Sharma, Himanshu (The University of Texas at Austin) | Panthi, Krishna (The University of Texas at Austin) | Ghosh, Pinaki (The University of Texas at Austin) | Weerasooriya, Upali (The University of Texas at Austin) | Mohanty, Kishore (The University of Texas at Austin)
Enhanced oil recovery (EOR) techniques involving surfactants such as surfactant floods, foam floods, and wettability alteration have been studied to recover remaining oil after primary and secondary floods. In these processes, a surfactant solution is injected to promote one (or more) of the following: lowering of capillary forces, improvement in sweep efficiency, and wettability alteration. Although significant advances have been made in designing surfactant molecules to achieve the above mentioned objectives efficiently, surfactant price is often the key limiting factor for a field-scale operation. Most surfactant molecules have a hydrocarbon chain (for example alkyl chain) or an aromatic ring as the main hydrophobe. The hydrocarbon chain (or ring) imparts hydrophobicity (and surface activity) to the surfactant molecule. However, these hydrophobes also result in additional cost. In this study, we discuss low-cost surfactants developed without hydrocarbon chains (or rings) for chemical EOR processes in general. The focus of this paper, however, is on their application in surfactant floods. These novel surfactants were developed by using methanol as the starting material, followed by the addition of propylene oxide (PO) and ethylene oxide (EO) groups, and an anionic head group. The surface tension and critical micelle concentration (CMC) values of these surfactants were measured. A screening study was performed to identify promising candidates; which showed ultralow interfacial tension (IFT) with various crude oils as well as aqueous stability at reservoir conditions. A comparison between novel surfactants with traditional surfactants was made based on the screening study. Oil recovery corefloods were performed in Berea and Boise sandstone cores to test the ultralow IFT formulations. These surfactants were found to have very low CMC values, and lowered the surface tension to about 32 dynes/cm. Their aqueous stability at a given temperature was found to be dependent on the number of PO and EO groups. Phase behavior experiments showed low IFT formulations with different crude oils by using these surfactants by themselves as well as in combination with internal olefin sulfonates (IOS). Moderate oil recoveries were obtained in coreflood experiments using these surfactants.
Carbonate rocks are typically heterogeneous at many scales; hence foams have the potential to improve both oil displacement efficiency and sweep efficiency in carbonate rocks. However, foams have to overcome two adverse conditions in carbonates: oil-wettability and low permeability. This study evaluates several foam formulations that combine wettability alteration and foaming in low permeability oil-wet carbonate cores. Contact angle experiments were performed on oil-wet calcite plates to evaluate the wettability altering capabilities of the surfactant formulations. Static foam stability tests were conducted to evaluate their foaming performance in bulk. Finally, oil displacement experiments were performed using Texas Cream and Estaillades Limestone cores with crude oil. Two different injection strategies were studied in this work: alternating gas-surfactant-gas injection and co-injection of wettability alteration surfactant with gas at a constant foam quality. Cationic surfactants DTAB and BTC altered the wettability of the oil-wet calcite plate to water-wet, but were ineffective in forming foam. The addition of a non-ionic surfactant Tergitol NP helped in the foaming ability of these formulations. In-house developed Gemini cationic surfactant GC 580 was able to alter the wettability from oil-wet to water-wet and also formed strong bulk foam. Static foam tests showed increase in bulk foam stability with the addition of zwitterionic surfactants to GC 580. Oil displacement experiments in oil-wet carbonate cores revealed that tertiary oil-recovery with injection of a wettability-altering surfactant can recover a significant amount of oil (about 20–25% OOIP) over the secondary water flood and gas flood. The foam rheology in the presence of oil suggested propagation of only weak foam in oil-wet low permeability carbonate cores.