Wettability and rate effects on immiscible displacement: Lattice Boltzmann simulation in microtomographic images of reservoir rocks

Pore networks derived from synchrotron X-ray microtomographic images of reservoir rocks were used to provide realistic geometries for simulation of oil displacement by water. The Lattice Boltzmann Method was used to compute two-phase flow dynamics with constant pore geometry—but different pressure driving forces and imposed wettability distributions at the pore wall boundaries. Such examinations can be used to formulate more physically meaningful functional forms for multiphase flow relations to replace empirical constructs of relative permeability with only implicit dependencies on wettability and pore structure. Each simulation started with the same low initial water saturation distribution achieved by primary drainage network simulation in an initially strongly water-wet, reasonably homogeneous pore system. Mixed-wet scenarios were produced by altering the boundary conditions of those surfaces contacted by the non-wetting phase in the initial water distribution image. In one set of simulations comparing water-wet and mixed-wet boundary conditions, capillary numbers in the vicinity of 10−4 were not low enough for differences in wettability to give markedly different displacement dynamics. In a second set of simulations, as the driving force for flow was reduced further, we saw differences in the fluid distributions, but only minor changes in the relative permeabilities extracted from simulation output. The biggest difference between waterflood simulations with different wettabilities appears to be the additional recovery possible after breakthrough in the mixed-wet scenario. Capillary pressure, and not relative permeability, controls whether or not a particular saturation state can be observed. It is therefore concluded that capturing wettability effects in capillary pressure relations is probably more important than trying to roll such effects into relative permeability functions. Wettability is expected to play a more important role in heterogeneous systems.