Comparisons of fracturing mechanism of tight sandstone using liquid CO2 and water

CO2 is recently applied as fracturing liquid in exploitation of tight oil and gas resources and could significantly increase the oil and gas production. However, the changes in micro-scale pore and crack and in macro-crack due to liquid CO2 fracturing are still not completely understood and need multi-scale investigation. In this study, liquid CO2 and water fracturing tests were performed on tight sandstone under different confining pressure to analyze their different fracturing mechanism. Three methods (CT scanning, 3-D fracture morphology scanning and NMR measurement) were applied to identify pore and crack characteristics of fractured samples from micro-to macro-scale. Comparing with water fracturing, liquid CO2 fracturing can significantly reduce the breakdown pressure under the same confining pressure. The breakdown pressures using both water fracturing and liquid CO2 fracturing increase with increasing confining pressure, while the increasing ratio of the latter one is larger. For liquid CO2 fracturing, CT scanning results show that fractures tend to deflect and produce secondary branched cracks in the direction of propagation, and fracture tortuosity is greater than that of water fracturing. Moreover, NMR results show water fracturing mainly produces large-scale macro-cracks, whereas both large-scale macro-cracks and micro-scale cracks could be observed for liquid CO2 fracturing. The main reason is that the liquid CO2 permeation generates more the micro-scale cracks, and induces irreversible crack initiation and preferred propagation, resulting in more tortuous fracture plane and lower breakdown pressure. The test results could improve our knowledge of CO2 fracturing in unconventional oil/gas exploitation.