The workshop contains presentations in the following areas: (1) reservoir engineering research; (2) field development; (3) vapor-dominated systems; (4) the Geysers thermal area; (5) well test analysis; (6) production engineering; (7) reservoir evaluation; (8) geochemistry and injection; (9) numerical simulation; and (10) reservoir physics. (ACR)

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Meager Creek is perhaps the most intensely explored geothermal system occurring in the Cascade and Garibaldi Volcanic Belts. This paper describes the results of new lithologic, petrographic, X-ray, isotopic, and geochemical investigations of core and cuttings from the Meager Creek wells. The data demonstrate that alteration related to the present geothermal system is superimposed on basement rocks which were metamorphosed and intruded by dioritic stocks prior to the onset of volcanism. The geothermal alteration developed mainly after emplacement of hypabyssal dikes associated with Meager Mountain volcanism and is characterized by mineral assemblages consisting primarily of sheet silicates, quartz, carbonate, hematite, iron oxides, pyrite, and minor epidote, potassium feldspar, actinolite and biotite. Permeabilities within the upper portions of the reservoir are low, reflecting filling of the fracture systems by carbonate. Petrographic observations suggest that sealing of the fractures accompanied hydrothermal brecciation and boiling of the fluids.

Thermal 15 was drilled in November, 1983, to a TD of 700 feet. A steam entry encountered at 490 feet was found to communicate with the high-permeability upflow zone of the Thermal Shallow Reservoir. A low-flow-rate, higher-pressure steam entry at 600 feet was not detected while drilling but was indicated during a subsequent spinner survey. The pressure, flowrate, and enthalpy of the five wells completed in the upflow zone, including the Thermal 4 blowout, were monitored and recorded over a four month period before, during and after Thermal 15 was drilled. It was found that the Thermal 4 blowout communicates with the upflow zone of the Thermal Shallow Reservoir, the Thermal 4 flowrate is controlled by the shallow reservoir pressure, and the high permeability of the upflow zone allows such strong interference effects that three of the four commercial production wells will maximize production from this reservoir. A simple model was developed which describes the pressure-production characteristics of the reservoir over the normal range of operating conditions.

The Hawaii Geothermal Project (HGP-A) well, located in the Kilauea volcano east rift zone, was drilled to a depth of 6450 feet in 1976. It is considered to be one of the hot-test producing geothermal wells in the world. This single well provides 52,800 pounds per hour of 371 F and 160 pounds per square inch-absolute (psia) steam to a 3-megawatt power plant, while the separated brine is discharged in percolating ponds. About 50,000 pounds per hour of 368 F and 155 psia brine is discharged. Geothermal energy development has increased steadily in Hawaii since the completion of HGP-A in 1976: (1) a 3 megawatt power plant at HGP-A was completed and has been operating since 1981; (2) Hawaiian Electric Company (HECO) has requested that their next increment in power production be from geothermal steam; (3) three development consortia are actively, or in the process of, drilling geothermal exploration wells on the Big Island; and (4) engineering work on the development of a 400 megawatt undersea cable for energy transmission is continuing, with exploratory discussions being initiated on other alternatives such as hydrogen. The purpose for establishing the Puna Geothermal Research Facility (PGRF) is multifold. PGRF provides a facility in …

In bringing you up to date on DOE's geothermal R and D programs, they are going to emphasize first those elements that may be of special interest to a reservoir engineering audience, because the activities in support of an improved understanding of hydrothermal reservoirs deserve attention. Reservoir definition, brine injection, and reservoir stimulation technologies are major elements of the Hydrothermal Research Program, and in total they account for nearly 50% of the fiscal year (FY) 1985 hydrothermal research budget. These elements fall into the essential R and D category; that is, while some basic technologies have been borrowed from the petroleum industry for geothermal service, they are often ill-suited to geothermal requirements, and cannot be used without significant technological innovations. Into this category fall the current reservoir technology, brine injection, and reservoir stimulation projects that are listed in Table 1. The reservoir technology projects include: (1) development of methods for characterizing and mapping reservoir parameters, processes, and spatial dimensions; (2) development of methods to predict and monitor reservoir changes from fluid extraction; (3) evaluation of existing methods and development of new methods for predicting the location and mapping faults and fractures in geothermal reservoirs; and (4) testing of new analysis …

