A Study of Displacement Efficiency in the Redwater Field

Abstract

WILLMON, G.J., IMPERIAL OIL LTD., EDMONTON, ALTA., CANADA Abstract The Redwater field, Alberta, Canada, discovered by imperial Oil Ltd. in 1948, has now produced approximately 25 percent of the oil originally in place. The present position of the oil-water contact in this strong bottom-water drive pool has been interpreted from borehole contact measurements in observation wells and from an analysis of producing well behavior. Based on this information, and material balance calculations of water influx, the displacement efficiency of the encroaching water is 64 percent. This value has been confirmed by a study of the residual oil and connate water saturations of the two principal rock types in the reservoir. Connate water saturations above the present oil-water contact and residual oil saturations in the water-invaded region were measured directly in cores obtained from the reservoir. in this operation we used a low-invasion, oil-base coring fluid especially developed to minimize flushing of the core by mud filtrate, with a pressure-retaining core barrel to prevent loss of fluids due to depressuring as the core was brought to the surface. Connate water saturations were also determined from conventional oil-base cores and from restored-state capillary pressure tests run at reservoir temperature using reservoir fluids. Introduction The Redwater field (Fig. 1) which is located 35 miles northeast of the City of Edmonton in Alberta, Canada, was discovered in 1948. Development was rapid and by the end of 1951 over 900 wells had been drilled on 40-acre spacing. Most of these wells were completed openhole and generally had productivities in excess of 1,000 B/D following a small acid treatment. Production in Redwater is from the reefal Leduc limestone formation of the Woodbend group which is Upper Devonian in age. The Redwater reef covers an area of some 200,000 acres, but only the eastern or updip portion is productive. This part of the reef may be generally divided into three sections: a fore-reef zone, a biohermal zone in the center of the reef and a back-reef zone (Fig. 2). The fore-reef consists of two types of porosity: a zone of infilled clastics which has poor granular and chalky porosity, and a fragmental zone which has good granular porosity. The algal-biohermal zone has vuggy porosity while the back-reef zone has vuggy as well as some granular and chalky porosity. A detailed geological study of the core material from wells that penetrated the reef section through to the oil-water contact shows that 76 percent of the reservoir is composed of vuggy material which contains 60 percent of the total pore space. The remaining granular material contains 40 percent of the total pore volume. JPT P. 449ˆ