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DOE-project on geothermal reservoir engineering computer code comparison and validation: evaluation of results for Problem 6

Description: Three of the four simulators used in computing a difficult three-dimensional problem show excellent quantitative agreement. This demonstrates that numerical simulators are capable of producing accurate results for field-wide reservoir depletion problems, involving phase transitions, gravitationally induced steam/water counterflow, and recharge.
Date: December 1, 1980
Creator: Pruess, K.
Partner: UNT Libraries Government Documents Department

Quantitative model of vapor dominated geothermal reservoirs as heat pipes in fractured porous rock

Description: We present a numerical model of vapor-dominated reservoirs which is based on the well-known conceptual model of White, Muffler, and Truesdell. Computer simulations show that upon heat recharge at the base, a single phase liquid-dominated geothermal reservoir in fractured rock with low matrix permeability will evolve into a two-phase reservoir with B.P.D. (boiling point-for-depth) pressure and temperature profiles. A rather limited discharge event through cracks in the caprock, involving loss of only a few percent of fluids in place, is sufficient to set the system off to evolve a vapor-dominated state. The attributes of this state are discussed, and some features requiring further clarification are identified. 26 refs., 5 figs.
Date: March 1, 1985
Creator: Pruess, K.
Partner: UNT Libraries Government Documents Department

GMINC: a mesh generator for flow simulations in fractured reservoirs

Description: GMINC is a pre-processor computer program for generating geometrical meshes to be used in modeling fluid and heat flow in fractured porous media. It is based on the method of multiple interacting continua (MINC) as developed by Pruess and Narasimhan. The meshes generated by GMINC are in integral finite difference form, and are compatible with the simulators SHAFT79 and MULKOM. Applications with other integral finite difference simulators are possible, and require slight modifications in input/output formats. This report describes methodology and application of GMINC, including preparation of input decks and sample problems. A rather comprehensive overview of the MINC-method is also provided to make the presentation self-contained as a guide for modeling of flow in naturally fractured media.
Date: March 1, 1983
Creator: Pruess, K.
Partner: UNT Libraries Government Documents Department

Proceedings of the TOUGH workshop

Description: A workshop on applications and enhancements of the TOUGH/MULKOM family of multiphase fluid and heat flow simulation programs was held at Lawrence Berkeley Laboratory on September 13--14, 1990. The workshop was attended by 62 scientists from seven countries with interests in geothermal reservoir engineering, nuclear waste isolation, unsaturated zone hydrology, environmental problems, and laboratory and field experimentation. The meeting featured 21 technical presentations, extended abstracts of which are reproduced in the present volume in unedited form. Simulator applications included processes on a broad range of space scales, from centimeters to kilometers, with transient times from seconds to geologic time scales. A number of code enhancements were reported that increased execution speeds for large 3-D problems by factors of order 20, reduced memory requirements, and improved user-friendliness. The workshop closed with an open discussion session that focussed on future needs and means for interaction in the TOUGH user community. Input from participants was gathered by means of a questionnaire that is reproduced in the appendix. 171 refs., 91 figs., 16 tabs.
Date: September 1, 1990
Creator: Pruess, K.
Partner: UNT Libraries Government Documents Department

TOUGH User's Guide

Description: This document contains a technical description of the TOUGH computer program, which was developed at Lawrence Berkeley Laboratory for simulating the coupled transport of water, vapor, air and heat in porous and fractured media. The physical processes taken into account in TOUGH are discussed, and the governing equations actually solved by the simulator are stated in full detail. A brief overview is given of the mathematical and numerical methods, and the code architecture. The report provides detailed instructions for preparing input decks. Code applications are illustrated by means of six sample problems.
Date: August 1, 1987
Creator: Pruess, K.
Partner: UNT Libraries Government Documents Department

