A series of experiments was conducted on crushed tuff at 90{sup 0}C and 150{sup 0}C and on core wafer samples at 150{sup 0}C. The results show the following: increasing the ratio of rock to water increases the rate of approach to steady-state concentrations in solution. Surface outcrop samples of Bullfrog tuff contain a minor component of highly soluble material believed to be a residue from the evaporation of surface runoff water in the pores of the rock. This material can be removed by shaking the crushed rock with water at room temperature and subjecting it briefly to heat with fresh water. Solution analyses for unfiltered samples that have reacted for short periods show higher concentrations of Al and Fe than do analyses for filtered samples; results for other elements are independent of filtration. This difference probably exists because of particulate matter in the solutions that dissolves when the samples are acidified prior to analysis. Agitation of samples during reaction produces sub-0.1 {mu} particles in the solutions. These particles dissolve when samples are acidified, resulting in abnormally high concentration values for some elements, such as Al and Fe. Comparison of the results for crushed rock with those for core wafers shows …

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 …

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In the development of geothermal sources for power generation, production of geothermal fluids as well as reinjection becomes an important aspect for significant heat extraction from the reservoir rock. The purpose of this work was to understand how cold water injection in five spot pattern affected the temperature distributions and production pressures in a physical model with a constant temperature heat source. The production and injection rates were varied as well as their respective depths. The model is a hot water dominated system with crushed limestone of 0.6-0.9 cm particle size as the reservoir rock, which had 40% porosity, 58 darcy permeability. The analysis revealed that injection rate should be at least 2/3 of production rate (measured as condensed water) so that the pressure decline at the producing end was stopped. Heat extraction from the system was high when injection was done towards the top of the model while production horizon was deeper.

For a description and analysis of the flow they consider the conservation equations of the two phases separately, but in thermal and mechanical equilibrium, coupled by the itnerface shear forces (two fluid model, drift flux model). Coupling may be weak or strong, depending on Froude and Mach numbers of the flow. The fluid is highly compressible, not because the individual phases move at such speeds that their individual density changes are significant but because evapiration (phase change) results in large density changes of the system at moderate pressure or temperature changes once flashing occurs. The slip between the phases is caused by unequal wall shear stress, acceleration of the fluid or gravitational forces and is hindered by the interface interaction. if they denote by {gamma} the ratio of the liquid density to the vapor density and by {sigma} the ratio of the vapor speed to the liquid speed they find that in horizontal flows {sigma} = {gamma}{sup 1/2} yields the maximum slip (neglecting acceleration effects) that can be reached with no interface force acting (assuming equal friction coefficients for both phases at the wall). If one investigates the conditions of thermodynamic flow similarity between different substances in two phase flow, …

The concept of relative permeability is the key concept in extending Darcy's law for single phase flow through porous media to the two-phase flow regime. Relative permeability functions are needed for simulation studies of two-phase geothermal reservoirs. These are poorly known inspite of considerable theoretical and experimental investigations during the last decade. Since no conclusive results exist, many investigators use ad hoc parametrization, or adopt results obtined from flow of oil and gas (Corey, 1954). It has been shown by Reda and Eaton (1980) that this can lead to serious deficiencies. Sensitivity of the relative permeability curves for prediction of mass flow rate and flowing enthalpy into geothermal wells has been studied by many investigators (e.g. Eaton and Reda (1980), Bodvarsson et al (1980), Sun and Ershagi (1979) etc.). It can be concluded from these studies that the beehavior of a two-phase steam/water reservoir depends greatly on the relative permeability curves used. Hence, there exists a need for obtaining reliable relative permeability functions.

