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2 Geothermal Resources Council TRANSACTIONS, Vol. 14, Part 11, August 1990 NUMERICAL SIMUlATION OF THE KAMOJANG GEOTHERMAL FIELD, INDONESIA Michael J O'Sullivan Brian G Barnett (2) M Yunus Razali (3) 1. University of Auckland, New Zealand 2. Geothermal Energy New Zealand Limited 3. Dinas Geotermal, Pertamina, Jakarta, Indonesia ABSTRACT A three dimensional model of the vapour dominated, Kamojang Geothermal Field has been constructed. The model has been refined and calibrated by matching the natural, pre-exploitation state of the reservoir and by reproducing reservoir pressure declines observed in individual production wells during the first seven years of exploitation. Uncertainties in the simulation results due to the choice of the initial vapour saturation, boundary conditions and model thickness have been addressed and quantified. I. I 1 INTRODUCTDN The Kamojang Geothermal Field is a vapour dominated field located about 45 km southeast of the city of Bandung in West Java, Indonesia. A 30 MW turbine has been generating electricity since October An additional 110 MW plant (ie 2 x 55 MW turbines) was installed in late 1987 and since that time, the field has supported 140MW(e) of electricity production. A total of 26 production wells are connected to the steam collection network and are available for production. A further 4 wel1.s (KMJ-15, 21, 23 and 32) are available for reinjection of the excess condensate produced in the power plant. Figure 1 shows the locations of all deep wells drilled in the field (including 5 exploration wells, KMJ-6, 7, 8, 9 and 10). Numerical simulation was initially carried out by the steamfield developers (Pertamina) to help quantify the maximum level of steam extraction that the reservoir can sustain over a 25 year production period. This paper describes detailed numerical models developed to match both the natural, pre-exploitation reservoir conditions and the observed field performance during the first seven years of exploitation. Simulations were carried out using a version of the MULKOM code (Pruess, 1983) extensively modified at the University of Auckland. This code is based on a combination of equations of state with Darcy's Law and conservation of mass and energy to create differential equations describing fluid flow in the reservoir. Finite difference techniques are used to solve these second order, partial differential equations in three dimensions with additional constraints imposed by the thermodynamic and physical properties of water, steam and carbon dioxide. PREVIOUS MODELS OF VAPOUR DOMINATED RESERVOIRS A basic conceptual model for vapour dominated geothermal fields was proposed by White and others Figure 1. Kamojang Well Locations (1971) but there have been only a few numerical modelling studies. Schubert and Straus (1979, 1980), and Straus and Schubert (1981) used analytical methods to study simplified, one-dimensional, steady-state versions of vapour dominated systems. Pruess and Truesdell (1980) used a simple radially symmetric model to simulate the evolution of a vapour dominated system. However their model produced only a relatively thin and wet vapour dominated zone in which the pressure gradient was close to hydrostatic. Pruess and others (1983) set up a detailed three-dimensional model of the vapour dominated zone at Serrazano, Italy and used it to simulate 15 years of production. They did not study the evolution of the natural state and their model was closed to recharge from surrounding low-permeability, cooler, water-saturated rock. Sorey and lngebritsen (1983) and lngebritsen and Sorey (1985) have carried out numerical simulations of a "parasitic" vapour zone above a lateral outflow of hot water. The model was based on data from the Lassen Volcanic National Park. lngebritsen (1986) has carried out a number of numerical simulations of the evolution of 1317

3 O'Sullivan, Barnett and Razali vapour dominated systems. A two-dimensional (vertical) model was used and the effects of either decreasing mass flow or increasing conductive heating at the base of the model were investigated. Both mechanisms produced vapour dominated zones under some conditions. However it is not clear from the reported results whether or not the vapour dominated systems obtained were stable steady states. Pressure (Bar) Pruess and Narasimhan (1982) developed a multiple interacting continuum method (M INC) for numerically simulating two-phase flow of a homogeneous fluid in a fractured porous medium. They used it for investigating the fluid reserves present in a vapour dominated system. Pruess (1985) also used the MlNC method for investigating the evolution of a vapour dominated