other geothermal systems, are common at KTB. The broad range of salinities reflects extensive boiling and concentration of salts. These observations indicate that temperatures were >25OoC at the start of chalcedony deposition. The pressure gradient, calculated from the temperatures and salinities was -20 bad100 m. This gradient, which is at least double a hot hydrostatic gradient and between 60% and 80% llc*oool* --I - -2 of the lithostatic gradient, indicates that the early liquid-dominated system was strongly over-pressured. As pressures subsequently declined, the vapor dominated regime developed over the declining liquid zone, and SO4- rich steam condensate began to percolate downward from relatively shallow depth within the volcanic complex. Interactions between the condensate and wall rocks produced advanced argillic alteration assemblages and veins dominated by the successive appearance of chlorite minerals, anhydrite & tourmaline, pyrite, calcite and fluorite. Fluid inclusions trapped in anhydrite, calcite and fluorite record the evolution of these descending fluids. At depths shallower than 800 m, the ice- melting temperatures increase with depth from -2.8" to -1.5"C (4.7 to 2.6 weight percent NaCl equivalent) as temperatures increase from 160" to 205 C. The melting temperatures then remain approximately constant as temperatures increase further to 235 C. The decrease in the freezing point depressions is due to the incorporation of SO4 into newly formed minerals, whereas the minimum value represents the residual salinity of the fluid, which is controlled primarily by CI and unreacted C02. At depths greater than 800 m, boiling off of the descending condensate produced a progressive increase in its salinity. Hypersaline brines containing daughter crystals of halite (3 1 weight percent NaCl equivalent) were trapped at 300 C in anhydrite. Further boiling and the circulation of chloride-bearing steam resulted in the deposition of NaCl, KCl, FeCI, and Ti-Si-Fe scales. The presence of these precipitates demonstrates that the rocks had dried out prior to drilling. The Conceptual Model of Karaha-Telaga Bodas The conceptual model that emerges is that of a relatively rapidly convecting vapor-dominated reservoir surrounded and fed by a poorly mixed liquid system consisting of a mixture of deep groundwater and steam condensate. The liquid phase boils to steam in the heart of the system, in what is probably a narrow N-S fissure zone aligned along the chain of vents that make up the volcanic highlands of the prospect. Rising steam condenses at the top of the system and flows laterally away from the fissure zone, to combine and mix with deep groundwater flowing in from the east and west to complete the convection cycle. Figure 2 shows this conceptual model of the system in a schematic east-west cross-section, oriented transverse to the N-S alignment of the system. Boron and chloride in the liquid phase appear controlled by steam condensate, suggesting that condensate formed at the top of the system re-emerges in the deep brine phase, completing the convection cycle. Evidence for evaporation of downward percolating condensate shows that some condensate descends into the reservoir and is subsequently evaporated. Most of the liquid return flow to the system, then, must occur outside the 3 70

4 West ff Fumaroles East return flow / Figure 2. Schematic E-W cross-section of the Karaha-Telaga Bodas hydrothermal system. Regional groundwater and steam condensate flow into the system, mix with magmatic vapor and are vaporized. Superheated steam rises through a fracture network to shallow level, feeding a vapor reservoir. Steam condenses at the margin of this reservoir and flows downward to mix with inflowing groundwater, completing the convection cycle. vapor reservoir. Mixing with deep groundwater in the return flow of liquid provides the overall meteoric isotope signature to the reservoir fluids and the source of crustal gases in the steam. The Heat Pipe Model of The Geysers A number of features of the KTB vapor system have analogs in the consensus heat pipe conceptual model of The Geysers steam field, first proposed by White, et. al., (1971) and later refined by Truesdell, et. al., (1993). The model is shown schematically in Figure 3, from the latter reference. In the model, geothermal steam rises through a fracture network from a source at depth, initially thought to be a deep brine layer but later recognized to be hot, dry rock of the HTR. The steam condenses upon encountering the top of the reservoir, and then percolates downward in the same reservoir to start the convection cycle surface over again. Heat is moved vertically through the system by boiling liquid water at