Microemulsion-Assisted Fracturing-Flooding: An Eco-Technical Alternative to Thermal Recovery for Marginal Extraheavy Oil

Summary Low-permeability, water-sensitive extraheavy oil reservoirs with abundant reserves in China remain unapproved for commercial development using thermal recovery or other existing technologies due to their carbon emission problems and/or techno-economic limitations. This study proposes injecting microemulsions via fracturing-flooding to unlock the production potential of these challenging reservoirs. Alkyl polyglucoside and sodium dodecyl diphenyl ether disulfonate were used to formulate multifunctional microemulsions specifically tailored for harsh reservoir conditions. High-rate/high-pressure fracturing-flooding technology was used to establish significant pressure gradients for enhanced oil displacement. Experimental results demonstrated that the bicontinuous microemulsions effectively cleaned the oil sands and inhibited clay swelling, thereby improving permeability. The key mechanism for improving extraheavy oil mobility involved two sequential steps: (1) the oil phase of the microemulsions reduced the extraheavy oil viscosity to below 5,000 mPa·s through dilution, followed by (2) the surfactant components subsequently emulsifying the diluted crude oil into oil-in-water droplets with viscosity <500 mPa·s. Coreflooding tests revealed that moderately elevated injection rates enhanced pore-throat connectivity and achieved higher oil recovery, whereas excessive rates reduced chemical retention time, leading to a sharp decline in ultimate oil recovery. A development strategy centred on microemulsion-assisted fracturing-flooding, comprising four key steps, was implemented in a marginal heavy oil reservoir of Sinopec Shengli Oil Field, achieving a multifold increase in crude oil production and being assessed as economically viable at oil prices above 48 USD/bbl. This milestone achievement represents a groundbreaking advancement in the development of marginal heavy oil resources under escalating carbon emission regulations.