The objectives of the physical modeling effort are to: (1) evaluate injection-backflow testing for fractured reservoirs under conditions of known reservoir parameters (porosity, fracture width, etc.); (2) study the mechanisms controlling solute transport in fracture systems; and (3) provide data for validation of numerical models that explicitly simulate solute migration in fracture systems. The fracture network is 0.57-m wide, 1.7-m long, and consists of two sets of fractures at right angles to one another with a fracture spacing of 10.2 cm. A series of injection-backflow tests, similar to those performed at the Raft River Geothermal field, was conducted. These included variable volume injection and injection-backflow tests with varying quiescent periods between injection and backflow. This latter series of tests was conducted with a range of flow fields passing through the model. recovery is related to the flow field in the physical model and model parameters. Longer quiescent times and greater flow fields result in a lower tracer recovery. A plot of the fractional tracer recovery against quiescent time results in a straight line. This relationship, combined with classical reservoir engineering data, can be used to predict aquifer flow rate and porosity from known injection volumes and tracer recovery.

The Krafla geothermal field in northeastern Iceland consists of several zones, which contain fluids of different composition and thermodynamic state (Stefansson, 1981). In this paper they examine production data from wells which are completed in two-phase zones. Transient changes in flow rate and flowing enthalpy are analyzed to obtain insight into relative (liquid and gas phase) permeabilities, and other reservoir parameters. Numerous studies have shown that predictions of geothermal reservoir behavior are strongly dependent upon the choice of relative permeability functions. There is an extensive literature on gas-oil and oil-water relative permeabilities, but steam-water relative permeabilities which are needed for geothermal reservoir analysis are poorly known. Laboratory experiments by Chen et al. (1978) and Counsil and Ramey (1979) have provided some data which, however, seem to be at variance with relative permeability characteristics deduced from field data by Grant (1977) and Horne and Ramey (1978). The differences may reflect uncertainties in the analysis methods used, or they may reflect ''real'' differences in relative permeability behavior of fractured reservoirs from that of porous medium-type laboratory cores. Recent theoretical work by Menzies (1982) and Gudmundsson et al. (1983) has substantiated the relative permeability characteristics obtained by Horne and Ramey (1978) for Wairakei …

Five deep wells have been drilled on the Island of Milos, Greece, identifying a high-temperature, high-enthalpy geothermal reservoir. The thermodynamic properties of the fluid, and the estimated porosity and presumed thickness of the formation suggest a fluid and heat storage capacity that could support a 60 MWe power plant for 85 years or a 120 MWe for half that time. The existing five wells can deliver 180 t/h of steam at 10 bar abs pressure, capable of generating a maximum electric power output of slightly less than 20 MWe. This paper describes the geology, the drilling and the well testing results pertaining to the five wells, and discusses the reservoir potential for a 60 MWe geothermal power plant.

General features of the geometry of Los Azufres reservoir have been defined through the mapping of hydrothermal mineral alteration zones. Hydrothermal alteration has been studied in cuttings and drill cores from most of the active wells. X-ray diffraction microprobe analysis and classical optical methods have been employed for the identification of primary and authigenic minerals in fresh and altered samples. Observed patterns of alteration have been correlated with temperature and patterns of fluid circulation. The resulting model depicts a body of geothermal fluid at depth, which ascends and discharges through two main fracture systems. These two circulation zones are characterized by concentric aureoles of increasing hydrothermal alteration towards quasivertical axes. The overall pattern could be described as a dome structure produced by the abnormal thermal gradient, distorted by the effects of active upward circulation of the fluids.

Fracture systems may be generalized in terms of number and orientation of sets of parallel fractures and the distribution of length, width, thickness and separation. Borehole measurements may be used to particularize these parameters for a specific site. Global flow and dispersion in an aquifer occur in the interconnected fractures and may be related to specific fracture elements. A fluid dynamics code named SALE has been used to solve the Navier-Stokes equations for laminar flow in these elemental geometries. A marker particle calculation has been added to characterize longitudinal dispersion due to the velocity profile across the fracture and lateral dispersion due to flow disturbances at junctions. Local flow and dispersion in the matrix occur in the finer fracture structure and are evaluated using porous media approaches. These results or models are integrated in a 2D isothermal reservoir simulator named FRACSL. Discrete fractures are superimposed on the edges or diagnoals of rectangular grid elements. Water may flow from node to node through the matrix or through the fracture. The heads are found by iterating for the distribution which conserves the appropriate local mass. Marker particles are used to monitor the tracer dispersion due to motion in the fractures, in the …

