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Numerical Studies of the Energy Sweep in Five-Spot Geothermal Production/Injection Systems

Description: Most recent interest in the injection of cold water into a geothermal reservoir has been related to the disposal of geothermal brines. Injection also offers the potential benefit of prolonging the useful life of a vapor-dominated system by providing additional water to extract energy out of the rock matrix. In a liquid-dominated reservoir injection may help to maintain pressures near the production wells by pushing the hot water toward them and preventing too much local boiling. Pressure maintenance can also be achieved for superheated steam zones, because injection will cause pressures to increase towards the saturation pressure (Schroeder et a l . (1980)). The general physical principles governing these processes are understood but no quantitative information is available. The present work is aimed at helping to improve the qualitative and quantitative understanding of injection into a geothermal reservoir by considering a few idealized problems. First a vapor-dominated, single layer reservoir is considered, next a vapor-dominated, four layer reservoir, and finally a liquid-dominated, single layer reservoir. In each case varying injection rates are considered and in some cases the injectionis changed at different times.
Date: December 16, 1980
Creator: O'Sullivan, M.J. & Pruess, K.
Partner: UNT Libraries Government Documents Department

Sampling Geopressured Fluids: Some Contributions from the Properties of the H{sub2}O-CH{sub4} System

Description: This paper is devoted t o predicting and quantitatively estimating geopressured fluid behavior during sampling of reservoirs in their natural, unperturbed conditions. Both the fluid in the sampler and the wellbore fluid are considered. To that end, I have developed a simple model ( an ''equation of state") t o estimate thermophysical properties of geopressured fluids. This model is briefly described. The "equation of state" is applied to compute and discuss fluid properties associated with the different stages of the sampling process. Questions explored include: the probable range of CH4 content of the samples; pressure, phase transitions, fraction of total volume corresponding to each phase, and composition of each phase present in the sample, over the expected range of temperatures; whether and under what conditions the fluid collected at wellhead in a flowing w e l l provides a representative sample of the bottomhole fluid composition; the expected range of fluid pressures in the lubricator; and the expected range of differential stresses on the sampler. Bottomhole temperatures and pressures generally increase with depth in the geopressured formations of the Gulf Coast (e.g., Dorfman and Fisher, 1979). Thus, two well depths, representing approximatley the top and the bottom of the geopressured zone, were considered in detail to assess effects associated with depth. Finally, r e s u l t s and recommendations are summarized in Section I V .
Date: August 18, 1980
Creator: Iglesias, Eduardo R.
Partner: UNT Libraries Government Documents Department

The Sensitivity of Geothermal Reservoir Behavior to Relative Permeability Parameters

Description: In the present work three problems are considered; (1) the sensitivity of {nu}{sub t} and h{sub f} to variations in the relative permeability functions; (2) the determination of {nu}{sub t} and h{sub f} from well-test data, following which a method of is suggested to use these results together with theoretical plots of k{sub rl} and k{sub rv} versus h{sub f} to deduce the general shape of the relative permeability functions is suggested; and ( 3 ) the effect of the relative permeability functions on the pressure decline and flowing enthalpy build-up during a constant rate production test. It is shown that the characteristic rise in the flowing enthalpy from its initial value to a stable value after a moderate time is strongly influenced by the dependence of h{sub f} on S{sub 1} (and hence k{sub rl} and k{sub rv} on S{sub 1}) .
Date: December 16, 1980
Creator: Bodvarsson, Gudmundur S.; O'Sullivan, Michael J. & Tsang, Chin Fu
Partner: UNT Libraries Government Documents Department

Conceptual Model of the Klamath Falls, Oregon Geothermal Area

Description: Over the last 50 years significant amounts of data have been obtained from the Klamath Falls geothermal resource. To date, the complexity of the system has stymied researchers, leading to the development of only very generalized hydrogeologic and geothermal models of the area. Recently, the large quantity of available temperature data have been re-evaluated, revealing new information on subsurface heat flow and locations of faults in the system. These inferences are supported by borehole, geochemical, geophysical, and hydrologic data. Based on re-evaluation of all available data, a detailed conceptual model for the Klamath Falls geothermal resource is proposed. 1 tab., 8 figs., 21 refs.
Date: January 20, 1987
Creator: Prucha, R.H.; Benson, S.M. & Witherspoon, P.A.
Partner: UNT Libraries Government Documents Department

