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Development of a dual-porosity model for vapor-dominated fractured geothermal reservoirs using a semi-analytical fracture/matrix interaction term

Description: A new type of dual-porosity model is being developed to simulate two-phase flow processes in fractured geothermal reservoirs. At this time it is assumed that the liquid phase in the matrix blocks remains immobile. By utilizing the effective compressibility of a two-phase water/steam mixture in a porous rock, flow within the matrix blocks can be modeled by a single diffusion equation. This equation in turn is replaced by a nonlinear ordinary differential equation that utilizes the mean pressure and mean saturation in the matrix blocks to calculate the rate of fluid flow between the matrix blocks and fractures. This equation has been incorporated into the numerical simulator TOUGH to serve as a source/sink term for computational gridblocks that represent the fracture system. The new method has been compared with solutions obtained using fully-discretized matrix blocks, on a problem involving a three-dimensional vapor-dominated reservoir containing an injection and a production well, and has been found to be quite accurate.
Date: January 28, 1993
Creator: Zimmerman, Robert W.; Hadgu, Teklu & Bodvarsson, Gudmundur S.
Partner: UNT Libraries Government Documents Department

A dual-porosity reservoir model with an improved coupling term

Description: A new dual-porosity model is developed for single-phase flow in fractured/porous media. As in the commonly-used approach, flow is assumed to take place through the fracture network, and between the fractures and matrix blocks. The matrix blocks are treated in a lumped-parameter manner, with a single average pressure used for each matrix block. However, instead of assuming that fracture/matrix flux is proportional to the difference between the fracture pressure and matrix pressure at each point, as in the Warren-Root model, a nonlinear equation is used which accurately models the flux at both early and late times. This flux equation is verified against analytical solutions for spherical blocks with prescribed pressure variations on their boundaries. This equation is then used as a source/sink term in the numerical simulator TOUGH. The modified code allows more accurate simulations than the conventional Warren-Root method, and with a large savings in computational time compared to methods which explicitly discretize the matrix blocks.
Date: January 1, 1992
Creator: Zimmerman, Robert W.; Chen, Gang & Bodvarsson, Gudmundur S.
Partner: UNT Libraries Government Documents Department

Theoretical studies of flowrates from slimholes and production-size geothermal wells

Description: The relationship between production rates of large diameter geothermal production wells, and slimholes, is studied. The analysis is based on wells completed in liquid-dominated geothermal fields, where flashing occurs either in the wellbore or at the surface. Effects of drawdown in the reservoir, and pressure drop in the wellbore, are included; heat losses from the wellbore to the formation are not presently included in our analysis. The study concentrates on the influence of well diameter on production rate. For situations where the pressure drop is dominated by the reservoir, it is found that the mass flowrate varies with diameter according to W - D{sup {alpha}}, where the exponent {alpha} is a function of reservoir outer radius, well diameter and skin factor. Similarly, when pressure drop in the wellbore is dominant, the scaling exponent was found to be a function of well diameter and pipe roughness factor. Although these scaling laws were derived for single-phase flow, numerical simulations showed them to be reasonably accurate even for cases where flashing occurs in the wellbore.
Date: January 20, 1994
Creator: Hadgu, Teklu; Zimmerman, Robert W. & Bodvarsson, Gudmundur S.
Partner: UNT Libraries Government Documents Department