system from a liquid dominated one as a result of the sudden rupture of the low permeability cap rock. Pruess and others (1987) used a one-dimensional fracture-matrix model to investigate fluid and heat transfer in deep zones of vapour dominated reservoirs. McGuinness and Pruess (1 987) investigated the importance of the boundary conditions in numerical simulations of vapour dominated systems. They showed for a simple one-dimensional model that a vapour dominated system must be pressure controlled at depth. Recently Bodvarsson and others (1989) have carried out simulations of the behaviour of the Geysers using a single layer two-dimensional fracture-matrix model. With their two-dimensional horizontal model it was not possible to carry out any simulations of the natural state of the Geysers. The results obtained show that the model performance is remarkably similar for a range of fracture and matrix permeabilities. THE NATURE OF THE KAMOJANG RESERVOIR The Kamojang geothermal field lies within the Pangkalan Caldera (Healy, 1973). Its surface expressions are located in the eastern part of the caldera and include fumaroles, boiling water and mud pools and areas of steaming ground. Collectively, the surface features are called the Kawahs. Hochstein (1975) estimates a natural heat discharge of about 97 J/s or approximately 35 kg/s of reservoir steam. The pre-exploitation state of the reservoir has been defined by the drilling and testing of some 41 deep wells (KMJ-6 to KMJ-47). KMJ-1 to 5 are shallow wells (maximum depth 150m) drilled in Pressure distributions in the vertical and horizontal planes are shown in Figures 2 and 3. Temperatures within the reservoir are at boiling point for the measured pressures thereby resulting in temperature distributions identical in shape to those for pressure. The observed temperature and pressure distributions in the vertical plane indicate that formations saturated with liquid water overlie a vapour dominated reservoir in which steam is the dominant, continuous phase and temperature and pressure gradients closely correspond to vapour static conditions. The vapour dominated nature of the reservoir is further confirmed by the observation that all deep wells discharge dry steam. The fact that the discharge fluid is not superheated at the wellhead indicates the existence of some mobile liquid water in the reservoir. Similarly, the absence of any observable pressure response radiating from discharging wells supports the contention Figure 2. Vertical reservoir pressure distribution. that the reservoir contains a two-phase mixture of water and steam in which pressure disturbances are effectively damped. While the coexistence of water and steam in the reservoir is clearly demonstrated, the enthalpy or dryness of the in-situ fluids has not been accurately defined. Although a number of authors have addressed this problem (Grant, 1979a, GEMZL, 1981 and Simatupang, 1987) their estimates of 25-35% liquid water by volume are considered rather speculative, Figure 2 shows pressures measured in KMJ-9 and 10 that exceed those in the underlying vapour dominated reservoir and thus indicate the existence of a low permeability 'cap rock" separating the liquid and vapour dominated zones in at least some areas. To date, the reservoir cap is the only clearly identified boundary to the system. It has probably been formed as a result of mineralisation processes sealing the reservoir pores and fractures and is either absent or breached in the region of the Kawahs allowing some vertical leakage of steam to feed the natural discharge features. The side or lateral boundaries have not been defined by drilling but are likely to consist of low permeability flow barriers which isolate the low pressure, vapour dominated reservoir from the surrounding liquid saturated formations. Effective isolation of the reservoir from its surroundings is necessary to inhibit 1318

4 OSullivan, Barnett and Razali of the field and to coincide with the permeability variations observed within the reservoir. In each layer a total of 45 blocks are located within the 10 ohm metre resistivity contour. A further 12 blocks make up the boundary regions extending approximately 2 km in all directions beyond the resistivity boundary. The configuration of blocks in the AAA layer (identical for all layers) is shown in Figure 4. o 500 iooom 1 Figure 3. Reservoir pressure distribution at 625 masl. the inward flow of liquid water into the vapour dominated reservoir. A resistivity boundary (10 ohm meter contour at AB12 = 1 km) has been identified by surface geophysical surveys. This boundary probably represents the extent of the shallow hot water overlying the reservoir and should not be considered as