Improved Oil Recovery from Carbonate Reservoirs by Chemical Stimulation

Abstract

Abstract Oil reserves from shallow-shelf carbonate reservoirs account for 22% of the original oil in place (OOIP) of the entire U.S. oil resource. Many of these reservoirs are naturally fractured. A pressure pulsing technique is often used in fractured fields to improve oil recovery. In some situations, imbibition of water can be promoted by chemical stimulation to alter the reservoir wettability towards water-wetness such that oil is expelled at an economic rate from the rock matrix into fractures. Class II shallow shelf carbonate reservoirs typically produce less than 10% OOIP during primary recovery and respond poorly to water injection. In this work, promotion of imbibition was determined for a cationic surfactant, cocoalkyltrimethyl ammonium chloride, and a nonionic surfactant, an ethoxylated alcohol. Cores from three dolomitic Class II reservoirs, Cottonwood Creek, Dagger Draw, and Lustre, were used in the laboratory tests. After preparing core samples using the corresponding reservoir crude oil and brine, spontaneous expulsion of oil was measured in glass imbibition cells at elevated temperature for over 50 core samples. When reservoir brine was used as the imbibition fluid, oil recovery was in the range of 0 - 35% OOIP. After imbibition of reservoir brine had ceased, the cores were transferred into surfactant solutions at or somewhat above the critical micelle concentration to test for enhanced recovery by further imbibition. Typically, immersion in the surfactant solution resulted in additional recovery of 5 to 10% OOIP. The increased recovery is mainly ascribed to increased water wetness. The effect of acidization prior to surfactant treatment was also tested and found to be detrimental to oil recovery. Introduction About 22% of the original oil in place of the entire U.S. oil reserves resides in shallow shelf carbonate reservoirs. Most of such reservoirs are heterogeneous and naturally fractured. Attempts to displace oil by waterflooding pose significant problems because the fractures lead to bypassing of the rock matrix and early breakthrough. Spontaneous imbibition can be of special importance to oil recovery from fractured reservoirs.1,2,3 However, spontaneous imbibition only occurs when the pore surfaces are effectively water-wet so that water imbibes into the rock matrix and oil is expelled into the fractures. The oil can then be flushed along the fractures towards the production wellbore. The significance of spontaneous imbibition as a recovery mechanism was first recognized for the naturally-fractured water-wet Spraberry field of west Texas in the early 50's.4 Promotion of oil recovery by spontaneous imbibition from the Spraberry field is still being pursued.5 Oil recovery by spontaneous imbibition into fractured chalk is recognized as the key production mechanism for North Sea Chalk reservoirs.6 Willingham et al.7 reported that pressure pulsing, intended to stimulate imbibition, increased the oil producing rate and ultimate recovery from a number of individual wells in a Class II reservoir in Wyoming. Laboratory studies provide direct demonstration of spontaneous imbibition as a recovery mechanism. If rocks are oil-wet or neutral wet, imbibition does not occur. Use of surfactants to alter the rock surface to more water-wet to promote imbibition of water into the matrix of fractured reservoirs was recognized as an important mechanism for enhanced recovery.8,9 Extensive laboratory research on improved oil recovery from chalk by imbibition of cationic surfactant solutions has been reported by Austad and coworkers.10 The surfactant interacts with and removes the adsorbed organic materials from the rock surface. The rock surface becomes water-wet and imbibition is enhanced. Chen et al.11 reported on the use of nonionic surfactants to stimulate oil wells in the Yates San Andres reservoir. The field results were encouraging. Average oil-production rate for one well increased from 35 barrels per day to 67 barrels to give an incremental 17,000 barrels of oil at the time of publication. Improved recovery was ascribed to altering the rock surface wettability and/or gravity segregation of oil and water between the fracture and the matrix.