Determining Relative Permeability In Shales By Including The Effects Of Pore Structure On Unsaturated Diffusion And Advection

Abstract

Abstract Existing models for shale permeability couple advection and diffusion but do not include the effects of fluid saturations on the permeability. Using a combination of critical path analysis, effective medium theory, and percolation theory, we modeled shale relative permeability by considering the effects of pore structure on unsaturated advection and diffusion. Relevant percolation properties were determined from N2 adsorption-desorption measurements. We considered samples of Green River Shale, Woodford Shale, and Cameo Coal, and isolated kerogens from the Green River and Woodford. We considered gas to be the wetting phase in the kerogens and coal and the nonwetting phase in the bulk shales. We found that the nonwetting phase relative permeability scales linearly with the nonwetting phase saturation, while the wetting phase relative permeability exhibits a highly nonlinear relationship with wetting phase saturation due to the large pore space fractal dimensions (>2.68) of the shales. Based on our model, we predict that water production from the shales is expected to be minor until the gas phase reaches its residual saturation, and that the rate of gas transport out of the kerogen and into the shale matrix should decrease rapidly following the onset of production. This may explain in part why shale gas wells typically produce little formation water and exhibit rapid production decline rates.