The Influences of CO2–Brine–Rock Interaction on Supercritical CO2 Fracturing Mechanism and Pattern in Typical Ferroan Carbonate Shale Reservoirs
摘要
During shale fracturing, interactions among CO2, brine, and rock significantly govern hydraulic fracture initiation and propagation, thereby affecting stimulation outcomes. This research elucidates the impact of CO2–brine–rock interactions on the fracturing mechanisms and failure patterns during ScCO2 fracturing in ferroan carbonate shale reservoirs. Experimental results suggest that the generated dissolution pores and microfractures weaken the mechanical properties of the rock, reducing breakthrough pressure. With decreasing rock elasticity, the failure pattern transitions from mixed tensile–shear to tensile and composite failure. Short-term reaction is dominated by the rapid dissolution of carbonate minerals, which enlarges primary pores and generates new dissolution pores, increasing total pore volume. However, increased pore surface roughness and limited microfracture development restrict ScCO2 migration. Consequently, fracture propagation, dominated by the stress field, results in a single main fracture with a simple pattern. Following the long-term reaction, intensified feldspar dissolution leads to smoother pore surfaces, along with increased macroporosity and microfracture. These changes improve pore connectivity, thereby facilitating fluid pressure transmission. Mineral dissolution also creates numerous fracture initiation points and weak planes, reducing the rock’s elastic modulus and strength while enhancing its plasticity. Consequently, fracture propagation shifts from simple double-wing patterns to complex radial networks. In addition, pre-fracturing CO2 injection enhances reservoir stimulation, with reaction duration directly governing fracture complexity. The high reactivity of ferroan carbonate shales makes them ideal for ScCO2 fracturing by promoting beneficial microstructural alterations. To accurately predict fracturing outcomes under field conditions, the micro-mechanisms revealed in this study must be integrated with dominant field-scale factors.
Highlights Explains how CO2–brine–rock interactions affect fracturing mechanisms and failure patterns in ScCO2 fracturing. Comparison of microfracture and pore size, and fractal dimension changes after reaction. Prolonged reaction shifts failure from mixed tensile–shear to tensile and composite. Dissolution pores and microfractures weaken rock and accelerate fracture initiation. Original pore expansion enhances connectivity and fracture propagation.