Modeling of fluid and heat flow in fractured geothermal reservoirs

Description: In most geothermal reservoirs large-scale permeability is dominated by fractures, while most of the heat and fluid reserves are stored in the rock matrix. Early-time fluid production comes mostly from the readily accessible fracture volume, while reservoir behavior at later time depends upon the ease with which fluid and heat can be transferred from the rock matrix to the fractures. Methods for modeling flow in fractured porous media must be able to deal with this matrix-fracture exchange, the so-called interporosity flow. This paper reviews recent work at Lawrence Berkeley Laboratory on numerical modeling of nonisothermal multiphase flow in fractured porous media. We also give a brief summary of simulation applications to problems in geothermal production and reinjection. 29 refs., 1 fig.
Date: August 1, 1988
Creator: Pruess, K.
Partner: UNT Libraries Government Documents Department

Modeling of geothermal reservoirs: Fundamental processes, computer simulation, and field applications

Description: This article attempts to critically evaluate the present state of the art of geothermal reservoir simulation. Methodological aspects of geothermal reservoir modeling are briefly reviewed, with special emphasis on flow in fractured media. Then we examine applications of numerical simulation to studies of reservoir dynamics, well test design and analysis, and modeling of specific fields. Tangible impacts of reservoir simulation technology on geothermal energy development are pointed out. We conclude with considerations on possible future developments in the mathematical modeling of geothermal fields. 45 refs., 4 figs., 2 tabs.
Date: September 1, 1988
Creator: Pruess, K.
Partner: UNT Libraries Government Documents Department

TOUGH2. Unsaturated Groundwater and Heat Transport Model

Description: TOUGH2 is a new and improved version of TOUGH. TOUGH2 offers added capabilities and user features, including the flexibility to handle different fluid mixtures (water, water with tracer; water, CO2; water, air; water, air, with vapor pressure lowering and water, hydrogen), facilities for processing of geometric data (computational grids), and an internal version control system to ensure referenceability of code applications. TOUGH2 is a multi-dimensional numerical model for simulating the coupled transport of water, vapor, air, and heat in porous and fractured media. The program provides options for specifying injection or withdrawal of heat and fluids. Although primarily designed for studies of high-level nuclear waste isolation in partially saturated geological media, it should also be useful for a wider range of problems in heat and moisture transfer, and in the drying of porous materials. For example, geothermal reservoir simulation problems can be handled simply by setting the air mass function equal to zero on input. The TOUGH2 simulator was developed for problems involving strongly heat-driven flow. To describe these phenomena a multi-phase approach to fluid and heat flow is used, which fully accounts for the movement of gaseous and liquid phases, their transport of latent and sensible heat, and phase transitions between liquid and vapor. TOUGH2 takes account of fluid flow in both liquid and gaseous phases occurring under pressure, viscous, and gravity forces according to Darcy`s law. Interference between the phases is represented by means of relative permeability functions. The code handles binary, but not Knudsen, diffusion in the gas phase and capillary and phase adsorption effects for the liquid phase. Heat transport occurs by means of conduction with thermal conductivity dependent on water saturation, convection, and binary diffusion, which includes both sensible and latent heat.
Date: May 1, 1991
Creator: Pruess, K.
Partner: UNT Libraries Government Documents Department

Heat pipe effects in nuclear waste isolation: a review

Description: The existence of fractures favors heat pipe development in a geologic repository as does a partially saturated medium. A number of geologic media are being considered as potential repository sites. Tuff is partially saturated and fractured, basalt and granite are saturated and fractured, salt is unfractured and saturated. Thus the most likely conditions for heat pipe formation occur in tuff while the least likely occur in salt. The relative permeability and capillary pressure dependences on saturation are of critical importance for predicting thermohydraulic behavior around a repository. Mineral redistribution in heat pipe systems near high-level waste packages emplaced in partially saturated formations may significantly affect fluid flow and heat transfer processes, and the chemical environment of the packages. We believe that a combined laboratory, field, and theoretical effort will be needed to identify the relevant physical and chemical processes, and the specific parameters applicable to a particular site. 25 refs., 1 fig.
Date: December 1, 1985
Creator: Doughty, C. & Pruess, K.
Partner: UNT Libraries Government Documents Department

Performance Matching and Prediction for Serrazzano Geothermal Reservoir by Means of Numerical Simulation