accurately defining the size of the deeper vapour dominated reservoir. (Refer Figure 1). Drilling to a maximum depth of 2000 m has yet to identify the lower reservoir boundary. A correlation of well feed zones suggests that most steam production is obtained from that region between sea level and 1000 metres above sea level (masl). Some variation in permeability in the horizontal plane has been identified by Robert (1988) on the basis of well lithologies and productivities. A band of low permeability running north-north-east to south-southwest from the Kawahs and including Wells KMJ-20, 23, 21, 45 and 32 has been proposed. Similarly, poor productivity observed in KMJ-13, 24, 25, 39 and 47 suggests a region of low permeability to the north. Values of reservoir permeability-thickness product calculated from pressure buildup and falloff tests are in the range of 0.5 to 120 Darcy metres with productive wells displaying values generally greater than 5 Darcy metres. Wells located in the relatively impermeable regions described above demonstrate low values of permeability-thickness (generally less than 3 Darcy metres). MODEL DESCRIPTION The numerical model consists of ten layers (AAA to JJJ) each containing 57 blocks. The shape and orientation of the blocks were chosen to best match the size and shape Figure Model block structure. In the absence of more definitive information on the spatial limits of the vapour dominated reservoir, it was decided to restrict the productive reservoir to that area contained within the resistivity boundary shown in Figure 1. The top of the model is at 1500 mas1 and approximately coincides with the ground surface. All layers above sea level (ie. AAA to FFF) are 250 m thick and those below sea level (ie. GGG to JJJ) are 500 m thick. The model base is at masl. The permeability distribution in the model reflects that observed in the field, the principal features of which include: i) ii) iii) iv) Low permeability rocks (k = 0.02 md) in layer BBB and part of layer CCC form a cap rock to the reservoir. A high permeability zone (k = 150 md) located in parts of the CCC, DDD, EEE and FFF layers corresponds to the highly productive zone, from which most production wells feed. Low permeability rocks (kx = ky = 0.05 md, kz = 0.02 md) completely surround the reservoir. Rocks of medium permeability (k = 15 md) make up the remainder of the reservoir and extend down to masl. 1319

5 OSullivan, Barnett and Razali v) At deeper levels (-1000 to mast) the reservoir rocks are of lower permeability (k = 1.5 md) The boundary conditions applied to the model are: i) w iii) The upper boundary is the atmosphere and is constrained to realistic ambient conditions; namely a pressure of 1 bar abs and a temperature of I 5OC. All lateral boundarjes are impermeab~e and allow no fluid to enter or leave the model through its sides. These boundaries are sufficiently remote from the reservoir that the no-flow ~ndition does not signifi~ntly affect model results. The lower boundary is located at the base of the JJJ layer at mast. In the blocks directly beneath the reservoir, this boundary is set at a constant pressure and a constant immobile water saturation (selected after e~perim~ntation as 60.3 bar and 30% respectively which corresponds to a temperature of 276%). The lower boundary beneath the outer, boundary blocks is impermeab~e allowing no verticat flow into or out of the model. The thickness of the model has been chosen to ensure that the lower boundary condition does not unduly influence the model performance during exploitation predictions. The effect of varying the depth of the bottom boundary has been assessed and is reported in the Discussion section of this paper. The model used here was a uniform porous medium model rather than a fracture-matrix model (see Pruess and Narasimhan (1982), for example). The best means of representing flow in a fractured vapour dominated geothermal reservoir, either by a suitable choice of relative permeability functions or by using a fracturelmatrix model, is the subject of ongoing research. However it appears from numerical experiments conducted by the authors and results reported by Bodvarsson and others (1989) that for modelling long term production the extra complexity of the fracture-matrix model may be unnecessary. NATURAL STATE RESULTS Natural state simulations were carried out by running the model with all blocks initially containing cool water and with the specified boundary conditions and heat and mass withdrawals and injections driving the model. Simulation continued until the reservoir pressures and temperatures reached a steady state. In this case, natural state models were run for 30 time steps which corresponds to