the reservoir base and condensing it at the top. It is proposed by Truesdell, et. al., (1993) that some recharge also enters from the base of the reservoir. This model has withstood the test00 of time, but its consistency with the geology and geochemistry of the field has not been thoroughly reviewed. Some of the predictions of the heat pipe model have been difficult to reconcile with the known system: 1) Non-condensible gas should accumulate at the top of the reservoir. McKibbin and Pruess ( 1988) conducted simulations of gravity-driven heat pipes including CO2 gas. Results for representative circulation rates show that, without significant mass throughput, a high-gas wedge forms at the top of the reservoir. Gunderson (1 989) showed that gases in the normal Geysers reservoir do decrease with depth, doubling every 250 to 450 meters (800 to 1500 ft) throughout the reservoir, but this is a relatively minor concentration gradient compared to the heat pipe model predictions. The gas wedge predicted by the heat pipe model affects only the upper few tens of meters and typically achieves very high concentrations (>30 mole percent). These types of gas wedges are common features in naturally occurring steam caps in liquid systems, but have not been observed at The Geysers. When mass throughput to the cap is included in the numerical model, results show a gas concentration gradient similar to that observed. 2) Fitzgerald, et. ul., (2001) suggest that the downward counterflow of dilute condensate predicted by the heat pipe model would enhance vertical permeability within the reservoir by rock dissolution. While dissolution textures have been observed in cores from top of the NW Geysers (Moore, et. al., 1998) it is locally accompanied by evidence of condensate boiling, like at KTB. 3) Conversely, the downflow of condensate would be expected to deposit a zone of evaporite at the top of the HTR, where the heat pipe model predicts re-boiling. This has not been observed. To date, no mineralogic transition has been observed at the boundary between the normal and HTR that would be consistent with sustained condensate boiling. 4) There is evidence of a rapid boiling event at KTB, in the form of widespread, chacedonic quartz, which presumably formed when the system converted from liquid-dominated to steam-dominated. Although heat pipe models seem to require a similar mechanism to explain the switch from liquid-dominated to steam-dominated conditions at The Geysers (Pruess, 1985), no evidence for such an event has been seen in the mineralogic record. If a discharge event like the one at KTB had occurred at The Geysers, features like those observed at KTB would have been seen. Figure 3. Schematic conceptual model of The Geysers from Truesdell et. a/., (1 993). Steam flow is represented by open arrows, and liquid condensate and recharge water by solid arrows. HTR denotes the high-temperature reservoir. New Conceptual Model for The Geysers To explain the inconsistencies in the heat pipe model with respect to some of the characteristics of The Geysers, we pro- 371

5 pose a hybrid model that shares some characteristics of an open system model like that proposed for KTB. This model is structurally similar to thet~esde11, et. at., (1993) model, with modified reservoir caprock properties and a different return flow. The caprock proposed for this model is semi-permeable and allows the passage of liquid water and gas. This has important implications for fluid circulation in the system. The caprock at The Geysers is different from the clay-rich caprock observed in volcanic hosted geothermal reservoirs. A clay-rich caprock arguably creates an effective permeability barrier at the top of the geothermal system. In many areas of The Geysers, the caprock is similar to the reservoir rocks, with relatively minor alteration related to current reservoir processes. Reservoir greywacke doubles as cap rock in the abandoned PG&E Unit 15 area, for example. The difference appears to be that the Franciscan Formation Geysers rocks were metamorphosed during their Mesozoic emplacement and are not prone to further alteration by the hydrothermal system. In the open system model, deep groundwater is drawn into the shallow heat anomaly at The Geysers from the surrounding region. At the margin of the vapor reservoir, the boundary to the HTR, this water encounters a permeability barrier. (The low pressure of the vapor reservoir at The Geysers requires some type of permeability barrier, otherwise the reservoir would be quickly ~ooded by groundwater from su~ounding rock). The pressure difference between the surrounding rock and HTR is great and water leaks through. Upon encountering the low pressure vapor reservoir it