Mechanisms of rock failure in compression--axial splitting and shear failiure-- are studied, based on a microscopic consideration. Analytical models are constructed and model experiments on plates of a brittle polymer are performed. It is shown that unstable growth of tension cracks which propagate from the tips of pre-existing cracks and curve towards the maximum compressive direction, is the fundamental mechanism that produces axial splitting of a uniaxially compressed rock specimen, whereas shear failure of a triaxially compressed specimen is a result of sudden growth of tension cracks at tips of a suitably arranged interacting set of microcracks. The simultaneous out-of-plane unstable growth of a suitably oriented row of cracks is analyzed and, on the basis of this model, the variations of the ''ultimate strength'' and the orientation of the overall fault plane with the confining pressure are estimated. The brittle-ductile transition is discussed with the aid of a model which includes both tension crack extnesion and plastic zone development from the pre-existing cracks.

Pressure analysis for a slug test which corresponds to the flow period of a Drill Stem test is extended to wells in reservoirs with double-porosity behaviour. Solutions are obtained for either pseudo-steady state or transient interporosity flow. The distinctive specific features of both solutions are identified. Results presented are applicable to both naturally-fractured and layered reservoirs with the more permeable layer connecting to the wellbore. Type curves based on the pseudo-steady or transient interporosity flow are presented. These type curves are similar to the existing homogenous single layer type curve with addition of interporosity flow lines indicating double-porosity behaviour.

The Cerro Prieto geothermal field is a liquid-dominated geothermal reservoir of complex geological and hydrological structure. It is located at the southern end of the Salton-Mexicali trough which includes other geothermal anomalies as Heber and East Mesa. Although in 1973, the initial power plant installed capacity was 75 MW of electrical power, this amount increased to 180 MW in 1981 as field development continued. It is expected to have a generating capacity of 620 MW by the end of 1985, when two new plants will be completely in operation. Questions about field deliverability, reservoir life and ultimate recovery related to planned installations are being presently asked. Numerical modeling studies can give very valuable answers to these questions, even at the early stages in the development of a field. An effort to simulate the Cerro Prieto geothermal reservoir has been undergoing for almost two years. A joint project among Comision Federal de Electricidad (CFE), Instituto de Investigaciones Electricas (IIE) and Intercomp of Houstin, Texas, was created to perform reservoir engineering and simulation studies on this field. The final project objective is tosimulate the behavior of the old field region when production from additional wells located in the undeveloped field zones will …

In this paper we develop and demonstrate a method to estimate the reservoir pressure and a productivity index for vertical steam wells, from its production characteristic (also called output) curves. In addition, the method allows to estimate the radius of influence of the well, provided that a value of the reservoir transmisivity is available. The basic structure of the present method is: first, the measured well head mass flowrates and pressures are transformed to downhole conditions by means of a numerical simulator; then, the computed downhole variables are fitted to a simple radial model that predicts the sandface flowrate in terms of the flowing pressure. For demonstration, the method was applied to several steam wells from the Los Azufres Geothermal field. We found excellent agreement of the model with this ample set of field data. As a bonus, the processed data allowed several inferences about the steam producing zone of the reservoir: that the wells considered produce from relatively isolated pockets of steam, which are probably fed by near-by inmobile water; and that these feed zones are in poor hydraulic communication with the field surface waters. our method are that it provides a way to retrieve important reservoir information from …

The 400{degree}F liquid dominated Desert Peak geothermal reservoir produces from fractures associated with intersecting north-northeast and east-northeast trending normal faults. Fractures occur in intrusive basement rocks, pre-Tertiary metasedimentary and metavolcanic rocks, and Tertiary volcanic rocks. Static temperature surveys from six deep wells indicate that the reservoir has both recharge and discharge in the vicinity of wells B21-1 and 86-21. Interference data, from a 30-day flow test of 86-21 show high reservoir connectivity. the calculated transmissivity is an order of magnitude higher in a north-south direction than in an east-west direction. A reservoir thickness on the order of thousands of feet and disturbed reserves in excess of 7 billion barrels are estimated. A conceptual model of the Desert Peak system contains meteric water derived from the Carson and Fernley Sinks. Heated at depth, water rises up along normal faults into highly fractured rocks between the depths of 3000 and 9000 feet, forming a geothermal reservoir. the thermal water naturally rises or leaks out of the reservoir up normal faults to within a few hundred feet of the surface until it has reached hydrostatic equilibrium or is blocked by discontinuous impermeable lacustrine sedimentary rocks. In the latter case it spreads out laterally …