approximately 2 x lo7 years. The steady state conditions thus attained are then campared to the preexploitation state of the reservoir. In this manner the validity and accuracy of the model are demonstrated. Pressure Bar) The constant pressure. bottom boundary condition leads to a vertical flow into the reservoir at this depth. The flow rates across this boundary are not restrained but depend on the pressure in the JJJ layer. The thickness of the model was chosen to ensure that inflows through its base during exploitation do not greatly exceed those in the natural state. Similarly, fluid is free to enter or leave the model through the upper, atmospheric boundary. These flows are also unrestrained and depend on the pressures in the AAA layer. In addition to the pressure induced flows acres$ the upper and iower boundaries, the model includes an injection of heat at a rate of 24.7 x lo5 Jls distributed evenly throughout the basal boundary blocks (JJJ 46 to JJJ 57). This corresp nds to a typical, non-geothermal heat flow of 60 mw/m B. The natural surface discharges of steam at the Kawahs are represented by a variable rate, pressure dependent discharge from blocks CCC 22 and CCC 23. This discharge is proportional to the block pressure and therefore declines as the reservoir pressure decreases. In addition to ~rmeabiiities each block has specifled rock parameters as listed below: Rock density = 2500 kglm3 Rock Porosity = 0.1 (for high and medium permeabil~ty rocks) Rock Porosity = 0.01 (for tow permeabi~ity rocks) Heat Conductivity = 2.5 W/m C Specific Heat of Rocks = 1000 J/kg C Residual Immobile Water Saturation = 0.3 Residual Immobile Steam Satur~ion = 0 Perfectly Mobile Steam Saturation = 0.7 Straight line refative permeability curves were used. Figure 5. Pressure distribution in the reservoir and in column 30 of the natural state model. 1320

6 OSullivan, Barnett and Rarali The natural state model results are as follows: In the natural state, all the reservoir blocks contain a mixture of water and steam with a liquid water fraction marginally greater than 30% giving rise to steam static pressure gradients and liquid water that is on the threshold of mobility. The blocks in and above the cap rock and the boundary blocks outside the reservoir contain liquid water only. The permeabjlities in the natural state model were adjusted to give a good match of the pressures and temperatures in the model to the observed reservoir conditions prior to exploitation. Figure 5 is a plot of the natural state vertical pressure distribution in column 30 (near the centre of the reservoir) compared to the measured well feed zone pressures. Excellent agreement between the simufated and measured pressures in the vertical plane are achieved. Figure 6 shows the isobars in layer DDD (ie. 625 mast) of the natural state model. When this distribution is compared with that shown in Figure 3, it can be seen that the model pressures closely match the real field pressures. I' I /- :1] I, for the large heat transfer from depth through the system to the steam vents and conductive losses at the surface. HISTORY MATCHING The best natural state model was then used to simulate the past seven years of exploitation at Kamojang. The initial state of the reservoir used at the beginning of each history matching simulation was the steady state preexploitation conditions obtained from the natural state simulation. Wells were located in the blocks in the model closest to their estimated feed points and production rates were chosen and varied according to the actual production during the period 1 October 1982 to 1 September The pressure declines predicted by the model in the relevant blocks were then compared to those measured over the same period to de~onstrate the accuracy of the model. Two typical comparisons are shown in Figures 7 and 8 for KMJ-17 (block DDD 31) and KMJ-18 (block CCC 31) respectively. These figures demonstrate a reasonably accurate match to the observed field data. When it is considered that the field data has been calculated from a series of pressure buildup tests with an anticipated error of *5OI0 of the reservoir pressure (ie. A 1.5 bar) then the matches obtained by modelling are quite acceptable. +DDD 31 EIKNIJ~?, 4% 10 Ohm-m at A812 2 iooom -4 I Yea r Figure 7. History match for well KMJ-17. I * I o 590 iooom Figure 6. Pressure distribution in the natural state model at 625 mast. The vapour zone in Kamojang acts like a large heatpipe with steam ascending and condensed water falling. ' The model parameters were adjusted to produce a discharge of steam to the fumaroles of 32.2 kgls which is very close to that measured at the Kawahs. This ioss of mass is replenished in the model by 20.7 kg/s of water entering the top of the model, from the atmosphere boundary, representing infiltration of rain water into the shallow groundwater, The remaining 11.5 kg/s of mass enters the base of the model. The low permeability of the rock surrounding the vapour zone was adjusted to give this small upflow at depth (discussed fu~her in a later section). The nett upflow of mass of 11.5 kgts was made up of 54.0 kg/s of steam rising and 42.5 kg/s of water trickling back down through the base of the model. This counterflow of steam and water is the mechanism 2 m W Q 0 L '0 S # v) E n Figure Year History match for well KMJ-18. e KMJ18 -c ccc