Data from a seven week pressure interference test in the Klamath Falls, Oregon geothermal resource have been analyzed. The data indicate that productive wells are fed by a highly permeable fracture network and that the less permeable matrix blocks contribute significantly to the reservoir storage capacity. Detailed analysis of data from two wells is presented. Data from both of the wells yield a reservoir permeability-thickness (kh) of approximately 1.3x10{sup 6} md-ft and a storativity of 6.8x10{sup -3} ft/psi. The parameters ({lamda} and {omega}), which are determined by the distribution of permeability and storativity between the matrix and fractures, vary by more than an order of magnitude. A sensitivity study shows that for these wells, the pressure transients are not very sensitive to the distribution of permeability and storativity between the fractures and matrix blocks. No hydrologic boundaries were detected during the test. This indicates that the fault which supplies hot water to the shallow hydrothermal system does not behave according to the cassical model of either a barrier or constant potential boundary.

Large volume hydraulic injections into the Carnmenellis granite have been completed at the CSM Hot Dry Rock Project during the period October 1982 to July 1983, with small volume injections before and after this period. The effects of the injections on the hydraulic properties of the rock mass have been estimated by pressure transient analysis. The growth of the reservoir zone was tracked with microseismic locations, and the growth mechanism modeled with the computer program FRIP. The limited duration of the transients amenable to analysis and the tendency for growth below the injection zone meant that the interpretations could only describe hydraulic conditions within about 100 m of the wellbores. The effect of the large volume injections was to increase permeability values from less than 100 {micro}d to greater than 5 md, and to decrease skin values from about -3 to about -6. The FRIP modeling explained the observed reservoir growth in plan with reference to measured in-situ stresses, jointing and rock properties and showed some of the limitations of continuum modeling.

Five new gas geothermometers are introduced. They are useful for predicting subsurface temperatures in water dominated geothermal systems. The geothermometers use data on CO{sub 2}, H{sub 2}S and H{sub 2} concentrations in fumarole steam as well as CO{sub 2}/H{sub 2} and H{sub 2}S/H{sub 2} ratios. It is demonstrated that the gas composition of fumarole steam may be used with or withour drillhole data to evaluate steam condensation in the upflow zones of geothermal systems. Uncertainty exists, however, in distinguishing between the effects of steam condensation and phase separation at elevated pressures. The gas content in steam from discharging wells and the solute content of the water phase can be used to evaluate which boiling processes lead to "excess steam" in the discharge and at which temperature this "excess steam" is added to the fluid moving through the aquifer and into the well. Examples, using field data, are given to demonstrate all the mentioned applications of geothermal chemistry.

In geothermal wells injection tests are commonly used to obtain well and reservoir data. These tests are typically conducted in a series of step rates followed or preceded by a complete shutin. Usually the temperature of the injected fluid is different from that of the reservoir fluid. Because of the strong temperature dependence of fluid viscosity and to a lesser extent, fluid density, nonisothermally related pressure responses must be considered. The nonisothermal isjectivity index obtained from these tests depends on the mobility ratio of the cold region to the hot reservoir and the extent of the cold spot. This paper proposes a method which accounts for these effects and relates the nonisothermal injectivity index to the isothermal injectivity index.

The development of reliable spinner tools may help avoid much of the ambiquity which often accompanies well tests in geothermal wells, due to interlayer flows through the well bore. However, the use of both pressure and flow rate changes requires new methods of well test interpretation. The Stanford Geothermal Program has been developing microcomputer-based techniques for the simultaneous analysis of pressure and flow rate measurements. There are two key steps in the procedure. Firstly, the non-linear regression is achieved by calculating the gradients of the response (with respect to the unknown reservoir parameters) in Laplace space, and inverting numerically. Secondly, the variable flow rate is represented in terms of a superposition of many step changes - this was found to work better than a spline fit to the data. One problem was encountered when attempting to analyze data in which the spinner "stalled", causing a jump to zero flow rate. The method shows great promise in that the degrees of freedom on the interpretation are greatly reduced, the well bore storage effect disappears, and inter-feed flows do not affect the results.