immediately undergoes decompressional boiling and separates to steam and water. The mineral load in the water is concentrated by steam separation and undergoes rapid precipitation. Mineral precipitation formed in this manner would likely help to maint~n a permeability barrier by plugging fractures and may be responsible for the presence of the barrier itself. From here the steam-water mixture flows upward toward lower pressure, continuing to boil and precipitate solids. Conductive heat flow within the HTR supplies the enthalpy needed to heat the mixture to dry steam by the time it reaches the normal reservoir. The normal reservoir is at about 30 bar pressure, coincident with the enthalpy maximum of saturated steam, and this has important consequences for the development of the normal reservoir. Vapor rise through the normal reservoir becomes isothermal because at pressures above 30 bar the vapor extracts heat from the rock and below 30 bar it transfers heat back to the rock. The rising steam al~matively cools the reservoir below and heats the reservoir above, extending the vertical extent of isothermal reservoir. Steam rises into the cap rock, where it encounters more limited permeability, cooler temperatures and groundwater, and condenses. Condensate and gas enter the cap and are swept to the margins of the reservoir, with shallow groundwater, to complete the convection cycle. Figure 4 shows the path of this water on an enthalpy-pressure diagram. In the figure water heats and gains enthalpy as it approaches the vapor reservoir, perhaps to the critical point. Upon crossing the permeability boundary into the HTR de-pressurization causes it to separate into steam and water. Heat transfer to the rising two-phase mixture supplied by conductive heat flow in the HTR boils remaining water to vapor. Rise of vapor continues to extract heat from the rock until reaching the enthalpy ~ imum, after which continued rise results in heat transfer back to the rock. C~culations show that this mechanism yields an approximate heat balance between heat entering the system at the base and exiting the system in the cap. A representative surface heat flow of 1 W/m2 at The Geysers requires condensation of 56 g/ day-m2 of steam at the top of the normal reservoir. The heat flow needed to boil this quantity of pure water from the critical point to steam at 235 C translates to temperature gradient of 155 Ckrn in Geysers greywacke. This is within the observed range of temperature gradients in the HTR (90-18OoC/km, Walters, et. at., 1992). In other words, the thermal boost supplied by conductive heat flow in the HTR is sufficient to supply the steam condensing in the system s cap. This model predicts a different origin and behavior for a number of the chemical constituents in The Geysers reservoir: I) Steam and noncondensible gas in the reservoir have their origin in the deep groundwater that flows into the system. The isotopically light steam in the southeastern part of the system reflects a predominantly groundwater source, whereas the heavier isotopes of the northwest would reflect the predominance of deep met~o~hic water, as originally proposed by Haizlip, et. al. (1985). Low gas in the southeast would reflect the pr~o~nantly igneous (felsite) basement rock, whereas the high gas in the northwest reflects sedimen~ rocks of the Franciscan Formation. The ammonia excess in Geysers steam identified by Lowenstern, et. al., (1999) would result from the relatively short residence time of gases in the reservoir. 350 Heat added by Critical Fluid I d a 200 g! 2 150! M Enthalpy - J/gm Figure 4. Enthalpy versus pressure diagram showing the path of fluid entering the HTR at The Geysers. The fluid depicted enters the reservoir near the critical point and immediately separates to liquid and steam. Conductive heat flow in the HTR continues to heat and boil the liquid and add heat to the steam phase as it flows to lower pressure. 372

6 Although the model does not predict a boundary between the HTR and normal reservoir, it does predict that the HTR will generally show greater pressure than the normal reservoir. Steam at HTR temperatures and normal reservoir pressure would be superheated and less dense than it s saturated counterpart. This superheated steam would be expected to rise quickly through the system, heating reservoir rock as it rises. This would, with time, form superheated pipes through the normal reservoir. On the other hand, the extra enthalpy requirements of heating some steam to superheat would undoubtedly tax the conductive heat supply in the HTR, causing it to merge with normal reservoir. The boundary between the HTR and normal reservoir, then, may be a natural one that simply separates