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Matrix-fracture interactions in dual porosity simulation

Description: A new method for simulating flow in fractured media is presented which uses a truncated version of the analytical solution to resolve pressure transients in the rock matrix. The point at which the series solution may be truncated is a known function of the problem, and may therefore be readily determined. Furthermore, the functional form of the method is essentially dimension-independent, and implementation of the method requires only minimal modification to an existing dual porosity simulator. Three test cases are presented comparing results from fine grid simulations, Warren and Root simulations, and the new formulation. In each of the three cases presented, excellent agreement with the fine grid simulations is obtained using the new method. The W&R formulation exhibits excessive error throughout the simulated time, first underpredicting outflow rates, and then overpredicting rates. The error using the W&R formulation is largest for 3-D fracture networks, but is large for all cases tested.
Date: January 1, 1996
Creator: Shook, G.M.
Partner: UNT Libraries Government Documents Department

Generalization of vapor pressure lowering effects in an existing geothermal simulator

Description: Thermodynamic properties of pore water are shown to be different from those of bulk water because of interfacial forces between the aqueous and solid phases. This {open_quotes}vapor-pressure lowering{close_quotes} (VPL) effect is described through Kelvin`s equation, which relates VPL to properties of the liquid phase. An algorithm that accounts for VPL had previously been implented in the geothermal simulator TETRAD. This algorithm applies to a narrow range of reservoir properties, and in some cases leads in inconsistencies. This report presents a generalization of the VPL algorithm which removes many of its limitations. The governing equations for the generalization are presented, assumptions and limitations of the method are discussed, and the modifications are validated.
Date: June 1, 1993
Creator: Shook, G. M.
Partner: UNT Libraries Government Documents Department

An integrated approach to reservoir engineering at Pleasant Bayou Geopressured-Geothermal reservoir

Description: A numerical model has been developed for the Pleasant Bayou Geothermal-Geopressured reservoir. This reservoir description is the result of integration of a variety of data, including geological and geophysical interpretations, pressure transient test analyses, and well operations. Transient test analyses suggested several enhancements to the geologic description provided by University of Texas Bureau of Economic Geology (BEG), including the presence of an internal fault not previously identified. The transient tests also suggested water influx from an adjacent aquifer during the long-term testing of Pleasant Bayou; comparisons between transient test analyses and the reservoir description from BEG suggests that this fault exhibits pressure-dependent behavior. Below some pressure difference across the fault, it remains a no-flow barrier; above this threshold pressure drop the barrier fails, and fluid moves across the fault. A history match exercise is presented, using the hypothesized {open_quotes}leaky fault.{close_quotes} Successful match of 4 years of production rates and estimates of average reservoir pressure supports the reservoir description developed herein. Sensitivity studies indicate that the degree of communication between the perforated interval and the upper and lower sands in the reservoir (termed {open_quotes}distal volume{close_quotes} by BEG) impact simulation results very little, whereas results are quite sensitive to storage and transport properties of this distal volume. The prediction phase of the study indicates that Pleasant Bayou is capable of producing 20,000 STB/d through 1997, with the final bottomhole pressure approximately 1600 psi above abandonment pressure.
Date: December 1, 1992
Creator: Shook, G. M.
Partner: UNT Libraries Government Documents Department

Validation of a Geothermal Simulator

Description: A geothermal simulator, TETRAD, is validated against the Stanford Geothermal Problem Set. The governing equations, formulation, and solution technique employed by TETRAD are first outlined. Each problem in the Stanford Problem Set is then discussed in detail, and results from the simulations are presented. The results obtained using TETRAD are compared against several other geothermal simulators. Favorable comparison between results indicates that TETRAD is capable of solving the highly non-linear equations describing the flow of mass and energy in porous media. This validation exercise allows for the use of TETRAD in studying geothermal problems with a high degree of confidence.
Date: October 1, 1991
Creator: Shook, G.M. & Faulder, D.D.
Partner: UNT Libraries Government Documents Department

Field-scale simulation of matrix-fracture interactions

Description: Simulation of flow in fractured media continues to be among the most challenging problems faced in geothermal reservoir engineering. Because of a lack of information regarding specific matrix-fracture characteristics (e.g., fracture distribution, spacing, and aperture, and interfacial area for exchange of fluid), explicit representation of the reservoir is generally not feasible. Instead, a multiple (but usually dual) continua model is used. In multiple continua models, specific details of the reservoir are replaced with averaged properties (average fracture spacing, for example). Such averaging facilitates the simulation of fractured reservoirs; however, field-scale simulation remains numerically intensive. For example, it has been stated that 5-10 nested shells are required in the Multiple Interacting Continua formulation in order to adequately resolve transient pressure and saturation gradients between the fracture and matrix domains. While this results in a large amount of additional work (compared with a single porosity system of the same dimension), it should be noted that the MINC method is capable of resolving such transients, whereas most dual porosity simulators cannot.
Date: May 1997
Creator: Shook, G. M.
Partner: UNT Libraries Government Documents Department

Preliminary Efforts to Couple TETRAD with Geophysics Models

Description: The Geothermal Program at the Idaho National Engineering and Environmental Laboratory is enhancing our reservoir simulation capabilities by writing new subroutines with TETRAD that write necessary files for use with SAIC's geophysics models, including DC Resistivity, SP, and microgravity. This is part of long-term efforts to develop reservoir models that take advantage of various observations that are - or can be - made on both existing fields or during exploration efforts. These new routines will be made available to the TETRAD user community in 2002 through the next release of TETRAD 2002.
Date: February 19, 2002
Creator: Shook, G.M. & Renner, J.L.
Partner: UNT Libraries Government Documents Department

