Smart mobility control agent for enhanced oil recovery during CO_2 flooding in ultra-low permeability reservoirs

摘要

The development of natural/artificial fractures leads to significant differences of the physical properties between the matrix and the fractures, which usually causes serious channeling and low sweep efficiency during CO2 flooding in ultra-low permeability reservoirs, the use of a CO2-responsive smart mobility control system to generate bulk gel by wormlike micelles (WLMs) to mitigate gas channeling has great potentials for enhanced oil recovery (EOR) in ultra-low permeability reservoirs. In this study, five kinds of chemicals with CO2-sensitive groups are screened to measure the apparent viscosity using a rheometer. The experimental results show that the optimum system consists of 4.4 wt% N, N-dimethyl octylamide-propyl tertiary amine (DOAPA) and 2.0 wt% sodium p-toluenesulfonate (SPTS). Subsequently, the plugging capacity and EOR performance of the system are systematically evaluated using core flooding experiments. The optimized system (DOAPA/SPTS) exhibits outstanding plugging capacity for gas channeling with a plugging efficiency of 99.2%. The oil recovery of the CO2 flooding increases by 20.0%. In addition, the thickening mechanism of the CO2-responsive system is studied using rheological experiments and a cryogenic transmission electron microscopy (Cryo-TEM). The shear-thinning behavior demonstrates that the thickening effect of the high-viscosity WLMs is strong in the DOAPA/SPTS-CO2 solution, and the Cryo-TEM results indicate a transition from spherical micelles to the WLMs. The protonation contributes to the formation of the WLMs in the solution during phase transformation process. The results of this study are expected to provide benchmark to select the mobility control agent for CO2 flooding in ultra-low permeability reservoirs.