Insights on the fracture behavior of mudstone subjected to water and liquid CO2 fracturing

Abstract

Hydraulic fracturing stands as the predominant method for the commercial development of reservoirs. However, maintaining the integrity of the mudstone caprock during fracturing is critical for ensuring the long-term stability of reservoir production. In this case, we conducted fracturing tests on downhole mudstone cores using both water and L-CO₂. Advanced techniques, including micro-computed tomography (μCT), nuclear magnetic resonance (NMR), and scanning electron microscopy (SEM), were utilized to characterize the differences in fracture response between the two methods and to further elucidate the fracturing mechanisms of mudstone. The findings reveal that the breakdown pressure in L-CO₂ fracturing was consistently lower than that in water fracturing, with both being lower than theoretical predictions. In water fracturing, the injection pressure curve showed pronounced fluctuations during the failure phase. The induced cracks exhibited a stepped morphology macroscopically and sliding traces microscopically. This phenomenon is attributed to the infiltration of water into the layered particles of the mudstone, which weakens inter-particle bonds and creates numerous micro-pores, leading to a multi-scale cracking feature. In contrast, the perturbation of pore pressure in L-CO₂ fracturing contributed to the cracking along the weak plane, with a predominant in microscopic feature. The insights gained from this research are invaluable for the optimization of field-scale fracturing operations.