Extensive study of the Cerro Prieto geothermal field has provided much geologic and thermodynamic data of its structurally-complex, liquid-dominated reservoir. Several of the studies investigated the resource characteristics of fluid and energy flow. An early report by Mercado (1975) showed that the heat source for the part of the reservoir under development, now called Cerro Prieto I (CPI), originated in the eastern part of the field. Subsequent studies confirmed the flow of hot water from the east. A summary of several experimental and numerical studies of fluid and energy transport in the field was given by Lippmann and Bodvarsson (1983). The hydrogeologic model of Halfman et al. (1982) shows hot-water flow from the east divided into a shallow (alpha) aquifer at about 120Om and a deeper (beta) aquifer at about 170Om depth. A cross section along an east-west direction shows a central upflow to the two aquifers and uncertain geology beyond the western border of the field near well M-9. It also shows a fault dividing the line of border wells at M-29 from the inner wells at M-25 to the east. The hydrogeology of the field was described by Sanchez and de la Pena (1981) as an alluvial unit …

A large body of field data from Larderello shows striking temporal correlations between decline of well flow-rate, produced gas/steam ratio, chloride concentration and produced vapor fraction. The latter is inferred from measured concentrations of non-condensible gases in samples of well fluid, using chemical phase equilibrium principles. Observed temporal changes in the vapor fractions can be interpreted in term of a ''multiple source'' model, as suggested by D'Amore and Truesdell (1979). This provides clues to the dynamics of reservoir depletion, and to the evaluation of well productivity and longevity.

The large Thermal Landslide overlies the initial area of geothermal development at The Geysers. The landslide is waterbearing while the underlying Franciscan formation bedrock units are essentially non-waterbearing except where affected by hydrothermal alteration. Perched ground water moving through the landslide is heated prior to discharge as spring flow.

The boundaries of reservoir fluid convection cells are discreet and intricate zones, commonly sealed or reduced in permeabilities, which are often quite readily identifiable in many hydrothermal systems. Cell boundaries in the Geysers Steam Field are more vague; however, they are gradually being revealed by cumulative and extensive wellbore data. A profound example of a type of boundary has been revealed by drilling in one area of the steam field. A proposed model utilizes a sericitic alteration scheme to establish cell self-sealing. Mineralogical, permeability, and temperature properties all coincide so as to allow formation of a boundary model. This reinforces previously held views that the reservoir cell rock and hydrothermal system are greatly out of equilibrium. Such similar phenomena are suggested from drilling experiences in other parts of the steam field. Considerably, more work is required to better define and comprehend the nature and location of reservoir cell boundaries within the Geysers Steam Field.

The Nesjavellir High Temperature Geothermal Field is located in the Northern part of the Hengill Geothermal Area, which has been estimated to be one of the largest geothermal areas in iceland. Drilling started at Nesjavellir some 20 years ago with five wells. In 1982 a renewed exploration phase began and five additional wells have been drilled during the last three years. The pressure distribution within the geothermal system is very inhomogeneous in both horizontal and vertical directions. Variations in temperature are also considerable. The highest pressure and temperature is found in the southwestern part of the investigated area and both pressure and temperature decreases towards northeast. There seem to be four different zones of pressure potential in the system, which require the existence of both horizontal and vertical barriers in the system. Some parts of the geothermal system are in two-phase condition whereas other parts are in single phase liquid condition. The chemical composition of the fluid seem to be relatively uniform and a common origin of the fluid is assumed. The transmissivity of wells is in the range (1,3-3,5) 10{sup -8} m{sup 3}/Pa {center_dot} s whereas the flowing enthalpy ranges from 1200-2100 kJ/kg. The thermal output of wells are …

