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Eleventh workshop on geothermal reservoir engineering: Proceedings

Description: The Eleventh Workshop on Geothermal Reservoir Engineering was held at Stanford University on January 21-23, 1986. The attendance was up compared to previous years, with 144 registered participants. Ten foreign countries were represented: Canada, England, France, Iceland, Indonesia, Italy, Japan, Mexico, New Zealand and Turkey. There were 38 technical presentations at the Workshop which are published as papers in this Proceedings volume. Six technical papers not presented at the Workshop are also published and one presentation is not published. In addition to these 45 technical presentations or papers, the introductory address was given by J. E. Mock from the Department of Energy. The Workshop Banquet speaker was Jim Combs of Geothermal Resources International, Inc. We thank him for his presentation on GEO geothermal developments at The Geysers. The chairmen of the technical sessions made an important contribution to the Workshop. Other than Stanford faculty members they included: M. Gulati, E. Iglesias, A. Moench, S. Prestwich, and K. Pruess. The Workshop was organized by the Stanford Geothermal Program faculty, staff, and students. We would like to thank J.W. Cook, J.R. Hartford, M.C. King, A.E. Osugi, P. Pettit, J. Arroyo, J. Thorne, and T.A. Ramey for their valued help with the meeting arrangements and preparing the Proceedings. We also owe great thanks to our students who arranged and operated the audio-visual equipment. The Eleventh Workshop was supported by the Geothermal Technology Division of the U.S. Department of Energy through Contract DE-AS03-80SF11459. We deeply appreciate this continued support. January 1986 H.J. Ramey, Jr. P. Kruger R.N. Horne W.E. Brigham F.G. Miller J.R. Counsil
Date: January 23, 1986
Creator: Ramey, H.J. Jr.; Kruger, P.; Miller, F.G.; Horne, R.N.; Brigham, W.E. & Counsil, J.R. (Stanford Geothermal Program)
Partner: UNT Libraries Government Documents Department

Injectivity and Productivity Estimation in Multiple Feed Geothermal Wells

Description: A method is presented which allows individual zone injectivity/productivity to be determined without downhole flow measurements. Mass balance in conjunction with the specific pressure change measured at selected points in the well is related to the individual zone injectivity/productivity which can then be used to estimate productive capacity. A sample staged completion test program is presented to obtain the maximum information from a completed well without discharge or use of the downhole flow meter.
Date: January 21, 1986
Creator: Leaver, Jonathan D.
Partner: UNT Libraries Government Documents Department

Power Potential of Geothermal Wells Related to Reservoir Temperature

Description: Abstract: For equal flows of hot water wells, the electric power which can be generated increases with feed water temperature. However, high temperature wells discharge greater flows than that of lower temperature wells of similar permeability, with the result of enhanced power potential. In fact, where fluids are exploited utilizing two-stage flash, these factors combine to give a power potential which is proportional to the cube of the feed water temperature in degrees celsius. Hence a feed of 315 C would generate twice the power of that of water at 250 C for wells of good permeability and where the reservoir exists under conditions of boiling point with depth. Higher temperature water (exceeding 300 C) has, however, a commensurate higher tendency to mineral deposition in reinjection water lines and this disposes design to single-stage flash with slightly reduced power, compared with the two-stage alternative.
Date: January 21, 1986
Creator: James, Russell
Partner: UNT Libraries Government Documents Department

Significant Silica Solubility in Geothermal Steam

Description: Abstract: Although it is widely believed that silica solubility in low pressure (5 to 10 bar) geothermal steam is negligible, when one takes into account steam flows exceeding 10 million tonnes a year--at Wairakei, for instance--it is found that the amount transmitted in the vapor has the potential to give significant deposits on turbine nozzles and blades. A 150 MWe power station, when based on flows from a hot water reservoir at (a) 250 C or (b) 315 C, and with separator pressures of 6 bar, is found to carry about 100 and 200 kg/year respectively in the steam phase. In the case of a similar sized station exploiting a dry steam reservoir such as The Geysers, equivalent silica flows are obtained, dissolved in steam and carried as dust--the latter as solid particles precipitating from the vapor en route from source to turbine, and not preexisting in the formations as is commonly considered. Choking or coating of subterranean rock near such dry steam wells due to exsolving silica, may be the principal cause of declining steam discharge under production. Silica from completely dry or superheated steam can also seal the cap and sides of steam reservoirs when expanding below the criticus temperature (236 C) in a way previously thought possible only by hot water or wet steam.
Date: January 21, 1986
Creator: James, Russell
Partner: UNT Libraries Government Documents Department

