Reservoirs That Are Not in Capillary Pressure Equilibrium

Abstract

Summary URTeC 1618391 The physics of fluid flow in oil and gas reservoirs is determined by capillary forces and interfacial tension. Standard reservoir parameters are sufficient to understand fluid flow in many hydrocarbon productive reservoirs, but in other reservoirs additional information must be known to adequately determine the effects of capillary pressure and interfacial tension and to predict fluid flow. In conventional reservoirs the assumption is generally made that the reservoirs are in capillary pressure equilibrium. If this assumption is incorrect then our understanding of fluid flow in these reservoirs is invalid and we are dealing with unconventional reservoirs. Bennion, Thomas, and Ma (2000) recognized that there are reservoirs that were not in capillary pressure equilibrium. Post hydrocarbon uplift and erosion cause the water volumes to decrease in these reservoirs. Because of this deficit of water, if the reservoir is water wet and water is introduced to the well in the drilling or completion process, the reservoir will spontaneously imbibe water and water block the hydrocarbon pore network. If it is mixed wet but hydrophobic, then forced imbibition is required to get the water into the reservoir but the resulting water block is still very difficult to remove because it requires returning the reservoir to a non equilibrium state. The results of this relative permeability damage vary depending on the absolute permeability of the reservoir and the fluid saturations of the reservoir. The damaged hydrocarbon reservoir may flow nothing, a mixture of water and the hydrocarbons present, or even all water. The water produced can be greatly in excess of the water lost because it is water that is flowing from the water pore network in the reservoir that is no longer held in place by capillary forces because of the reservoir damage.