Reservoir technology research at the INEL

Description: Reservoir engineering research at INEL was aimed at developing a better understanding of The Geysers and developing better tools with which to study flow in fractured geothermal reservoirs in general. Two specific topics were studies in the last year: matrix fracture interactions and decline curve analysis. A third project, revisiting the behavior of the `high-temperature reservoir` (HTR), was started near the end of 1995. These projects are being conducted in collaboration with other researchers and/or private industry. For example, our HTR studies are motivated in part because of new isotopic analyses conducted elsewhere (Walters et al., in preparation). The ultimate goal of these projects is to improve predictive capabilities and reservoir management practices and to extend the commercial life of The Geysers. In addition to conducting engineering research for the Reservoir Technology Program, INEL also continued to assist the Geothermal Technology Organization (GTO) with the development and execution of cooperative research projects. In support of the overall mission of the Reservoir Technology program, INEL also entered into a broad program of subcontracts with industrial groups and universities. These programs support the Reservoir Technology mission by providing support for research topics considered particularly important by the geothermal industry. The GTO projects are summarized below.
Date: May 1, 1996
Creator: Renner, J. L.; Shook, G. M. & Faulder, D. D.
Partner: UNT Libraries Government Documents Department

Parametric Sensitivity Study of Operating and Design Variables in Wellbore Heat Exchangers

Description: This report documents the results of an extensive sensitivity study conducted by the Idaho National Engineering and Environmental Laboratory. This study investigated the effects of various operating and design parameters on wellbore heat exchanger performance to determine conditions for optimal thermal energy extraction and evaluate the potential for using a wellbore heat exchanger model for power generation. Variables studied included operational parameters such as circulation rates, wellbore geometries and working fluid properties, and regional properties including basal heat flux and formation rock type. Energy extraction is strongly affected by fluid residence time, heat transfer contact area, and formation thermal properties. Water appears to be the most appropriate working fluid. Aside from minimal tubing insulation, tubing properties are second order effects. On the basis of the sensitivity study, a best case model was simulated and the results compared against existing low-temperature power generation plants. Even assuming ideal work conversion to electric power, a wellbore heat exchange model cannot generate 200 kW (682.4e+3 BTU/h) at the onset of pseudosteady state. Using realistic conversion efficiency, the method is unlikely to generate 50 kW (170.6e+3 BTU/h).
Date: May 1, 2004
Creator: Nalla, G.; Shook, G.M.; Mines, G.L. & Bloomfield, K.K.
Partner: UNT Libraries Government Documents Department

Novel Application of Single-Well Tracer Tests to Evaluate Hydraulic Stimulation Effectiveness

Description: This paper presents a graphical method by which one can identify the number of fractures and their permeability distribution in the near-well region from single-well tracer tests. The method is an extension of tracer analysis methods developed previously to estimate flow geometry and relies on caluclating the relative fluid velocity from F-__ plots. A number of numerical examples show that high flow zones (fractures) are readily identified from the derivatives of an F-___ curve. The method can be used in evaluating well stimulation efforts by conducting a tracer test before and after the stimulation and comparing the velocity distributions.
Date: September 1, 2005
Creator: Shook, G. M. & Nalla, Gopi
Partner: UNT Libraries Government Documents Department

Design Considerations for Artificial Lifting of Enhanced Geothermal System Fluids

Description: This work evaluates the effect of production well pumping requirements on power generation. The amount of work that can be extracted from a geothermal fluid and the rate at which this work is converted to power increase as the reservoir temperature increases. Artificial lifting is an important issue in this process. The results presented are based on a configuration comprising one production well and one injection well, representing an enhanced geothermal system. The effects of the hydraulic conductivity of the geothermal reservoir, the flow rate, and the size of the production casing are considered in the study. Besides submersible pumps, the possibility of using lineshaft pumps is also discussed.
Date: July 1, 2005
Creator: Xie, Xina; Bloomfield, K. K.; Mines, G. L. & Shook, G. M.
Partner: UNT Libraries Government Documents Department

A semi-analytical model to calculate energy production in single fracture geothermal reservoirs

Description: The ultimate goal of geothermal reservoir management is to mine as much energy as possible from the reservoir. Since most of geothermal reservoirs are fractured, the development of improved techniques for monitoring, prediction and control of two-phase mass and energy transport in the fractures are needed. Tracer testing is a powerful tool for characterizing fractures properties such as swept pore volume. By incorporating the pore volume of the fracture estimated from tracer data with a semi-anaytical model of mass and energy transport in the fracture and matrix, the energy production rate can be calculated. Previous models to predict energey production assumed single-phase flow and semi-infinite matrix blocks and thus overestimate the energy that can be usufully produced from the reservoir. In this paper, a semi-analytical solution to the mass and energy balance equations is derived for two-phase flow in a fracture with heat conduction from a finite matrix. Calculations using the semi-analytical model are shown to be in good agreement with numerical simulations using the TETRAD simulator.
Date: September 1, 2005
Creator: Shook, G.M.; Wu, Xingru; Pope, G. A. & Srinivasan, S.
Partner: UNT Libraries Government Documents Department