Development of a Dual-Porosity Model for Vapor-Dominated Fractured Geothermal Reservoirs Using Semi- Analytical Fracture Matrix Interaction Terms

Abstract

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 waterhteam 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 fullydiscretized matrix blocks, on a problem involving a three-dimensional vapor-dominated reservoir containing an injection andí a production well, and has been found to be quite accurate.