Pore pressure versus confining pressure and their effect on oil-water relative permeability curves

Reservoir petrophysical properties such as relative permeability are influenced by the state of stresses acting on the reservoir rock. Production and water flooding operations induce changes in the net effective pressure due to changes in pore pressure that affects relative permeability curves. Previous studies conducted to investigate the effect of net pressure on relative permeability were conducted by varying the overburden pressure rather than pore pressure. The effect of pore pressure variation on oil-water relative permeability curves at fixed overburden pressures, and at two fixed net pressures was investigated. The effect of confining pressure on relative permeability curves at constant pore pressure was also studied. Experiments were conducted using unsteady state flow with Berea core samples, a 3% NaCl solution and crude oil (API=32). Water and oil relative permeabilities were calculated using JBN method. A significant decrease in oil relative permeability and minor effects of water relative permeability occurred as pore pressure increased at constant overburden pressure. Similar trends were observed as pore pressure was increased at fixed net effective pressure. The results may be attributed to rock wettability alteration from water-wet to mixed-wet to slightly oil-wet as the pore pressure increased at fixed overburden pressure or fixed net effective pressure. Oil relative permeability decreased and water relative permeability increased as overburden pressure increased at fixed pore pressure. The difference in behavior between the two cases (varying pore pressure and varying overburden pressure) is probably due to the direct application of pressure on fluids in the case of varying pore pressure and the indirect application of pressure on the fluids through the rubber sleeve and hence the radial surface the rock. (C) 2005 Elsevier B.V. All rights reserved.