Experimental Study on Pore Structure and Mechanical Properties of Stratified Coal

The most striking feature of liquid nitrogen (LN2) fracturing is to drastically reduce the temperature around the rock when LN2 is injected into the reservoir. The strong thermal gradient can significantly induce the internal thermal stress of the rock, which results in severe damage. In this study, the permeability, ultrasonic wave, and triaxial compression test were performed to investigate the influence of LN2 treatment on pore structure and mechanical properties of stratified coal. In the pore structure tests, the increasing range of permeability of LN2-treated samples is 100.3-149.6%, and the P-wave velocity decreases by 3.1-4.6%. In the triaxial compression tests, the compressive strength and elastic modulus of LN2-treated samples decrease by 11-39.6% and 18.2-32.1%, respectively. The experimental results indicate that LN2 treatment increases the connectivity of coal pores, improving its flow conductivity and permeability, which can effectively enhance the fracturing effect. Meanwhile, it also reduces the ability of coal to resist deformation and rupture, which contributes to the reduction of initiation pressure in the reservoir simulation. In addition, the analysis further suggests that the super low temperature thermal stress can greatly promote the generation of secondary cracks and the formation of complex fracture networks inside coal. The study provides an important reference for tight reservoir simulation of cryogenic fracturing.