Wettability Alteration with Weak Acid Assisted Surfactant Flood

Abstract

Oil-wetness and heterogeneity are two key reasons for low oil recovery by waterflooding in carbonate reservoirs. Surfactants have been effective in altering oil-wet matrix to a more water-wet condition and initiating spontaneous imbibition. Because it takes time for surfactant to alter wettability, oil recovery from the tight matrix is considered to be slow and sometimes not economically feasible. Acids have a potential of dissolving minerals, which may alter wettability. In this study, the EOR performance of acid-assisted surfactant water was evaluated for both low- and high-temperature applications. A set of acids and their acetates were tested. Bulk rock-acid reaction, wettability alteration (WA) tests and spontaneous imbibition measurements were conducted at both 35°C and 80°C to identify effective candidates. Coreflood tests were then performed to evaluate the selected acid-surfactant formulations. Before and after coreflood test, the core was scanned in a micro-CT to investigate pore structure alteration. Bulk reaction measurements showed that weak acids, especially acetic acid (AA), have the desired low reaction rates at 35°C. At 80°C, acetates exhibited a slow reaction. WA tests showed that, at 35°C, AA can remove the crude oil off the rock surface and alter wettability. Acetates showed wettability alteration potentials at 80°C. Spontaneous imbibition experiments showed that AA-surfactant solution results in the highest oil recovery at 35°C. Acetate-surfactant showed a high oil recovery and a long equilibrium time at 80°C. Coreflood tests showed that adding AA into surfactant water can significantly improve oil production and its rate through mineral dissolution and wettability alteration. Micro-CT showed that minerals were transported along the core and plugged vugs, which reduced permeability and diverted flow leading to improved oil recovery.