Natural State Model of the Nesjavellir Geothermal Field, Iceland

Description: The Nesjavellir geothermal system in southern Iceland is very complex from both a thermal and hydrologic point of view. There are large pressure and temperature gradients in the wellfield and zones with drastically different pressure potentials. Thus, natural fluid flow is substantial in the system and flow patterns are complex. We have developed a two-dimensional natural state model for the Nesjavellir system that matches reasonably well the observed pressure and temperature distributions. The match with field data has allowed determination of the energy recharge to the system and the permeability distribution. Fluids recharge the system at rate of 0.02 kg/s/m with an enthalpy of 1460 kJ/kg. The permeability in the main reservoir is estimated to be in the range of 1.5 to 2.0 md, which agrees well with injection test results from individual wells. Permeabilities in shallower reservoirs are about an order of magnitude higher. Most of the main reservoir is under twephase conditions, as are shallow aquifers in the southern part of the field. The model results also suggest that the low temperatures in the shallow part of the northern region of the field may be due to the young age of the system; i.e., the system is gradually heating up. If this is the case the estimated age of the system near the wellfield is on the order of a few thousand years.
Date: January 21, 1986
Creator: Bodvarsson, G. S.; Pruess, K.; Stefansson, V.; Steingrimsson, B.; Bjornsson, S.; Gunnarsson, A. et al.
Partner: UNT Libraries Government Documents Department

The Streaming Potential Generated by Flow of Wet Steam in Capillary Tubes

Description: For a constant pressure differential, the flow of wet steam generated electric potentials which increased with time and did not reach equilibrium values. These potentials were found to increase to values greater than 100 volts. The reason for this kind of potential build-up behavior was the presence of tiny flowing water slugs which were interspersed with electrically nonconductive steam vapor slugs. The measured electric potential for wet steam increased with pressure differential, but the relationship was not linear. The increase in potential with pressure drop was attributed both to an increase in fluid flow rate and changes in the wet steam quality.
Date: January 21, 1986
Creator: Marsden, S.S. Jr. & Tyran, Craig K.
Partner: UNT Libraries Government Documents Department

Pressure Testing of a High Temperature Naturally Fractured Reservoir

Description: Los Alamos National Laboratory has conducted a number of pumping and flow-through tests at the Hot Dry rock (HDR) test site at Fenton Hill, New Mexico. These tests consisted of injecting fresh water at controlled rates up to 12 BPM (32 {ell}/s) and surface pressures up to 7,000 psi (48 MPa) into the HDR formation at depths from 10,000-13,180 feet (3050-4000 m). The formation is a naturally fractured granite at temperatures of about 250 C. The matrix porosity is < 1%and permeability is on the order of 1 nD (10 m{sup 2}). Hence most of the injected fluid is believed to move through fractures. there has been no evidence of fracture breakdown phenomena, and hence it is believed that pre-existing joints in the formation are opened by fluid injection. Water losses during pumping are significant, most likely resulting from flow into secondary fractures intersecting the main fluid conducting paths. The pressure-time response observed in these tests can be interpreted in terms of non-isothermal, fracture-dominated flow. As the fluid pressure increases from small values to those comparable to fracturing pressures, the formation response changes from linear fracture flow to the highly nonlinear situation where fracture lift off occurs. A numerical heat and mass flow model was used to match the observed pressure response. Good matches were obtained for pressure build up and shut-in data by assigning pressure dependent fracture and leak-off permeabilities.
Date: January 21, 1986
Creator: Kelkar, Sharad; Zyvoloski, George & Dash, Zora
Partner: UNT Libraries Government Documents Department

Fracture Detection: Interpretation of Well Logs to Select Packer Seats and Locate Injection Intervals

Description: A wireline and mud logging program has been conducted in conjunction with redrilling operations in well EE-3 at the Fenton Hill Hot Dry Rock (HDR) site near Valles Caldera, New Mexico. The trajectory for the new bore, EE-3A, penetrated a fractured zone stimulated from adjacent well EE-2 and thereby established hydraulic communication. To test and stimulate selected zones in EE-3A inflatable open hole packers designed for high temperature service were used. Proper identification and selection of packer seats was crucial to the success of the project. The logging program successfully identified five competent packer seats in six attempts. Wireline temperature, caliper, sonic televiewer and natural gamma ray logs were used in conjunction with mud logs, drill cuttings and drilling parameter data to locate fractures, out-of-gage hole, temperature anomalies and mineralized zones which were avoided in selection of the packer seats.
Date: January 21, 1986
Creator: Dreesen, D. S.; Burns, K. L.; Chavez, P.; Dash, Z. V.; Kelkar, S.; Kolar, J. et al.
Partner: UNT Libraries Government Documents Department