Description: The simulator SHAFT79 of Lawrence Berkeley Laboratory has been applied to field-wide distributed parameter simulation of the vapor-dominated geothermal reservoir at Serrazzano, Italy. Using a three-dimensional geologically accurate mesh and detailed flow rate data from 19 producing wells, a period of 15.5 years (from 1959 to 1975) has been simulated. The reservoir model used is based on field measurements of temperatures and pressures, laboratory data for core samples, and available geological and hydrological information. The main parameters determined (adjusted) during development of the simulation are permeabilities and much of the initial conditions.
Date: October 20, 1980
Creator: Pruess, K. & Weres, O.
Partner: UNT Libraries Government Documents Department

Some effects of non-condensible gas in geothermal reservoirs with steam-water counterflow

Description: A mathematical model is developed for fluid and heat flow in two-phase geothermal reservoirs containing non-condensible gas (CO{sub 2}). Vertical profiles of temperature, pressures and phase saturations in steady-state conditions are obtained by numerically integrating the coupled ordinary differential equations describing conservation of water, CO{sub 2}, and energy. Solutions including binary diffusion effects in the gas phase are generated for cases with net mass throughflow as well as for balanced liquid-vapor counterflow. Calculated examples illustrate some fundamental characteristics of two-phase heat transmission systems with non-condensible gas. 14 refs., 3 figs.
Date: January 1, 1988
Creator: McKibbin, R. & Pruess, K.
Partner: UNT Libraries Government Documents Department

Enhancement of steam phase relative permeability due to phase transformation effects in porous media

Description: An experimental study of two-phase concurrent flow of steam and water was conducted and a set of relative permeability curves was obtained. These curves were compared with semi-empirical results and experimental results obtained by other investigators for two-phase, two-component flow (oil/gas; gas/water; gas/oil). It was found that while the wetting phase relative permeabilities were in good agreement, the relative permeability for the steam phase was considerably higher than the relative permeabilities of non-wetting phase (oil/water and non-condensing gas in gas/oil or gas/water) in two-component systems. This enhancement of steam relative permeability is attributed to phase transformation effects at the pore level in flow channels.
Date: January 1, 1986
Creator: Verma, A. & Pruess, K.
Partner: UNT Libraries Government Documents Department

Hydrothermal conditions and resaturation times in underground openings for a nuclear waste repository in the Umtanum flow at the Basalt Waste Isolation Project

Description: Numerical simulation techniques have been used to study heat flow and pore fluid migration in the near field of storage tunnels and canister storage holes in a proposed high-level nuclear waste repository in the Umtanum Basalt at the Basalt Waste Isolation Project site at Hanford, Washington. Particular emphasis was placed on evaluating boiling conditions in the host rock. Sensitivity studies were performed to determine the influence of variations in critical site-specific parameters which are not presently accurately known. The results indicate that, even when rather extreme values are assumed for key hydrothermal parameters, the volume of rock dried by boiling of pore fluids is negligible compared to the volume of excavated openings. The time required for saturation of backfilling materials is thus controlled by the volume of the mined excavations. When realistic values for the parameters of the natural and man-made systems are used resaturation is predicted to occur within less than two years after backfilling is placed. The approximations used in the analyses, and their limitations, are discussed in the body of the report. Recommendations are made for additional studies of the thermohydrological behavior of a high-level nuclear waste repository. 31 references, 76 figures, 7 tables.
Date: July 1, 1982
Creator: Pruess, K. & Bodvarsson, G.
Partner: UNT Libraries Government Documents Department

Effects of silica redistribution on performance of high-level nuclear waste repositories in saturated geologic formations

Description: Evaluation of the thermohydrological conditions near high-level waste packages is needed for the design of the waste canister and for overall repository design and performance assessment. Most available studies in this area have assumed that the hydrologic properties of the host rock do not change in response to the thermal, mechanical or chemical effects caused by waste emplacement. However, the ramifications of this simplifying assumption have not been substantiated. We have studied dissolution and precipitation of silica in thermally driven flow systems, including changes in formation porosity and permeability. Using numerical simulation, we compare predictions of thermohydrological conditions with and without inclusion of silica redistribution effects. Two cases were studied, namely, a canister-scale problem, a repository-wide thermal convection problem, and different pore models were employed for the permeable medium (fractures with uniform or non-uniform cross sections). We find that silica redistribution generally has insignificant effects on host rock and canister temperatures, pore pressures, or flow velocites.
Date: November 1, 1985
Creator: Verma, A. & Pruess, K.
Partner: UNT Libraries Government Documents Department

