Thermo- and CO2-triggered viscosifying of aqueous copolymer solutions for gas channeling control during water-alternating-CO2 flooding

Abstract

Water-alternating-CO2 gas (WAG) flooding is a common technique to enhance oil recovery via improving CO2 sweep efficiency, but serious gas channeling may still occur, especially in reservoirs with strong heterogeneity, due to a poor blocking capacity of aqueous slugs to the “channeling” pathway induced by their low viscosity. Herein, a copolymer whose aqueous solution viscosity can be increased by both heat and CO2 was developed and added into the aqueous slug to control gas channeling and increase oil recovery during enhanced WAG processes. The copolymer was synthesized by grafting poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) onto the triblock copolymer, Pluronic F127, PEO100-PPO65-PEO100. The aqueous copolymer solutions can be thickened/gelled by heating due to the formation of micelles as the physical cross-link and their entanglements even at a concentration as low as 0.2 wt%, and viscosified by bubbling CO2 thanks to expanded copolymer coils caused by electrostatic repulsion-induced chain extension of protonated PDMAEMA chains. Notably, thermo- and CO2-triggered viscosifying can work synergistically and they are completely reversible upon cyclically heating/cooling the copolymer solutions and bubbling/removing CO2 into/from the aqueous phase, respectively. In WAG flooding tests using two parallel cores, gas channeling was mitigated by blocking high-permeability cores due to in-situ thermo-induced gelation and CO2-triggered thickening of copolymer slugs, thus diverting chase fluids into unswept low-permeability cores and enhancing 21–22% total oil recovery than the conventional and HAPM-enhanced WAG modes. Importantly, owing to the characteristics of low initial viscosity, single component, and thermo-gelation, this dually-responsive copolymer possesses high injectivity and satisfactory gelation control. This work could enlighten the design of novel dual/multiple stimuli-thickening polymers used for channeling control and enhanced oil recovery.