Representative Element Modeling of Fracture Systems Based on Stochastic Analysis

Description: An important task associated with reservoir simulation is the development of a technique to model a large number of fractures with a single description. Representative elements must be developed before reservoir scale simulations can adequately address the effects of intersecting fracture systems on fluid migration. An effective element model will sharply reduce the cost and complexity of large scale simulations to bring these to manageable levels. Stochastic analysis is a powerful tool which can determine the hydraulic and transport characteristics of intersecting sets of statistically defined fractures. Hydraulic and transport characteristics are required to develop representative elements. Given an assumption of fully developed laminar flow, the net fracture conductivities and hence flow velocities can be determined from descriptive statistics of fracture spacing, orientation, aperture, and extent. The distribution of physical characteristics about their means leads to a distribution of the associated conductivities. The variance of hydraulic conductivity induces dispersion into the transport process. The simplest of fracture systems, a single set of parallel fractures, is treated to demonstrate the usefulness of stochastic analysis. Explicit equations for conductivity of an element are developed and the dispersion characteristics are shown. The analysis reveals the dependence of the representative element properties on the various parameters used to describe the fracture system.
Date: January 21, 1986
Creator: Clemo, T.M.
Partner: UNT Libraries Government Documents Department

Fracture Detection and Mapping

Description: Because the costs of drilling, completing, and testing a well can be extremely high, it is important to develop better tools and methods for locating high permeability zones prior to drilling, and to develop better tools and methods for identifying and characterizing major fracture zones during the drilling and well testing stages. At the recommendation of the LBL Industry Review Panel on Geothermal Reservoir Technology, we organized and convened a one-day workshop this past July to discuss various aspects of DOE's current and planned activities in fracture detection, to review the geothermal industry's near-term and long-term research needs, to determine the priority of those needs, to disseminate to industry the status of research in progress, and to discuss the possibility of future joint research between industry and DOE. In this paper we present a brief overview of the workshop from the perspective of those who participated in it and provided us with written comments to a questionnaire that was distributed.
Date: January 21, 1986
Creator: Goldstein, Norman E. & Iovenitti, Joseph L.
Partner: UNT Libraries Government Documents Department

Permeability Enhancement Using High Energy Gas Fracturing

Description: This paper reports the results of a preliminary study of using High Energy Gas Fracturing (HEGF) techniques for geothermal well stimulation. Experiments conducted in the G-tunnel complex at the Nevada Test Site (NTS) showed that multiple fractures could be created in water-filled boreholes using HEGF. Therefore, the method is potentially useful for geothermal well stimulation.
Date: January 21, 1986
Creator: Chu, T.Y.; Cuderman, J.F.; Jung, J. & Jacobson, R.D.
Partner: UNT Libraries Government Documents Department

The Influence of Groundwater Flow on Thermal Regimes in Mountainous Terrain

Description: Active circulation of cool groundwater in mountainous terrain can cause an advective disturbance of the thermal regime. This factor complicates interpretation of data collected in geothermal exploration programs. An isothermal free-surface model has been developed which provides qualitative insight into the nature of an advective disturbance as it is affected by topography, permeability and climate. A fully coupled model of fluid and heat transfer is being developed for quantitative study of idealized mountain hydrothermal systems.
Date: January 21, 1986
Creator: Forster, Craig & Smith, Leslie
Partner: UNT Libraries Government Documents Department

Hot Spring Monitoring at Lassen Volcanic National Park, California 1983-1985

Description: Data collected on several occasions between 1983 and 1985 as part of a hydrologic monitoring program by the U.S. Geological Survey permit preliminary estimation of the natural variability in the discharge characteristics of hydrothermal features in Lassen Volcanic National Park and the Lassen KGRA in northern California. The total rate of discharge of high-chloride hot springs along Mill Creek and Canyon Creek in the Lassen KGRA has averaged 20.9 {+-} 1.7 L/s, based on seven measurements of the flux of chloride in these streams. Measured chloride flux does not appear to increase with streamflow during the spring-summer snowmelt period, as observed at Yellowstone and Long Valley Caldera. The corresponding fluxes of arsenic in Mill Creek and Canyon Creek decrease within distances of about 2 km downstream from the hot springs by approximately 30%, most likely due to chemical absorption on streambed sediments. Within Lassen Volcanic National Park, measurements of sulfate flux in streams draining steam-heated thermal features at Sulphur Works and Bumpass Hell have averaged 7.5 {+-} 1.0 and 4.0 {+-} 1.5 g/s, respectively. Calculated rates of steam upflow containing, dissolved H{sub 2}S to supply these sulfate fluxes are 1.8 kg/s at Sulphur Works and 1.0 kg/s at Bumpass Hell.
Date: January 21, 1986
Creator: Sorey, Michael L.
Partner: UNT Libraries Government Documents Department