Numerical simulation of the natural evolution of vapor-dominated hydrothermal systems

Description: Simulation of the transient evolution of a cold hydrothermal system into a steady-state partially vapor-dominated system is described. Here the effects of salts and gases were neglected and it was assumed that rock properties are time independent and homogeneous within each part of the system. Despite these simplifications it is believed that the model demonstrates the essential features of a natural hydrothermal convection system. (MHR)
Date: December 1, 1980
Creator: Pruess, K. & Truesdell, A.H.
Partner: UNT Libraries Government Documents Department

Unstable heat pipes

Description: Heat pipes are an important feature of models of vapor-dominated geothermal reservoirs. Numerical experiments reveal that a vapor-dominated heat pipe is unstable if pressure is controlled at shallow levels. This instability is discussed in physical terms, and some implications for geothermal reservoirs are considered. 9 refs., 10 figs.
Date: October 1, 1987
Creator: McGuinness, M.J. & Pruess, K.
Partner: UNT Libraries Government Documents Department

A semi-analytical method for heat sweep calculations in fractured reservoirs

Description: An analytical approximation is developed for purely conductive heat transfer from impermeable blocks of rock to fluids sweeping past the rocks in fractures. The method was incorporated into a multi-phase fluid and heat flow simulator. Comparison with exact analytical solutions and with simulations using a multiple interacting continua approach shows very good accuracy, with no increase in computing time compared to porous medium simulations. 14 refs., 3 figs., 5 tabs.
Date: January 1, 1988
Creator: Pruess, K. & Wu, Y.S.
Partner: UNT Libraries Government Documents Department

Practical method for modeling fluid and heat flow in fractured porous media

Description: A Multiple Interacting Continua method (MINC) is presented which is applicable for numerical simulation of heat and multi-phase fluid flow in multidimensional, fractured porous media. This method is a generalization of the double-porosity concept. The partitioning of the flow domain into computational volume elements is based on the criterion of approximate thermodynamic equilibrium at all times within each element. The thermodynamic conditions in the rock matrix are assumed to be primarily controlled by the distance from the fractures, which leads to the use of nested grid blocks. The MINC concept is implemented through the Integral Finite Difference (IFD) method. No analytical approximations are made for the coupling between the fracture and matrix continua. Instead, the transient flow of fluid and heat between matrix and fractures is treated by a numerical method. The geometric parameters needed in a simulation are preprocessed from a specification of fracture spacings and apertures, and the geometry of the matrix blocks. The MINC method is verified by comparison with the analytical solution of Warren and Root. Illustrative applications are given for several geothermal reservoir engineering problems.
Date: February 1, 1982
Creator: Pruess, K. & Narasimhan, T.N.
Partner: UNT Libraries Government Documents Department

Basic theory and equations used in the two-phase multidimensional geothermal reservoir simulator, SHAFT79

Description: The algorithm of SHAFT79 is based on mass and energy balance equations for two-phase flow in a porous medium. These basic equations are formulated as Integrated Finite Difference equations. The latter formulation allows both regular and irregular discrete grid approximations of reservoir geometry. The present version of SHAFT79 solves the non-linear mass and energy equations simultaneously using an efficient linear algebra package. The computer program is being applied to a vareity of problems to study both real reservoir behavior and to better understand the physics of two-phase systems. The types of applications include idealized one- and two-phase reservoir depletion, two-phase reservoir behavior with distributed liquid, simulation of real reservoirs, matching production data, and estimating material parameters from well test data.
Date: July 1, 1979
Creator: Pruess, K. & Schroeder, R.C.
Partner: UNT Libraries Government Documents Department

