Experimental study on the oil recovery characteristics of CO2 energetic fracturing in ultralow-permeability sandstone reservoirs utilizing nuclear magnetic resonance

Abstract

CO2 energetic fracturing is effective for ultralow-permeability sandstone reservoirs, but research on fractures and CO2 effects at the pore scale is lacking. This study examines their impacts on oil recovery, mobilization sites, residual distribution, and oil mobility limits across pore types. This paper analyzed mineral composition and pore structure of Changji Oilfield cores via scanning electron microscopy (SEM) and X-ray diffraction (XRD). Accurate pore size distribution was determined by fitting mercury intrusion porosimetry (MIP) and nuclear magnetic resonance (NMR) curves using power functions. Fracture and CO2 effects on oil recovery characteristics across pore types were investigated using four sets of NMR experiments: depletion, CO2 energetic depletion, fractured depletion, and CO2 energetic fracturing depletion. The results show that Changji Oilfield cores are mainly composed of quartz (approximately 70 %) and contain clay minerals (about 10.2 %). The cores exhibit intergranular pores and curved lamellar throats. Pore types are categorized into micropores, macropores, and fractures, with diameters ranging from 10 to 100 nm, 0.1–5 μm, and 5–50 μm, respectively. The presence of CO2 reduces the lower limit of oil mobility to 10 nm compared to 30 nm in its absence, mainly by promoting oil expansion. CO2 primarily enhances oil recovery in micropores, while fractures increases recovery in macropores, mobilizing nearby oil and leaving minimal residual oil. In contrast, CO2 mobilizes most of the crude oil across the entire core, resulting in high residual oil content in mobilized areas. CO2 energetic fracturing combines benefits of fractures and CO2, achieving a 24.93 % final oil recovery rate. These findings underscore the importance of this technique for effective development strategies in ultralow-permeability sandstone reservoirs.