Effect of Liquid Nitrogen Freezing on the Mechanical Strength and Fracture Morphology in a Deep Shale Gas Reservoir

Liquid nitrogen (LN 2 ) can dramatically deteriorate the rock and create more fractures in high-temperature reservoirs, such as deep hydrocarbon reservoirs. The present study systematically investigates the mechanical strength change and fracture morphology evolution in a deep shale gas reservoir subject to LN 2 freezing. The mineral components of three shales were measured by X-ray diffractometry (XRD). The uniaxial compressive and tensile experiments studied the mechanical strengths of shales under different LN 2 freezing parameters. Scanning electron microscope (SEM) and nuclear magnetic resonance (NMR) analyse microstructural changes. The results indicated that LN 2 freezing could effectively increase pore volume and induce bedding planes and longitudinal fractures in the deep shale at a high temperature. The freezing cycle will generate complex fracture networks at micro and macro levels. Intergranular cracks easily occur between quartz particle boundaries. The shale with fewer clay minerals is a rational target for LN 2 fracturing. Two regression models were developed to predict the uniaxial compressive and tensile strengths of deep shale after LN 2 freezing. The crucial findings are expected to provide fundamental guidance for developing deep shale resources in a feasible and environmentally acceptable way.