Improved oil recovery from carbonate reservoirs by chemical stimulation

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 has been 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 toward water-wetness such that oil is expelled at an economic rate from the rock matrix into fractures. Shallow-shelf (i.e., Class II) 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 (CAC), and a nonionic surfactant, an ethoxylated alcohol (POA). 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 more than 50 core samples. When reservoir brine was used as the imbibition fluid, oil recovery was in the range of 0 to 35% OOIP. After imbibition of reservoir brine had ceased, the cores were transferred into surfactant solutions at or somewhat above the critical micelle concentration (CMC) to test for enhanced recovery by further imbibition. Typically, immersion in the surfactant solution resulted in an additional recovery of 5 to 10% OOIP. The increased recovery is mainly ascribed to increased water-wetness. The effect of acidization before surfactant treatment was also tested and found to be detrimental to oil recovery.