7 O'Sullivan, Barnett and Razali DISCUSS ION The accuracy of any prediction of reservoir performance based on numerical simulation depends on our ability to assign rock parameters and model conditions that reflect the real reservoir situation. For some parameters (eg. rock permeability), direct measurement or calculation from well tests can provide adequate precision for the model. Similarly, estimates of many parameters can be iteratively refined during natural state modelling and history matching. During the course of this study it became obvious that with the currently available data, some parameters or features of the model could not be determined by direct observation or testing nor could they be constrained by history matching or natural state modelling. The principal indefinable unknowns were identified and considered in detail as follows: i) Vapour Saturation of Reservoir Fluids The wells at Kamojang discharge dry or very nearly dry steam indicating that the liquid water in the reservoir is immobile. However it is not known at exactly what liquid saturation (volume fraction of water) the water becomes immobile. Grant (1979a) has suggested a method for deducing the in-place liquid saturation from measurements of proportions of different gases in discharging wells but the reliability of the technique in practice has not been fully investigated. Most of the simulations discussed here were carried out with an immobile residual water saturation of 0.3, and straightline relative permeability curves. To test the importance of the immobile water fraction simulations of 30 years of exploitation at 140MW(e) were repeated using initial volumetric liquid water fractions of 0.3 and 0.5 in order to show the dependence of reservoir performance on the value of this parameter. Figures 9 and 10 show the predicted pressure distributions at 625 mas1 after 30 years of production assuming liquid water fractions of 0.3 and 0.5 respectively. Pressures remain substantially higher in the case of high initial reservoir liquid water content. from the principal production zones and thus reduce its impact on long term reservoir performance. Trial simulations were carried out to determine a depth to the bottom boundary which would provide a conservative degree of recharge during exploitation simulations Ohm-m at.58 AB12 = tooom 1 I L 0 SgO iooom Figure 9. Pressure distribution at 625 mas1 after 30 years production at 140 MW - residual immobile water 0.3. As liquid water in the reservoir is about 50 times denser than steam, the mass of fluid available and hence the reservoir's ability to sustain discharge of fluids from storage (as opposed to recharge) is dired!y related?n the proportion of liquid water initially present in the format ion. ii) Model Thickness Numerous attempts at generating a feasible natural state model demonstrated that a constant pressure bottom boundary is necessary in order to be able to maintain vapour dominated conditions in a truly steady state. Attempts at using a constant flow condition at the base of the model, resulted in a non-steady vapour dominated model which periodically reverted to liquid dominated Conditions. This phenomenon has been discussed in more detail by Blakeley, (1986) and McGuinness and Pruess, (1987). While the constant pressure base condition is satisfactory for simulations of the natural state, serious problems arise when exploitation conditions are imposed on the model. As pressures in the reservoir decline in response to fluid extraction, the constant pressure bottom boundary is able to act as an unlimited source of recharge. Provided the pressure response is able to reach this boundary then a significant proportion of the production may be supported by the pressure induced vertical recharge. It was therefore considered desirable to locate the boundary at sufficient depth to isolate it 10 Ohm-m at AB12 =: 1OOOm I 1 ' I 0, 590 iooom Figurelo. Pressure distribution at 625 mas1 after 30 years production at 140 MW - residual immobile water