The caldera environment represents a complex interaction of volcanic, structural, and often, hydrothermal processes. As a result calderas are often targets for geothermal exploration and development. From the standpoint of the reservoir engineer, such geothermal systems would be hosted by rocks that display a complex interplay of stratigrphic permeability, structural permeability, and changing permeability which results from the process of hydrothermal alteration and new fracture generation. The purpose of this paper is to present a geolgic model of the Baca geothermal reservoir which is situated in New Mexico. The geologic history of the Valles caldera is presented in Smith and Bailey (1968). The data we present is largely based on our studies of subsurface samples from Union Oil Company's Baca project area. Additional results of our work have been published previously (Hulen and Nielson, 1982, 1983; Nielson and Hulen, in press).

Mofete field, located near Naples, in southern Italy, lies within the large Campi Flegrei caldera. Drilling for geothermal fluids was carried out unsuccessfully in 1939-1954. AGIP, in joint venture with the national utility ENEL, after intensive exploration efforts, resumed drilling at the end of 1978; several new deep wells indicate the presence of a water dominated field in Mofete with three reservoirs (only the shallowest of which was reached by previous wells). The deepest aquifer, tapped by well Mofete 5 at the depth of about 2700 m, contains hypersaline fluids (about 516000 ppm TDS at atmospheric conditions corresponding to about 150000 ppm in the reservoir) with a bottom hole temperature of about 360{degrees}C. The intermediate level, reached by well Mofete 2 at 1900 m depth, is characterized by low salinity fluids (about 38000 ppm TDS at the surface corresponding to 18000 ppm calculated in the reservoir) with a reservoir temperature of 340{degrees}C. The uppermost reservoir, tapped by wells Mofete 1, 3D, 7D, 8D and 9D ranges between 550 and 1500 m depth and has water with salinity ranging from 40000 to 76000 ppm TDS at the surface corresponding to 28000 to 52000 ppm in the reservoir with a bottom temperature …

A tracer test was carried out in a geothermal doublet system to study the injection behavior of a developed reservoir known to be fractured. The doublet produces about 320 gpm of 160 F water that is used for space heating and then injected; the wells are spaced 250 ft apart. Tracer breakthrough was observed in 2 hours and 45 minutes in the production well, indicating fracture flow. However, the tracer concentrations were low and indicated porous media flow; the tracers mixed with a reservoir volume much larger than a fracture.

Injection-backflow tracer testing on a single well is not a commonly used procedure for geothermal reservoir evaluation, and, consequently, there is little published information on the character or interpretation of tracer recovery curves. Two field experiments were conducted to develop chemical tracer procedures for use with injection-backflow testing, one on the fracture-permeability Raft River reservoir and the other on the matrix-permeability East Mesa reservoir. Results from tests conducted with incremental increases in the injection volume at both East Mesa and Raft River suggests that, for both reservoirs, permeability remained uniform with increasing distance from the well bore. Increased mixing during quiescent periods, between injection and backflow, at Raft River suggest an area near the well bore that has a hydrologic character different from the far well bore environment. Increased flow rates for East Mesa testing resulted in a general decrease in mixing. Comparison of recovery curves from the Raft River reservoir with those from the East Mesa reservoir suggests that mixing is greatest, and therefore permeability is greatest, in the fractured reservoir. These test results indicate that injection-backflow testing with tracers can be used successfully to characterize flow in the near-well bore environment.

The current state of development of the Tongonan and Puhagan geothermal fields in the Philippines is presented and the nature of the reservoirs is described. In the latter part of the paper, reservoir engineering techniques which have been found to be particularly valuable are discussed and some aspects which give rise to problems are identified.

Considering various possible chemical reactions between N{sub 2} and other gas species present in the geothermal fluid, the following reaction has been individuated: C + 1/2 N{sub 2} + 7/2 H{sub 2} = NH{sub 3} + CH{sub 4} which is generally satisfied for plausible thermodynamic reservoir conditions (temperature and the relative contributions of steam and liquid to fluid production) at Larderello and Cerro Prieto.

