A Study of Variation in the Initiation Pressure and Fracture Distribution Patterns of Raw Coal in SC-CO2 Fracturing Under the True Tri-axial System

Supercritical CO2 (SC-CO2) fracturing is a water-less fracturing technology that has attracted increasing attention in the coalbed methane mining industry. However, the mechanisms of fracture initiation and propagation have not been studied in detail. In this study, SC-CO2 and hydraulic fracturing of coal was conducted using a true tri-axial testing system. The change in injection pressure and in fracture morphology, both before and after fracturing, was analyzed and results presented. In comparison to hydraulic fracturing, using SC-CO2 as a fracturing fluid reduces the pressure required to initiate fractures by approximately 32-41%. This reduction is due to the increased percolation and pore pressure effects arising from the use of SC-CO2. The low viscosity and high diffusivity of SC-CO2 in conjunction with the highly developed pore/fracture of coal result in the formation of a complex fracture network. The fractal dimension is positively correlated with the injection rate and temperature during SC-CO2 fracturing, indicating that using SC-CO2 as fracture fluid under high injection flows and temperature can achieve increased Coalbed methane production.

Automated summary

This study conducted experiments on the fracturing of coal using a true tri-axial testing system with supercritical CO2 (SC-CO2) as the fracturing fluid. The results showed that compared to hydraulic fracturing, SC-CO2 reduces the pressure required to initiate fractures by approximately 32-41%. This reduction is attributed to the increased percolation and pore pressure effects of SC-CO2, which in combination with the highly developed pore/fracture structure of coal, leads to the formation of a complex fracture network. The fractal dimension is positively correlated with the injection rate and temperature during SC-CO2 fracturing, indicating the potential for increased coalbed methane production under high injection flows and temperature.