8 OSullivan, Barnett and Ratali Simulations of 30 years of exploitation at 140MW(e) were carried out with a model containing 10 layers, AAA to JJJ (ie mas1 to masl) and an alternative model containing 8 layers, AAA to HHH (ie masl to masl). The thin 8 layer model displayed a substantial increase in steam recharge through its base during 30 years of exploitation. In this model, the natural state recharge of 48.9 kg/s of steam steadily increased during exploitation to a final rate of kg/s. In contrast, the 10 layer deep model displayed no increase in recharge through its bottom, constant pressure boundary. In fact, a slight decrease (-5%) in vertical recharge was observed after 30 years of exploitation. This phenomenon is probably due to the effect of reinjection and liquid water recharge increasing the water saturation in the deeper layers and thereby slightly inhibiting steam movement. reservoir during future exploitation will be slow and most production will have to come from storage within the reservoir. Pressure (Bar) 1000~ I 1 ' ' 1 iii) Low Permeability Surrounds The interface between the vapour dominated reservoir and the liquid saturated formations outside the field is of interest. Figure 11 shows the natural state pressure profiles in columns 45 and 51 of the model. Column 51 is a liquid filled, boundary column immediately adjacent to Column 45 which is located within the reservoir (refer to Figure 4). Water is descending in the boundary area (Column 51). Permeability in the boundary is so low that hydrostatic pressure gradients are substantially reduced by the downflow. At levels above -600 mas1 steam flows out of the reservoir and into the lower pressure boundary, condensing as it does so. Conversely, below -600 masl, boundary block pressures exceed those in the reservoir and water flows into the vapour dominated zone. Deep recharge of liquid water leads to a slight increase in the liquid water fraction in the deep layers (particularly Layer JJJ) within the reservoir resulting in downward drainage of mobile liquid through the reservoir base. It is of interest to note that at the depths commonly explored by drilling (ie m), a well located outside the boundary would display pressures lower than those observed within the reservoir. For the main vapour dominated zone at Kamojang the permeability was estimated from well test data and then further calibrated in the natural state modelling and history matching processes. Very little information is available on the permeability of the capping rock and low permeability rock surrounding the vapour zone. Obviously the permeability of both these zones must be very low to prevent colder water flooding the low pressure vapour dominated reservoir. As described above the best natural state model has a recharge of cold water from the atmosphere of 20.7 kg/s and a nett influx from depth of 11.5 kg/s. To test the sensitivity of the natural state to the permeability of the cap and the surrounding rock another natural state simulation was carried out with all permeabilities the same except for the cap and surrounding rock whose values were doubled. In this case the recharge from the atmosphere increased from 20.7 kg/s up to 36.5 kg/s and the discharge from the Kawahs declined to 27.4 kg/s. The difference of 9.1 kg/s flowed out the base of the model where there was a counterflow of 58.7 kg/s of steam up and 67.8 kg/s of water down. This nett downflow of mass at the base of the model is physically incorrect and means that the permeability of the cap and surrounding rock is too high in this case allowing too much recharge of cold water. The very low permeability of the cap rock and the surrounding rock means that recharge to the Kamojang Figure 11. Pressure gradients in the reservoir and boundary of the natural state model. SUMMARY AND CONCLUSIONS A three dimensional natural state numerical model has been produced which closely approximates the steady, pre-exploitation state of the Kamojang reservoir. Distributions of temperature, pressure and fluid saturation match those of the conceptual model developed from well measurements, testing and observation. The model consists of a highly permeable, productive zone m thick covering the entire reservoir. This zone is capped or confined by a m thick layer of low permeability rock. The rest of the vapour dominated reservoir consists of moderate permeability rocks. The reservoir is surrounded by low permeability lateral boundaries. Most model parameters have been' assigned on the basis of direct measurement or calculation and then refined to provide the best match to the natural state. Some parameters cannot be satisfactorily measured or calculated either directly.or by the modelling process. These parameters include the vapour saturation of reservoir fluids, model thickness and the permeability of the surrounding low temperature rocks. 1323