Numerical simulation of radon transport is a useful adjunct in the study of radon as an in situ tracer of hydrodynamic and thermodynamic numerical model has been developed to assist in the interpretation of field experiments. The model simulates transient response of radon concentration in wellhead geofluid as a function of prevailing reservoir conditions. The radon simulation model has been used to simulate radon concentration response during production drawdown and two flowrate transient tests in vapor-dominated systems. Comparison of model simulation with experimental data from field tests provides insight in the analysis of reservoir phenomena such as propagation of boiling fronts, and estimates of reservoir properties of porosity and permeability thickness.

Numerical simulators developed for geothermal reservoir engineering applications generally only consider systems which are saturated with liquid water and/or steam. However, most geothermal fields are in hydraulic communicatino with shallow ground water aquifers having free surface (water level), so that production or injection operations will cause movement of the surface, and of the air in the pore spaces above the water level. In some geothermal fields the water level is located hundreds of meters below the surface (e.g. Olkaria, Kenya; Bjornsson, 1978), so that an extensive so that an extensive unsaturated zone is present. In other the caprock may be very leaky or nonexistent [e.g., Klamath Falls, oregon (Sammel, 1976)]; Cerro Prieto, Mexico; (Grant et al., 1984) in which case ther eis good hydraulic communication between the geothermal reservoir and the shallow unconfined aquifers. Thus, there is a need to explore the effect of shallow free-surface aquifers on reservoir behavior during production or injection operations. In a free-surface aquifer the water table moves depending upon the rate of recharge or discharge. This results in a high overall storativity; typically two orders of magnitude higher than that of compressed liquid systems, but one or two orders of magnitude lower than that …

DOE's geothermal program continues to emphasize a range of reservoir-related programs in reservoir definition, brine injection, stimulation, hot dry rock, geopressured resources and, now, magma resources. These programs are described briefly. Programs in international cooperation between the U.S. and 23 other countries on hydrothermal research have produced important gains in knowledge over the past ten years. Although the activity has diminished, a resurgence is anticipated.

This report is about perceptions of national energy problem

Supporting data are presented for the 1982 low-temperature geothermal resource assessment of the United States. Data are presented for 2072 geothermal sites which are representative of 1168 low-temperature geothermal systems identified in 26 States. The low-temperature geothermal systems consist of 978 isolated hydrothermal-convection systems, 148 delineated-area hydrothermal-convection systems, and 42 delineated-area conduction-dominated systems. The basic data and estimates of reservoir conditions are presented for each geothermal system, and energy estimates are given for the accessible resource base, resource, and beneficial heat for each isolated system.

Over 1982-83 there have been substantial increases in the reservoir engineering contribution to the New Zealand geothermal programme. This has taken four forms: more extensive and detailed analysis of pre- and post-production performance, primarily at Broadlands; the extensive use of high-resolution pressure gauges to carry out interference tests at Rotorua, Ngawha and Broadlands; the installation of an extensive monitoring system at Rotorua; and the commencement of detailed simulation of Broadlands.

The attendance at the Workshop was similar to last year's with 123 registered participants of which 22 represented 8 foreign countries. A record number of technical papers (about 60) were submitted for presentation at the Workshop. The Program Committee, therefore, decided to have several parallel sessions to accommodate most of the papers. This format proved unpopular and will not be repeated. Many of the participants felt that the Workshop lost some of its unique qualities by having parallel sessions. The Workshop has always been held near the middle of December during examination week at Stanford. This timing was reviewed in an open discussion at the Workshop. The Program Committee subsequently decided to move the Workshop to January. The Tenth Workshop will be held on January 22-24, 1985. The theme of the Workshop this year was ''field developments worldwide''. The Program Committee addressed this theme by encouraging participants to submit field development papers, and by inviting several international authorities to give presentations at the Workshop. Field developments in at least twelve countries were reported: China, El Salvador, France, Greece, Iceland, Italy, Japan, Kenya, Mexico, New Zealand, the Philippines, and the United States. There were 58 technical presentations at the Workshop, of …

Mineral deposition or alteration is commonly found at fracture-block interfaces is fissured, geothermal reservoirs. In response to pressure reduction in the fissures such mineralization, if less permeable than the matrix rock, will retard the flow of fluid from the blocks to the fissures and is termed fracture skin in this paper. The problem of fluid flow to a production well in a double-porosity reservoir with fracture skin was analyzed theoretically. One of the findings of the analysis was that fully transient block-to-fissure flow can be approximated by pseudo-steady state flow if fracture skin permeability is sufficiently low. Type curves generated by numerical inversion of Laplace transform solutions are used to cooroborate the results of a finite-difference model of steam transport to a well in a naturally fissured, geothermal reservoir with fracture skin.