One of the major concerns of electric utilities in installing geothermal power plants is not only the longevity of the steam supply, but also the potential for changes in thermodynamic properties of the resource that might reduce the conversion efficiency of the design plant equipment. Production was initiated at Los Azufres geothermal field with wellhead generators not only to obtain electric energy at a relatively early date, but also to acquire needed information about the resource so that plans for large central power plants could be finalized. Commercial electric energy production started at Los Azufres during the summer of 1982 with five 5-MWe wellhead turbine-generator units. The wells associated with these units had undergone extensive testing and have since been essentially in constant production. The Los Azufres geothermal reservoir is a complex structural and thermodynamic system, intersected by at least 4 major parallel faults and producing geothermal fluids from almost all water to all steam. The five wellhead generators are associated with wells of about 30%, 60%, and 100% steam fraction. A study to compile existing data on the chemical and reservoir conditions during the first two years of operation has been completed. Data have been compiled on mean values …

The FRACSL flow and transport code is under development as part of an effort to improve reservoir characterization techniques. The present version simulates a two-dimensional, isothermal reservoir composed of a global fracture network imbedded in a porous media. FRACSL simulates the hydraulic response of a reservoir to injection or backflow. The code simulates the movement of injected tracers within the reservoir by adding advective and random dispersive motions of discrete particles. FRACSL has been benchmarked against theoretical flow and transport responses in simple systems. It has been used to simulate a benchscale physical model and to correlate flow and dispersion data from the East Mesa Hydrothermal Injection Test Program. Correlation of East Mesa data has provided an estimate of an anisotropic hydraulic conductivity, a natural drift in the reservoir, and dispersivity.

An extensive study of the Thermal Shallow Reservoir at The Geysers was performed in 1982 to improve our understanding of the source and flow patterns of steam in the shallow anomaly and how they relate to the Thermal 4 blowout. This project included gathering and analyzing pressure transient, enthalpy, tracer and chemical data and developing a reservoir model that was consistent with this data. Following the pressure transient testing and analysis, a convection-plume with lateral-flow model was proposed. Subsequent analysis of enthalpy, tracer and chemical data corroborated this model. The high flowrate wells--Thermal 4, Thermal 10, Thermal 11 and Magma 1--produce from the high-pressure, high-permeability upflow zone. The source of this upflow is a limited fracture system connecting the shallow anomaly with the underlying main reservoir. The outlying low-pressure, low-permeability wells are supplied by lateral flow of steam from the central area. The pressure gradient from the core to the periphery is caused by condensation in the flanks.

There have been two major high-temperature geothermal field developments in Iceland in the last decade; Krafla in the north-east, and Svartsengi in the south-west. These and other geothermal developments have recently been reported by Palmason et al. The Krafla field will not be discussed here, but details about the field are available in Stefansson and the power plant in Eliasson et al. Several reservoir engineering studies of the Krafla field have been published. The Svartsengi field is one of several fields on the Reykjanes Peninsula in south-west Iceland. About 15 km west of Svartsengi, on the tip of the Peninsula, the Reykjanes field is now under development, primarily for seawater chemicals production. The recently drilled Eldvorp field is located in line between these two fields, about 5 km west of Svartsengi. There are also several fields to the east of Svartsengi, at 15-20 km distance. The Svartsengi, Eldvorp, and Reykjanes fields exist in the same tectonic-volcanic environment, and are surrounded by similar geohydrological conditions, as discussed by Georgsson; see also Gudmundsson et al. and Franzson. Optimum development of these and other fields on the Reykjanes Peninsula, requires an understanding of their depletion behavior with time; that is, how the reservoir …

This paper presents the combined approaches of reservoir geology and engineering to a geothermal field where geological characteristics are highly complex and heterogeneous.Especially,the concrete approaches are discussed for the case of geothermal reservoir performance studies with a developed numerical model, by showing example cases accompanied with reinjection of produced disposal hot water into underground in an object geothermal reservoir. This combined approach will be a great help in solving complicated problems encountered during the development of a geothermal field.

A break in electrical power generation from the Krafla geothermal plant was planned from beginning of May to early September 1984. Early in June most of the production wells were shutin and their pressure recovery monitored. A regular monitoring of the pressure buildup was carried out on a well to well basis until mid-August, when the wells were put back into production except for wells 12 and 16. They were used to monitor the pressure drawdown due to the start of production. This was abruptly brought to an end by a nearby volcanic eruption in early September. The pressure buildup in the two-phase geothermal reservoir at Krafla is described and the first results presented. The results are compared with parameters determined on the completion of the wells and with predictions from numerical simulations of the reservoir. Finally the status of the Krafla geothermal system is discussed with regard to the comparison.