high permeability shallow reservoir from lower permeability deep reservoir. The high reservoir pressures in the Aidlin project, which is developed entirely within the HTR, suggest the possibility of higher pressure in some areas of the HTR versus that in normal reservoir (M. Stark, personal communication). Conclusions It is clear that simple heat pipe models don t explain all of the features at The Geysers. Non-condensible gases do not show a buildup at the top of the reservoir, as predicted. The absence of a layer of evaporite minerals at the top of the HTR calls into question the HTR s role in re-boiling downflowing condensate. Other features, such as the absence of indications of system-wide boiling event needed to change the system from liquid to steam dominated, are problems for the heat pipe model. Many features at The Geysers are similar to KTB, suggesting a similar nature. Liquid downflow in the KTB reservoir is minor due to evidence of the evaporation of downflowing liquid, similar to features observed at The Geysers. The chemistry of the deep liquid phase at KTB shows a significant steam condensate contribution, suggesting return flow of condensate around the margins of the vapor system. The evolution of KTB from liquid to vapor-dominated is similar to The Geysers, except that The Geysers lacks evidence of the liquid boil-off event seen at KTB. When applied to The Geysers, the open system model derived from the KTB system suggests that steam condensate flows around the vapor reservoir rather than through it. This addresses a number of these problems with the heat pipe model. No gas wedge builds up at the top of the normal reservoir because steam condensate and gas enter the cap and flow away from the reservoir. No lithologic change would be expected at the HTR-normal reservoir transition because it marks a thermodynamic boundary rather than a zone of boiling. Still unresolved in this discussion is the mechanism by which The Geysers transformed from a liquid to vapor-dominated system. References Allis, R., Moore, J., McCulloch, J. and Petty, S., 2000, Karaha-Telaga Bodas, Indonesia: a partially vapor-dominated geothermal system; GRC Transactions Vol. 24. D Amore, E and Truesdell, A.H., 1979, Models for steam chemistry at Larderello and The Geysers; Proc. 4* Workshop on Geothermal Reservoir Engineering, Stanford, CA. Fitzgerald, S.D., Woods, A.W. and Truesdell, A., 2001, Theformation of permeability contrasts in geothermal reservoirs; Proc. 26 Workshop on Geothermal Reservoir Engineering, Stanford, CA, p Gunderson, R.P., 1989, Distribution of oxygen isotopes and non-condensible gas in steam at The Geysers; Geothermal Resources Council Trans. Vol. 13. Haizlip, J.R., 1985, Stable isotope composition of steam from wells in the northwest Geysers, The Geysers, California; Geothermal Resources Council Trans. Vol. 9, Part I, p.3 I Lowenstern, J.B., Janik, C.J., Fahlquist, L.S. and Johnson,L.S., 1999, Gas and isotope geochemistry of 81 steam samples from wells in The Geysers geothermal field, Sonoma and Lake counties, California, U.S.A. ; USGS Open-File Report McKibbin, R. and Pruess, K., 1988, Some effects of non-condensible gas in geothermal reservoirs with steam-water counteflow; Proc. 1 3 h Workshop on Geothermal Reservoir Engineering, Stanford, CA, p Moore, J.N., Anderson, A.J., Adams, M.C., Aines, R.D., Norman, D.T. and Walters, M.A., 1998, The fluid inclusion and mineralogic record of the transition from liquid- to vapor-dominated conditions in The Geysers geothermal system, California; Proc. 23d Workshop on Geothermal Reservoir Engineering, Stanford, CA. Moore, J., Renner, J., Mildenhall, D. and McCulloch, J., 2002, Petrologic Evidence for Boiling to Dryness in the Karaha-Telaga Bodas Geothermal System, Indonesia; Proc. 27 Workshop on Geothermal Reservoir Engineering, Stanford, CA. Powell, T., Moore, J., DeRocher, T. and McCulloch, J., 2001, Reservoir geochemistry of the Kuruha-Telaga Bodas Prospect, Indonesia; Geothermal Resources Council Trans. Vol. 25. Pruess, K., 1985, A quantitative model of vapor dominated geothermal reservoirs as heat pipes infiacturedporous rock; Geothermal Resources Council Trans. Vol. 9, Part 11, p Truesdell, A., Walters, M., Kennedy, M. and Lippmann, M., 1993, An integrated model for the origin of The Geysers geothermal>eld; Geothermal Resources Council Trans. Vol. 17, p Walters, M.A., Haizlip, J.R., Sternfeld, J.N., Drenick, A.F., and Combs, J., 1992, A vapor-dominated high-temperature reservoir at The Geysers California; Geothermal Resources Council Special Report No. 17, p White, D.E., Muffler, L.J.P. and Truesdell, A.H., 1971, Vapor-dominated hydmthermal systems compared with hot-water systems; &on. Geol. V0166, NO. 1, p