Analysis of Wellbore Wall Stress for SC-CO2 Horizontal Well Fracturing in Bedded Rocks and the Influence Mechanism of Bedding Structure: Development of a Fracture Initiation Criterion and a Theoretical Prediction Model for Initiation Pressure in SC-CO2 Fracturing
摘要
The bedding structure in unconventional reservoir rocks, coupled with the unique properties of supercritical CO2 (SC-CO2), results in significant differences in fracture propagation behavior during SC-CO2 fracturing in bedded rocks compared to water-based fracturing. However, due to the absence of reliable theoretical guidance for SC-CO2 fracturing, the on-site application and promotion of SC-CO2 fracturing technology have been substantially impeded. To address this issue, this study proposes a novel tensile failure criterion for bedded rocks, termed the “Y-L criterion", and verifies its accuracy by comparing it with existing criteria. By incorporating the effects of bedding structure and the fluid characteristics of SC-CO2, a theoretical solution for the stress field at the wellbore wall during SC-CO2 horizontal well fracturing in bedded rock was derived. Based on this theoretical solution and the proposed Y-L criterion, a fracture initiation criterion and a theoretical model for predicting fracture initiation pressure in SC-CO2 fracturing were established. Experimental validation using SC-CO2 fracturing on coal samples demonstrated that the error of the theoretical prediction model was less than 10%. Further research revealed that the horizontal in-situ stress difference was negatively correlated with fracture initiation pressure, whereas injection displacement was positively correlated with it. The influence of elastic modulus orthotropy is significantly greater than that of Poisson's ratio. Additionally, the bedding dip direction determines the orientation of the maximum effective tensile stress at the wellbore wall, while the dip angle primarily affects its magnitude. This research develops a theoretical foundation for advancing SC-CO2 fracturing theory in unconventional reservoirs and provides theoretical guidance for the design and optimization of SC-CO2 fracturing schemes.