Further investigation of CO2 energization fracturing in shale reservoir- from microscopic mechanism to field application

CO2 energization fracturing technology offers a novel approach to enhancing both carbon capture efficiency and hydrocarbon recovery in shale reservoirs. Despite its potential, the effects of different injection sequences of water-based fracturing fluids and CO2 remain unclear. To address this, we conducted experimental studies using nuclear magnetic resonance (NMR) and computed tomography (CT) imaging to investigate the fracture characteristics and microscopic oil recovery associated with different injection sequences. Additionally, field-scale numerical simulations were performed based on existing fracturing operations. Compared to CO2 postfracturing methods, CO2 pre-fracturing achieves superior oil recovery, particularly by mobilizing oil in micropores, while both methods exhibited similar recovery in macropores. Meanwhile, NMR and CT results revealed that fractures generated by CO2 pre-fracturing maintained higher conductivity under closure conditions. Numerical simulations further demonstrated that CO2 post-fracturing led to a more significant initial increase in reservoir pressure, resulting in higher short-term production. However, CO2 pre-fracturing promoted the development of a larger stimulated reservoir volume (SRV), achieved higher CO2 sequestration efficiency, and provided better long-term reservoir pressure maintenance, leading to higher oil production. The Enhanced oil recovery (EOR) mechanism of CO2 pre-was also evaluated. The increased permeability of branch fractures contributed most significantly to oil production, followed by pressure enhancement, larger SRV, and crude oil modification. Field test results confirmed that CO2 pre-injection generated more fractures and increased oil production by 3-7 times, underscoring its promise as a highly effective fracturing technology.