9 OSullivan, Barnett and Razati Trial simulations were carried out to demonstrate the sensitivity of the natural state and exploitation models to these parameters and to allow conservative parameter choice for exploitation predictions. The boundary permeability was found to control the vertical movement of water through the reservoir. The ma~ntenance of a realistic mass flow through the reservoir was used to define a maximum boundary permeabi{ity. Exploitation predictions were found to be strongly affected by the residual immobile water saturation as this parameter defines the mass of fluid present in the reservoir prior to explojtatjon. Model thickness or the depth of the model base was also found to have a profound influence on e%ploitation models. A~though the constant pressure nature of this boundary is inherently non-conservative, it was found that increasing the depth of the base, reduced its influence on reservoir performance during exploitation. Btakeley, M.R., 1986, "Geothermal reservoir modelling", PhD Thesis, University of Auckland. Bodvarsson, G.S., Gaulke, S. and Ripperda, M., 1989 'Some considerations on resource evaluation of the Geysers", Geothermal Resources Council Transactions, 13, p Dench, N.D., 1980, "Interpretation of fluid pressure measurements in geothermal wells", Proc Second New Zealand Geothermal Workshop, University of Auckland, Auckland p GENZL, ~Geotherma~ Energy New Zealand Ltd), 1981, "Kamojang geothermal power station stages II and If1 feasibility report for 2 x 55 MW extension", Report for PLN. Grant, M.A., 1979a, "Water content of the Kawah Kamojang geothermal reservoir", Geothermics, 8, p Grant, M.A., 1979b, "Mapping the Kamojang reservoir', GRC 3, p Weafy, J., 1973, "Qeologicai report on Kawah Kamojang geothermal field", GENZL report for NZ Government Colombo Plan Project. Hochstein, M.P., 1975, "Geophysj~f exploratjon of the Kawah Kamojang geothermal field, West Java", Proc. Second UN Symposium on the Development and Use of Geothermal Resources, p Ingebritsen, S.E. and Sorey, M.L., 1985, "A quantitative analysis of the Lassen hydrothermal system, North- Central California", Water Resources Research, 21, p lngebritsen, S.E., 1986, "The evoluttion and natural state of large scale vapor-dominated zones," Proc. Eleventh Wor~shop on Geothermal Reservoir Engineering, Stanford University, Stanford, p dominated hydrothermal systems", Proc. Sixth Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, p Pruess, K. and Narasimhan, T.N., 1982, "On fluid reserves and the produ~ion of superheated steam from fractured, vapor-dominated geothermal reservoirs., Water Resources Research, 87, p Pruess, K., 1983, "Deveiopment of the general purpose simulator MULKOM", Annual Report 1982, Earth Sciences Division, Report LBL-15500, Lawrence Berkeley Laboratory. Pruess, K., Weres, O., and Schroeder, R., 1983, "Distributed parameter modelling of a producing vapor-dominated geothermal reservoir: Serrazano, Italy", Water Resources Research, 19, p Pruess, K., 1985, "A quantitative model of vapordomi~a~ed geothermal reservoirs as heat pipes in fractured porous rock", Geothermal Resources Council Transactions, p Pruess, K., Celati, R, Calore, C., and Cappetti, G., 1987, 'On fluid and heat transfer in deep zones of vapordominated geothermal reservoirs", Proc. Twelfth Workshop on Geothermal Reservoir Engineering, Stanford Un~versity? Stanford, Robert, D , "Subsurface study of the opt~misation of the development of the Kamo'ang geothermat fief&, 8ElClP/Geoserv~~s Report /or Pertamina. Schubert, G. and Straus, J.M., 1979, "Steam-water counterfiow in porous media", Journal of Geophysi~l Research, 84, p Schubert, G. and Straus, J.M., 1980, *Gravitational stability of water over steam in vapor-dominated geothermal systems", Journal of Geophysical Research, 85, p Simatupang, R., 1987, "Saturasi air La pangan Kamojang", Pertamina Report, December Sorey, M.L. and lngebritsen, S.E., 1983, "Numeric~l simulations of the hydrothermal system at Lassen Volcanic National Park", Proc. Ninth Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, p Straus, J.M. and Schubert, G., 1981, "One dimensional model of vapor-dominated geothermal systems", Journal of Geophysical Research, 86, p White, D.E., Muffler, L.J.P. and Truesdell, AM., 1971, "Vapor-dominated hydrothermal systems compared with hot water systems", Economic Geology, 66, p McGuinness, M.J. and Pruess, K., 1987, "Unstable heat pipes", Proc. Ninth New Zealand Geothermal Workshop, Unjversity of Auckland, Auckland, p Pruess, K. and Truesdell, A.H., 1980, "A numerical simulation of the natural evolution of vapor- 1324