Development of a generalized Richards equation for predicting spontaneous imbibition of highly shear-thinning liquids in gas recovery applications

Abstract

A new generalized Richards equation (GRE) valid for highly shear-thinning liquids obeying the power-law model is developed using the concept of the effective viscosity. The mathematical model developed this way is validated against experimental data reported recently for one-dimensional spontaneous imbibition of two pusher liquids by a tight sandstone. The GRE model was then used for evaluating the applicability of shear-thinning liquids for enhanced gas recovery. For a homogenous tight sandstone, it is shown that shear-thinning can dramatically shorten the time needed for the gas recovery to reach equilibrium. Based on the obtained numerical results, the mass of the gas recovered using spontaneous imbibition is increased if use is made of highly shear-thinning liquids. At prolonged times, however, it is predicted that gas recovery might slightly drop below its Newtonian counterpart even for highly shear-thinning fluids. The effect was attributed to the fact that, in spontaneous imbibition, the viscosity of power-law fluids increases with time and can eventually become larger than its Newtonian counterpart. For a two-layered non-homogeneous system, numerical results suggest that depending on the microstructure of the two layers, the liquid mass uptake can be smaller than that of the homogenous case. It is predicted that if the liquid is sufficiently shear-thinning, gas recovery can reach levels much above the homogeneous case.