Effects of Gas Cap and Edge Water on Oil Recovery by Steamflooding in a Steeply Dipping Reservoir

Abstract

ABSTRACT A numerical simulation study was conducted to elucidate recovery mechanisms and find optimum operating strategies for a steeply dipping, heavy oil reservoir. A model representing a 20° dip reservoir with an updip gas cap and a downdip edge-water was used for the study. The study showed that gravity drainage of the heated oil is the main production mechanism in steamflooding steeply dipping reservoirs. Steamflood oil recovery for this type of reservoir is strongly affected by gas cap and aquifer pressures. This is because the gas-oil or water-oil contact can move closer to or away from the initial oil zone, depending on those pressures relative to that of the steam zone. High gas cap pressure is desirable because it slows the injected steam rising above the initial gas-oil contact where injected heat is wasted. A high aquifer pressure retards the growth of steam-zone downdip, thereby reducing the steamflood efficiency. The study further showed that this type of reservoir can best be exploited by: (1) initiating the steamflood as a line drive with steam injection into the oil zone away from the gas cap and edge-water, (2) placing the injector closer to downdip producers than to updip producers in order to distribute the injected heat evenly between the updip and downdip directions, and (3) continuing to operate both updip and downdip producers to capture the displaced, heated oil until an economic limit is reached. Maintaining a high gas cap pressure by injecting a noncondensable gas, if the gas cap volume is finite, is an additional operating strategy for improving steamflood oil recovery from this type of reservoir. If the aquifer pressure is low, infill drilling downdip from the original producer can be used to improve capture of the displaced oil.