Evaluation of Porous Medium Permeability by Acoustic Logging Finds Geothermal Applications

Description: In a well, after an acoustic waveform has circulated through the surrounding porous media, the study of its alteration can help in evaluating their permeability. The treatment of the acoustic compressional wave's first three cycles yields a unique parameter called I-c. The recording of this I-c log all along any open hole interval is now possible by respecting some practical rules known by logging companies. Large flows of fluid found in geothermal low-enthalpy operations have provided an opportunity to check the validity of this method. Cumulative I-c derived permeability with depth (''EXAFLO'' log) correlates with the flowmeter log, as examples will show. Some new aspects of the theory underlying the I-c/permeability relationship have been developed and are described here.
Date: January 21, 1986
Creator: Conche, B.; Lebreton, F. & Rojas, J.
Partner: UNT Libraries Government Documents Department

Noi'i o Puna: Geothermal Research in Hawaii

Description: Noi'i 0 Puna - The Puna Research Center (PRC), located on the grounds of the HGP-A power plant site in Puna, Hawaii, was dedicated on August 24, 1985. Research projects, supported by the U.S. Department of Energy (USDOE), State, County, utility, and the private sector have been initiated in the areas of geothermal reservoir engineering, silica utilization, and corrosion of materials. An international geothermal applications workshop was held in Hilo, Hawaii the day before the dedication to discuss common problems and methods of solution by cooperative research. The three main categories addressed were process chemistry design, reservoir engineering, and agriculture/aquaculture applications. The workshop identified how PRC might be used for these research purposes. The advantages provided by PRC include the availability of non-proprietary information, an operational power plant with adjacent laboratory, proximity of private wells, the Fellows in Renewable Energy Engineering program, and strong support from the State, County, and utility. A second workshop is in the planning stages to follow through on the recommendations and will be held in the Orient next year. The Community Geothermal Technology Program, featuring projects conducted by individuals and companies in the local community, has been funded and will actively initiate projects this month. This program received matching funds from the USDOE, County of Hawaii and the private sector.
Date: January 21, 1986
Creator: Seki, Arthur; Chen, Bill; Takahashi, Patrick & Woodruff, Jim
Partner: UNT Libraries Government Documents Department

Dual Permeability Modeling of Flow in a Fractured Geothermal Reservoir

Description: A three dimensional fracture system synthesis and flow simulation has been developed to correlate drawdown characteristics measured in a geothermal well and to provide the basis for an analysis of tracer tests. A new dual permeability approach was developed which incorporates simulations at two levels to better represent a discrete fracture system within computer limitations. The first incorporates a discrete simulation of the largest fractures in the system plus distributed or representative element simulation of the smaller fractures. the second determines the representative element properties by discrete simulation of the smaller fractures. The fracture system was synthesized from acoustic televiewer data on the orientation and separation of three distinct fracture sets, together with additional data from the literature. Lognormal and exponential distributions of fracture spacing and radius were studied with the exponential distribution providing more reasonable results. Hydraulic apertures were estimated as a function of distance from the model boundary to a constant head boundary. Mean values of 6.7, 101 and 46 {micro}m were chosen as the most representative values for the three fracture sets. Recommendations are given for the additional fracture characterization needed to reduce the uncertainties in the model.
Date: January 21, 1986
Creator: Miller, John D. & Allman, David W.
Partner: UNT Libraries Government Documents Department