Analysis of injection testing of geothermal reservoirs

Description: By introducing a similarity variable r/..sqrt..t a quasi-analytical method can be used to calculate the flow induced by the injection of cold water into a hot water or boiling geothermal reservoir. The results obtained are compared with those produced by the reservoir simulator SHAFT79 and show good agreement.
Date: May 1, 1980
Creator: O'Sullivan, M. & Pruess, K.
Partner: UNT Libraries Government Documents Department

SHAFT79 user's manual

Description: SHAFT79 (Simultaneous Heat And Fluid Transport) is an integrated finite difference program for computing two-phase non-isothermal flow in porous media. The principal application for which SHAFT79 is designed is in geothermal reservoir simulation. SHAFT79 solves the same equations as an earlier version, called SHAFT78, but uses much more efficient mathematical and numerical methods. The present SHAFT79 user's manual gives a brief account of equations and numerical methods and then describes in detail how to set up input decks for running the program. The application of SHAFT79 is illustrated by means of a few sample problems. (MHR)
Date: March 1, 1980
Creator: Pruess, K. & Schroeder, R.C.
Partner: UNT Libraries Government Documents Department

Thermal effects of reinjection in geothermal reservoirs with major vertical fractures

Description: Possibilities for characterizing the thermal properties of fast paths by means of different types of tests (tracers, pressure transients, non-isothermal injection) are discussed. Thermal breakthrough in vertical fractures is examined in some detail, using an idealized model for which an analytical solution is available. The model shows that rapid tracer returns are not necessarily indicative of rapid thermal interference. Thermal breakthrough predictions can be made from tracer data only, if both fluid residence time and tracer dispersion are taken into account. However, due to the geometric simplifications necessary in analyzing the tracer data, thermal interference estimates on this basis appear questionable. Pressure transient tests can provide additional parameters for thermal interference predictions, but they cannot resolve the problem of non-uniqueness. A more reliable determination of thermal characteristics of fast paths appears possible from non-isothermal injection tests, combined with numerical simulation. A mixed numerical/semi-analytical approach is employed to model the three-dimensional fluid and heat flow in injection-production systems in vertical fractures, with heat transfer to and from the adjacent rock matrix. Illustrative calculations of thermal recovery after different injection periods suggest that shutting-in an injection well can prevent unacceptable temperature declines at production wells.
Date: October 1, 1983
Creator: Pruess, K. & Bodvarsson, G.S.
Partner: UNT Libraries Government Documents Department

Modeling studies of geothermal systems with a free water surface

Description: A numerical simulator was developed for the modeling of air-steam-water systems. The simulator was applied to various problems involving injection into or production from a geothermal reservoir in hydraulic communication with a shallow free-surface aquifer. First, a one-dimensional column problem is considered and the water level movement during exploitation is studied using different capillary pressure functions. Second, a two-dimensional radial model is used to study and compare reservoir depletion for cases with and without a free-surface aquifer. Finally, the contamination of a shallow free-surface aquifer due to cold water injection is investigated. The primary aim of these studies is to obtain an understanding of the response of a reservoir in hydraulic communication with a unconfined aquifer during exploitation or injection and to determine under which circumstances conventional modeling techniques (fully saturated systems) can be applied to such systems.
Date: December 1, 1983
Creator: Bodvarsson, G.S. & Pruess, K.
Partner: UNT Libraries Government Documents Department

On Relative Permeability of Rough-Walled Fractures

Description: This paper presents a conceptual and numerical model of multiphase flow in fractures. The void space of real rough-walled rock fractures is conceptualized as a two-dimensional heterogeneous porous medium, characterized by aperture as a function of position in the fracture plane. Portions of a fracture are occupied by wetting and non-wetting phase, respectively, according to local capillary pressure and accessibility criteria. Phase occupancy and permeability are derived by assuming a parallel-plate approximation for suitably small subregions in the fracture plane. Wetting and non-wetting phase relative permeabilities are calculated by numerically simulating single phase flows separately in the wetted and non-wetted pore spaces. Illustrative examples indicate that relative permeabilities depend sensitively on the nature and range of spatial correlation between apertures. 30 refs., 7 figs., 1 tab.
Date: January 1989
Creator: Pruess, K. & Tsang, Y. W.
Partner: UNT Libraries Government Documents Department