The present situation of the geothermal field developments in Japan is such that eight geothermal power stations are being operated, while there are sill many geothermal areas to be explored. Up to this day, the target of geothermal exploration has mainly been the areas by surface geological survey and the existing geothermal reservoirs are located not deeper than 1,500m depth. Recent geothermal energy development shows a trend from the study on vapor dominated of liquid dominated hydrothermal resources in shallow zones to that on hydrothermal resources in deeper zones. Exploration wells of 3,000m depth class have been drilled in Japan.

Achievements and problem areas are reviewed with respect to various engineering implications of geothermal field development in the European Community (EC). Current and furture development goals address three resource settings. (a) low enthalpy sources (30-150{degrees}C), an outlook common to all Member states as a result of hot water aquifers flowing in large sedimentary units with normal heat flow, widespread thoughout the EC; (b) high enthalpy sources (<150{degrees}C) in areas of high heat flow which, as a consequence of the geodynamics of the Eurasian plate, are limited to Central and South-West Italy and to Eastern Greece; (c) hot dry rocks (HDR), whose potential for Europe, and also the difficulties in implementing the heat mining concept, are enormous. A large scale experiment conducted at medium depth in Cornwall (UK) proves encouraging though. It has provided the right sort of scientific inputs to the understanding of the mechanics of anisotropic brittle basement rocks.

A surface electrical potential system was fielded during the chemical stimulation of the Rossi 21-19 well in the Beowawe Geothermal Field. The technique, which measures variations in resistivity resulting from the flow of conductive fluid into the reservoir, was not only shown to be highly sensitive, not only to the chemical treatment, but also to the in situ conductive zones before any acid injection. A review of the experiment and a preliminary interpretation of the data are presented. The data provide convincing evidence that it should be possible to map the treated zone as well as the primary pretreatment in situ conductive zones.

The present exploitation conditions of the Ahuachapan field are discussed. The high well density in a small area has resulted in a significant reservoir pressure decrease due to the inherent reservoir over-exploitation. The average pressure in the exploitation zone has decreased from the 1975 value of 34 kg/cm{sup 2} to the May 1983 value of 23 kg/cm{sup 2}. The production decline characteristics of the Ahuachapan wells were examined, concluding that all wells but Ah-22 show exponential decline. The cumulative production-reinjection for the field up to April 1983 is 159.090 x 10{sup 6} tons, and 37.592 x 10{sup 6} tons, respectively. The effect of reinjection upon field behavior is evident when observing the pressure decline characteristics of the field. It is seen that for indection fraction related to total mass extracted above 30 percent, the average decline pressure in the production area becomes approximately stabilized. If this condition is not met the reservoir pressure decreases sharply. From this finding it is concluded that a careful and properly planned reinjection program is a must for the field. The observed temperature reduction in some of the wells seems to be the result of two operating mechanisms. First, we have the pressure decline that …

Geothermal wells discharging hundreds of tons/hour of steam-water mixtures may be supplied at depth from one very narrow crack of width 1 to 2 mm, or alternatively, from some hundreds of hairline cracks. In the former case, turbulent flow takes place out to tens of meters from the well while the sum of frictional and kinetic pressure-drop indicates the flashing distance to be of the order of 10 cm from the well wall for pressure-temperature equilibrium. However it is unlikely that equilibrium obtains because of the high water velocity (order of 100 m/s) near the well giving no time for bubble nucleation. Flashing and hence mineral deposition are therefore not at all likely in the crack but can occur within the well from the crack horizon upwards. In the case of a multitude of fine cracks giving the same total flow, streamline conditions prevail over the flow path with the flash front a meter or so from the well, hence deposition is a possibility.

A freshly flashed geothermal liquid, previously dosed with inhibitor and super-saturated with calcite was injected into another well where it displaced an unflashed counterpart of itself around the wellbore. Back-production of the injectate, and subsequently the native fluid, has yielded data for the rate that a scale inhibitor is degraded after injection. The circumstance also displays a novel mechanism whereby two fluids that do not physically mix never the less reactive with one another through the reservoir rock's serving a role of intermediary. The results have been further interpretated to conclude that in some circumstances a short lifetime for the scale inhibitor is not necessarily a problem for long-term injection.