Preliminary Studies of Two-Phase Effects on Pressure Transient Data

Description: Presently, there are few methods available for analyzing pressure transient data from two-phase reservoirs. Methods published in the oil and gas literature (Earlougher, 1977) have been adapted for analyzing data from geothermal reservoirs, assuming a uniform initial steam saturation. However, it is well known that two-phase conditions often prevail only in parts of the reservoir, primarily in the top portion, and that vapor saturations are not uniform. Thus, there is a need to examine the pressure behavior during well tests considering more realistic conditions. Two-phase effects are important in pressure transient analysis because the mobility of two-phase mixtures can differ significantly from that of single-phase fluids. Also, the compressibility of two-phase mixtures is orders of magnitude higher than for single-phase liquid and vapor (Grant and Sorey, 1979). In this paper we perform scoping calculations on the effects of two-phase zones on well pressure transients. Three different cases are considered (Figure 1). The first is that of a fully two-phase system (e.g. Krafla, Iceland; Stefansson, 1981). This problem has been studied by various authors, including Moench and Atkinson (1977, 1978), Grant (1978), Garg (1978, 1980), Grant and Sorey (1979), and Aydelotte (1980). Some of the complexities of this type of system are discussed. The second problem is that of a single-phase liquid reservoir with a localized two-phase zone. Possible field examples include Cerro Prieto, Mexico and Baca, New Mexico, USA. This problem was studied by Sageev and Horne (1983a,b) and Sageev (1985); they used a constant pressure approximation for the two-phase zone. In this paper we investigate the pressure transients in a well located near an isolated two-phase zone in a single-phase liquid reservoir, and compare them to type curves based upon the constant pressure approximation. The third problem considered is that of a two-phase layer overlying a single-phase liquid layer. ...
Date: January 21, 1986
Creator: Cox, B. Lee & Bodvarsson, Gudmundur S.
Partner: UNT Libraries Government Documents Department

Modeling Studies of Cold Water Injection into Fluid-Depleted, Vapor-Dominated Geothermal Reservoirs

Description: Reinjection experiments in the strongly fluid-depleted reservoir of Larderello have revealed the possibility of increasing production rates and overall heat extraction by injection into high permeability, low pressure zones of the reservoir (Giovannoni et al., 1981; Cappetti et al., 1983; Bertrami et al., 1985). A large fraction (over 80%) of the injected water was recovered as steam in the most favorable area and, despite the short distance between injection and producing wells (the minimum distance being about 150 m), no significant temperature change has been observed in the latter, after 3 years of injection at a rate ranging from 10 to 50 kg/s (Bertrami et al., 1985). The physical processes involved in cold water injection into a ''superheated'' fractured reservoir are not yet fully understood, and this insufficient knowledge of the fundamental mechanisms limits the possibility of forecasting future reservoir behavior and optimizing the heat extraction process. Numerical simulation can be a very effective tool in the study of the complex phenomena involved, allowing a rapid examination of different situations and conditions, a systematic investigation of the effects of various parameters on reservoir performance, and some insight into long term behavior. We have performed simulation experiments on simple one-dimensional, porous and fractured reservoir models in order to study the migration of injected water, thermodynamic conditions in the boiling zone, heat extraction, and vapor generation. A two-dimensional radial porous medium model, with some characteristics typical of the high productivity zones of Larderello, has, also been applied for studying the- evolution. of the shape and the thermodynamic conditions of the injection plume in the presence of gravity, reservoir heterogeneities and anisotropy.
Date: January 21, 1986
Creator: Calore, C.; Pruess, K. & Celati, R.
Partner: UNT Libraries Government Documents Department

Inert and Reacting Tracers for Reservoir Sizing in Fractured, Hot Dry Rock Systems

Description: Flow characterization and volumetric sizing techniques using tracers in fractured hot dry rock reservoirs are discussed. Statistical methods for analyzing the residence time distribution (RTD) are presented. Tracer modal volumes and RTD shape are correlated with reservoir performance parameters such as active heat transfer area and dispersion levels. Chemically reactive tracers are proposed for mapping advance rates of cooled regions in HDR reservoirs, providing early warning of thermal drawdown. Important reaction rate parameters are identified for screening potential tracers. Current laboratory research and field work is reviewed.
Date: January 21, 1986
Creator: Tester, J.W.; Robinson, B.A. & Ferguson, J.H.
Partner: UNT Libraries Government Documents Department

Initial Measurements of Petrophysical Properties on Rocks from the Los Azufres, Mexico, Geothermal Field

Description: Petrophysical properties of geothermal reservoir rocks are valuable information for many activities, including reservoir characterization, modeling, field test analysis and planning of exploitation techniques. Petrophysical data of rocks from geothermal reservoirs located in volcanic areas is in general very scarce. In particular, no petrophysical data of rocks from the Los Azufres geothermal field area has ever been published. This work presents the results of initial petrophysical studies on outcrop rocks and drill core samples from the Los Azufres geothermal field. These studies are the first part of an ongoing experimental program intended to establish a data-base about physical properties of the Los Azufres rocks, in support of the many reservoir engineering activities which require of such information. The experimental work carried out consisted of laboratory measurements of density, porosity, permeability, compressibility, thermal conductivity, thermal expansion, electrical resistivity and sonic wave velocities. Some of the experiments were aimed at investigation of the effects of temperature, pressure, saturation and other parameters on the physical properties of rocks.
Date: January 21, 1986
Creator: Contreras, E.; Iglesias, E. & Razo, E.
Partner: UNT Libraries Government Documents Department

