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 o f reservoir rocks were used to provide realistic,geometries for simula tion of oil displacement by water. The Lattice Boltzmann Method was us ed to compute two-phase flow dynamics with constant pore geometry-but different pressure driving forces and imposed wettability distribution s at the pore wall boundaries. Such examinations can be used to formul ate more physically meaningful functional forms for multiphase flow re lations to replace empirical constructs of relative permeability with only implicit dependencies on wettability and pore structure. Each sim ulation started with the same low initial water saturation distributio n achieved by primary drainage network simulation in an initially stro ngly water-wet, reasonably homogeneous pore system. Mixed-wet scenario s were produced by altering the boundary conditions of those surfaces contacted by the non-wetting phase in the initial water distribution i mage. In one set of simulations comparing water-wet and mixed-wet boun dary conditions, capillary numbers in the vicinity of 10(-4) were not low enough for differences in wettability to give markedly different d isplacement dynamics. Zn a second set of simulations, as the driving f orce for flow was reduced further, we saw differences in the fluid dis tributions, but only minor changes in the relative permeabilities extr acted from simulation output. The biggest difference between waterfloo d simulations with different wettabilities appears to be the additiona l recovery possible after breakthrough in the mixed-wet scenario. Capi llary pressure, and not relative permeability, controls whether or not a particular saturation state can be observed. It is therefore conclu ded that capturing wettability effects in capillary pressure relations is probably more important than trying to roll such effects into rela tive permeability functions. Wettability is expected to play a more im portant rob in heterogeneous systems. (C) 1998 Elsevier Science B.V. A ll rights reserved.