CO2 Injection-Induced Fracturing in Naturally Fractured Shale Rocks

This article delves into an in-depth investigation of gas and supercritical CO2 injection-induced fracturing in naturally fractured caprocks found in deep aquifers, depleted reservoirs, and fractured shale reservoirs. Utilizing Niobrara shale cubes sourced from Colorado, the study examines the fracturing induced when gas or supercritical CO2 is injected under tri-axial stresses. The fracturing process and resulting fracture patterns were observed through various techniques, including real-time monitoring of pressure and temperature, acoustic wave analysis, pressure decay, and fracture coloring. Interestingly, without any pore pressure, CO2-induced fracturing occurred rapidly, marked by a significant temperature drop. Fractures that were strongly bonded had minimal effect on the propagation of transverse fractures. In contrast, weakly bonded or open fractures initially impeded the fluid but eventually facilitated the creation of new fractures aligned perpendicularly to the least horizontal stress. The research also highlighted that breakdown pressures, in scenarios with pre-existing fractures, were substantially lower than anticipated, especially when supercritical CO2 was used—a result of the unconformity of the pre-existing fractures and CO2's inherent low viscosity. The study concludes that alterations in tri-axial stress levels and varying stress differences can influence breakdown pressure. These findings are crucial for addressing geomechanical challenges in CO2 geological storage and the fracturing of shale reservoirs.