Breakthrough Time for the Source-Sink Well Doublet

Description: A pressure transient analysis method is presented for interpreting breakthrough time between two constant rate wells. The wells are modeled as two line source wells in an infinite reservoir. The first well injects at a constant rate and the second well produces at a constant rate. We studied the effects of transient pressure conditions on breakthrough time. The first arrival of injected fluid at the production well may be significantly longer under transient condition than under steady state condition. A correlation of the deviation of the breakthrough time for transient pressure conditions from the steady state condition is presented.
Date: January 21, 1986
Creator: Menninger, Will & Sageev, Abraham
Partner: UNT Libraries Government Documents Department

The Evolution and Natural State of Large-Scale Vapor-Dominated Zones

Description: Numerical simulation is used to define the rather special conditions under which large-scale vapor-dominated zones can evolve. Given an adequate supply of heat, a vapor-dominated zone can evolve within low-permeability barriers without changes in rock properties or boundary conditions. However, the evolution of the system is accelerated in cases involving an initially high fluid throughflow rate that decreases with time. Near-steady-state pressures within the vapor-dominated zone are shown to vary with depth to the caprock.
Date: January 21, 1986
Creator: Ingebritsen, S.E.
Partner: UNT Libraries Government Documents Department

The Response to Exploitation of Rotorua Geothermal Field

Description: The concern about possible decline in Whakarewarewa spring activity led to the creation of the rotorua Monitoring Program and the rotorua Task Force. Both will shortly issue their final reports (Ministry of Energy 1985a,b). Under the Monitoring Program a considerable database was collected about aquifer, well and spring behavior. From this it has been concluded that exploitation has had the following effects: (1) geothermal aquifer pressures have fallen by up to 0.5 bar; geothermal aquifer pressures beneath Whakarewarewa have fallen by about 0.2 bar; and (3) geothermal flow from Whakarewarewa has been reduced by about half. The effects on the springs could be reduced by reducing withdrawal particularly near Whaka, reinjection (including the use of downhole heat exchangers), or a combination of these.
Date: January 21, 1986
Creator: Grant, Malcolm A.
Partner: UNT Libraries Government Documents Department

Natural Vertical Flow in the Los Azufres, Mexico, Geothermal Reservoir

Description: This work focuses on estimating the mass (M) and energy (E) flow rates, the permeability k, and the relative permeability functions R{sub L} and R{sub V} associated with the natural vertical flow in the reservoir. To estimate M and E we used the standard 1-D vertical equations for two-phase flow, complemented with boundary conditions at the boiling and dew interfaces. These boundary conditions were derived in an earlier stage of this study that established an approximate 1-D vertical model of the reservoir. The estimated values of M and E were then used together with the previously established liquid saturation vertical profile of the reservoir, and the differential equation expressing the pressure gradient, to fit, by trial and error, the observed natural pressure profile. The accuracy of the fit depends on the assumed value for the vertical permeability and on the chosen forms for the relative permeability functions. They estimated M {approx} 6.9 x 10{sup -8} kg m{sup -2} s{sup -1} and E {approx} 0.2 W m{sup -2}. These results lie well within the ample ranges of mass and energy flowrates per unit area found in geothermal fields worldwide. The estimated values of M and E support the previous inference that there is an extensive caprock in the reservoir. The best fit to the natural pressure gradient implies a vertical permeability of about 0.08 mD, residual water- and steam-saturations of about 0.04 and 0.00 respectively, and ''fracture relative permeabilities'' (i.e., R{sub L} + R{sub V} = 1). This work addresses a major obstacle for a successful analysis of the Los Azufres geothermal reservoir, which is characterized by an extensive two-phase region: the former unavailability of reasonably reliable relative permeability functions. Furthermore, the present characterization of the vertical natural flow provides important constraints for both lumped- and distributed-parameter models of the reservoir. ...
Date: January 21, 1986
Creator: Iglesias, E.R.; Arellano, V.M. & Ortiz-Ramirez, J.
Partner: UNT Libraries Government Documents Department