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A published method for calculating excess steam is modified in order to make it applicable to the case of a well producing fluid with a high non-condensable gas content. The method is applied to data from a number of wells from Los Azufres. The results show the expected linear relation between excess steam and the molar fraction of CO{sub 2} in the total discharge. The modified method also provides an estimate for the molar fraction of CO{sub 2} in the liquid phase of the reservoir. It is found that this concentration is relatively homogeneous in the southern section of the field, and seems to be slightly smaller in the deeper zones of the northern section. The calculated values of excess steam agree qualitatively, but not quantitatively, with values of steam quality in the undisturbed reservoir calculated by other authors. The calculation of excess steam tends to overestimate steam quality in cases where steam saturation in the reservoir is low, and has the opposite tendency when the steam quality in the reservoir is high. The first type of deviation is explained in terms of the effects of higher mobility of steam in the two-phase zone induced by pressure drawdown. The second …

Starting from the basic laws of conservation of mass and energy, the differential equations that represent the thermodynamic behavior of a simplified geothermal reservoir are derived. Its application is limited to a reservoir of high permeability as it usually occurs in the central zone of a geothermal field. A very practical method to solve numerically the equations is presented, based on the direct use of the steam tables. The method, based in one general equation, is extended and illustrated with a numerical example to the case of segregated mass extraction, variable influx and heat exchange between rock and fluid. As it is explained, the method can be easily coupled to several influx models already developed somewhere else. The proposed model can become an important tool to solve practical problems, where like in Los Azufres Mexico, the geothermal field can be divided in an inner part where flashing occurs and an exterior field where storage of water plays the main role.

We have refined a method used previously by several authors to estimate water-steam relative permeabilities. Our version provides also reasonable estimates of the corresponding saturations. It relies on production output data obtained by means of short tests. This constitutes a distinct advantage over the previous variants, which require long-term production data, because it allows early accurate assessments of reservoir reserves and producibility. Applying the refined version to data from 5 Cerro Prleto wells we obtained the corresponding relative permeabilities. These results indicate that the relative permeabilities are remarkably homogeneous over a wide area of the field, and over a considerable fraction of the reservoir thickness. They also provide independent evidence that fracture flow is important in Cerro Prieto. Finally, comparison of our results with those found for Wairakei shows that the relative permeabilities of geothermal reservoirs can be sensitively site-dependent.

Turkey is classified among countries which have high geothermal energy potential. Geological and geophysical explorations are continuing in promising areas. However primary emphasis was given to Western Anatolia during the last decade. As a result of these efforts Kizildere field was developed and power plant was put into operation in February 14, 1984. This paper summarizes the power plant, production problems encountered in wells during the last nine months and future field development plans.

We have devised a simple method to assess the natural thermodynamic state of two-phase reservoirs. This is usually a complex task. The method is based on inferring sandface flowing pressures and enthalpies from production output (deliverability) curves, and then extrapolating to shutin conditions in the pressure-enthalpy plane. The method was applied to data from 10 wells of the Los Azufres geothermal field. Comparison of the results with measured pressures and temperatures showed that the method is reliable. We present detailed thermodynamic properties of the unperturbed reservoir fluid in the neighborhood of the wells studied, in tabular form. Moreover, we present a match to these results with a very simple model that allows reasonable estimates of natural thermodynamic conditions as functions of height above sea level. The present results have important implications for the assessment of the fluid reserves, which are suggested to be greater than previously thought.

Downhole logging and workover operations on 12-20 year old wells in several high temperature, liquid-dominated geothermal fields in New Zealand has shown that severe corrosion has commonly occurred in the production casing string where this is unprotected by larger diameter casings. To date corrosion products from only one well have been examined in detail. These indicate that corrosion attack commences at the outer casing wall and continues at a rate as great as 0.8mm/year. Rapid corrosion has been attributed to neutral or slightly acid high bicarbonate waters formed by the absorption of steam and gas into shallow aquifers not directly connected to the deeper, high chloride reservoir.