Experimental study of the effect of liquid nitrogen pretreatment on shale fracability

When cryogenic liquid nitrogen (LN2) comes into contact with deep reservoirs, it will bring about serious thermal damage to rock pore structure. Therefore, LN2 pretreatment on reservoirs can contribute to further improving the stimulated reservoir volume (SRV) during large-scale hydraulic fracturing. To investigate the effect of LN2 pretreatment on the SRV of shale reservoirs, a series of physical and mechanical experiments related to shale reservoir fracability were conducted. A quantitative fracability evaluation model suitable for different confining pressures was developed from the perspective of fracturing mechanisms. The changes in shale fracability due to LN2 pretreatment were discussed in detail. The responses in P-wave velocities and T2 spectrum curves demonstrate that shale pore structure is subjected to serious damage after freeze-thaw cycles, and some micro-pores are aggregated to form micro-cracks or even macro-cracks. The decreases in tensile, compressive and shear strength and elastic modulus show that the mechanical properties of shales are deteriorated due to cryogenic damage effect. The changes in physical and mechanical properties of shales reveal respectively that LN2 pretreatment can lead to a slight reduction in shale brittleness, a significant improvement in micro-crack development, a moderate decrease in stability of the bedding planes, a relative increase in the propagation ability of fractures. Based on the result of the analytic hierarchy process, it can be concluded that shale fracability is efficiently enhanced due to LN2 pretreatment, especially under high confining pressure condition. Finally, the hydraulic fracturing experiments under tri-axial stress were carried out to further validate the feasibility of LN2 pretreatment to improve the SRV of shale reservoirs. Compared with the experimental results without LN2 pretreatment, it is found that the initiation pressure and initiation time decline by about 54% and 60% respectively. Both the size and complexity of the SRV of shales are enhanced obviously.