Automatic History Matching of Geothermal Field Performance

Description: We have developed inverse modeling capabilities for the multiphase multicomponent numerical simulator TOUGH2 to facilitate automatic history matching and parameter estimation based on data obtained during exploitation of geothermal fields. The ITOUGH2 code allows one to estimate TOUGH2 input parameters based on any type of observation for which a corresponding TOUGH2 output can be calculated. Furthermore, a detailed residual and error analysis is performed, and the uncertainty of model predictions can be evaluated. This paper focuses on the solution of the inverse problem, i.e. the determination of model-related parameters by automatically calibrating a conceptual model of the geothermal system against data obtained during field operation. We first describe the modeling approach used to simulate fluid and heat flow in fractured-porous media. The inverse problem is then formulated, followed by a brief discussion of the optimization algorithm. A sample problem is given to demonstrate the application of the method to geothermal reservoir data.
Date: January 1, 1995
Creator: Finsterle, S. & Pruess, K.
Partner: UNT Libraries Government Documents Department

Analysis of Thermally Induced Permeability Enhancement in Geothermal Injection Wells

Description: Reinjection of spent geothermal brine is a common means of disposing of geothermal effluents and maintaining reservoir pressures. Contrary to the predictions of two-fluid models (two-viscosity) of nonisothermal injection, an increase of injectivity, with continued injection, is often observed. Injectivity enhancement and thermally-affected pressure transients are particularly apparent in short-term injection tests at the Los Azufres Geothermal Field, Mexico. During an injection test, it is not uncommon to observe that after an initial pressure increase, the pressure decreases with time. As this typically occurs far below the pressure at which hydraulic fracturing is expected, some other mechanism for increasing the near-bore permeability must explain the observed behavior. This paper focuses on calculating the magnitude of the near-bore permeability changes observed in several nonisothermal injection tests conducted at the Los Azufres Geothermal Field. In order to evaluate the pressure transient data and calculate the magnitude of the thermally induced permeability changes, a new analytic solution for calculating pressure transients with time-varying sandface flowrates and temperatures has been developed. The effects of temperature-dependent fluid and rock properties, as well as a moving thermal front, are explicitly included in the calculations. Based on this new solution, a technique is developed for calculating the reservoir permeability, skin factor of the well, and near-bore permeability increases. The results of these calculations indicate that the permeability increases by a factor of 5 in the near-bore region during the 2 to 3 hour injection tests. A good correlation between the permeability increase and the sandface injection temperature indicates that the permeability increase is caused by cooling the formation. 9 figs., 9 refs.
Date: January 20, 1987
Creator: Benson, S. M.; Daggett, J. S.; Iglesias, E.; Arellano, V. & Ortiz-Ramirez, J.
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

On Fluid and Heat Transfer in Deep Zones of Vapor-Dominated Geothermal Reservoirs

Description: We have presented a preliminary analysis of permeability structure and fluid and heat flow conditions in the deeper horizons of the Larderello geothermal system. Our main observations and findings are: (1) Measurements in deep Larderello wells have indicated formation temperatures near 300 ยบ C at 3000 m depth, and even higher temperatures at greater depth. (2) From an analysis of heat transfer mechanisms we suggest that a transition from vapor-dominated to liquid-dominated conditions must have been present in the natural state of the Larderello geothermal system. No reliable determination of the depth at which this transition occurred has yet been made, but a depth of approximately 2000 m or more appears most likely. (3) From temperature-depth data in two-phase reservoirs it is in principle possible to estimate vertical permeability. (4) For exploited reservoirs such as Larderello, reconstruction of permeability and temperature trends with depth can be made indirectly, using numerical simulation. Our preliminary results indicate that production of high-enthalpy fluids can be explained from two-phase flow effects in a fractured-porous medium. 18 refs., 2 tabs., 7 figs.
Date: January 20, 1987
Creator: Pruess, K.; Celati, R.; Calore, C. & Cappetti, G.
Partner: UNT Libraries Government Documents Department