In order to evaluate geothermal reserves, it is necessary to estimate the porosity-thickness product of the reservoir. This paper deals with the method for estimating the porosity-thickness product of geothermal reservoirs by means of combining well interference and pressure buildup tests. A field study from the Chingshui geothermal area in Taiwan is given to illustrate the application of the method.

The hydrothermal system in the vicinity of Lassen Volcanic National Park contains a central region of fluid upflow in which steam and liquid phases separate, with steam rising through a parasitic vapor-dominated zone and liquid flowing laterally toward areas of hot spring discharge south of the Park. A simplified numerical model was used to simulate the 10,000-20,000 year evolution of this system and to show that under certain circumstances fluid withdrawal from hot-water reservoirs south of the Park could significantly alter the discharge of steam from thermal areas within the Park.

This paper contains the experience gained after one year operating five 5 MW portable, back-pressure, geothermal power plants at Los Azufres. A brief description of the field and te equipment is given. Cost figures of the whole installation and a list of what they believe are the advantages and disadvantages is also presented. The main conclusion is that the use of this type of turbogenerators is quite attractive in new undeveloped fields and also in countries with financial problems where initial capital cost investments must be kept as low as possible at the expenses of long term steam consumption.

Many problems concerning the origin and exploitation of geothermal reservoirs demonstrate the need for models of reactive-solute transport. Of particular interest to us is the coupling between dissolution/precipitation reactions and transient-flow behavior. In an effort to account for observed flow-rate reductions during experiments on samples of granite held in a temperature gradient (summarized at this meeting in 1981 by Moore and others), we examine the effect of quartz precipitation on fluid flow. Our results confirm earlier inerences that reactions responsible for porosity reduction were affected by kinetic factors. Although our results show substantial flow-rate reductions, we are unable to reproduce measured silica concentrations of the outlet fluid by considering the behavior of silica phases without regard for that of the feldspars and micas.

Test data from several wells where scale either formed in the wellbore or in the reservoir have been observed. Some generalizations can be made about the behavior of downhole and wellhead pressure during drawdown and build-up. In addition, estimates of the size of the obstruction can be made in the field. Reservoir parameters can be calculated from pressure build-ups, after the problem of downhole scaling has been identified.

Tracer tests performed at the geothermal reservoir at Wairakei, New Zealand have been analyzed, using a mathematical and physical model in which tracer flows through individual fractures with diffusion into the surrounding porous matrix. Model calculations matched well with the observed tracer return profiles. From the model, first tracer arrival times and the number of individual fractures (the principal conduits of fluid flow in the reservoir) joining the injector-producer wells can be determined. if the porosity, adsorption distribution coefficient, bulk density and effective diffusion coefficient are nown, fracture widths may be estimated. Hydrodynamic dispersion down the length of the fracture is a physical component not taken into account in this model. Future studies may be warranted in order to determine the necessity of including this factor. In addition to the tracer profile matching by the matrix diffusion model, comparisons with a simpler fracture flow model by Fossum and Horne (1982) were made. The inclusion of the matrix diffusion effects was seen to significantly improve the fit to the observed data.

Using numerical simulation techniques and the radial model developed for the study of the natural state of the Heber field (Lippmann and Bodvarsson, 1983b), the response of this geothermal system to exploitation is analyzed. In this study the generation rate in the field is allowed to build up over a period of 10 years; after that, 30 years of constant power production is assumed. Full (100%) injection of the spent brines is considered, the fluids being injected 2250 m ("near injection") or 4250 m ("far injection") from the center of the system. The study shows that a maximum of 6000 kg/s (equivalent to approximately 300 MWe) of fluids may be produced for the near injection case, but only 3000 kg/s (eqivalent to approximately 150 MWe) for the far injection case. The results indicate that the possible extraction rates (generating capacity) generally are limited by the pressure drop in the reservoir. The average temperature of the produced fluids will decline 10-18{degrees}C over the 40-year period.