Interaction Mechanisms between Artificial and Natural Fractures during Supercritical CO 2 Fracturing in Coal and the Conditions Governing Natural Fracture Activation

In fracturing stimulation operations for unconventional reservoirs, the primary objective is to establish effective communication between artificial fractures (AFs) and natural fractures (NFs) to enhance hydrocarbon migration. A comprehensive understanding of the interaction mechanisms between AFs and NFs─particularly the activation behavior of NFs and the mechanical conditions governing fracture network formation─is essential for optimizing fracturing treatment design. This study combines physical experiments with numerical simulations to systematically investigate the mechanisms of interaction between artificial and natural fractures in coal under supercritical CO2 (SC-CO2) fracturing conditions. It identifies key factors influencing fracture initiation pressure and elucidates the activation conditions of NFs induced by AFs across a range of approach angles and horizontal stress differences (HSDs), along with the associated stress evolution characteristics. Three distinct NF activation states are identified based on the approach angle. By introducing the concept of “activation rate” as a quantitative metric to evaluate NF activation extent, the study systematically evaluates the influences of HSD and approach angle on this parameter. Based on these findings, the mechanical conditions required for connecting AFs and NFs to form an interconnected fracture network are clearly identified. The results indicate that under SC-CO2 fracturing conditions in coal specimens, NF activation can be categorized into three distinct modes according to the approach angle: semilength activation, partial or local activation, and minimal or no activation. The maximum principal stress primarily governs tensile-induced NF activation, whereas shear stress controls shear-induced activation. The activation rate of NFs follows an approximately normal distribution with variations in the approach angle; as the HSD increases, the sensitivity of the activation rate to changes in the approach angle significantly increases. This work provides a theoretical foundation for understanding the mechanical mechanisms of AF-induced NF activation in reservoir rocks under SC-CO2 conditions.