CROSS-CUTTING IMPORTANCE OF FRACTURE-MATRIX INTERACTION TO GROUNDWATER, GEOTHERMAL, AND PETROLEUM PERFORMANCE

The question of fracture-matrix interaction is a key issue in subsurface fluid flow that strongly influences many practical problems of groundwater, subsurface storage, and hydrocarbon production. Matrix diffusion and the access it allows to matrix chemical interactions is the main process of attenuation for radioactive waste disposal and for contaminant transport in fractured porous rocks. Carbon sequestration depends on both the access that fractures allow and the storage capacity of the matrix rock. Enhanced geothermal energy production depends on a thermal diffusion from matrix to fractures. Perhaps the most important fractured oil and gas reservoirs are Nelson’s type II fractured reservoirs where storage is overwhelmingly in the porous matrix but the matrix permeabilities require transport through fractures. Oil and gas reservoirs have the additional complexities of multiphase flow and gravity effects in the displacement of oil from matrix blocks.

The behavior of flow in fractured porous systems is controlled by the relative flow and diffusive properties of the fractures and matrix as well as by the size and shape of matrix blocks. Classic formulations of fracture-matrix interaction assume constant fracture spacing and fracture properties. Realistic fracture networks have neither, which leads to systems with heterogeneous transfer rates. Discrete fracture network (DFN) models provide a useful experimental tool for exploring the effects of heterogeneous fracture properties and variable fracture spacing. A comparison of DFN results for a heterogeneous fracture network with analytical solutions for constant-spaced fractures shows a significant effect of heterogeneous block sizes on the thermal breakthrough behaviors. Pressure responses from the production of fluids from hydraulic fractures produce multiple flow regimes associated with fracture flow, matrix flow, and fracture interference. With heterogeneous block sized these regimes become blurred.