Numerical Modeling of Cracking Behaviors of Coal Reservoirs Subjected to Cryogenic Shock

ABSTRACT Thermally-induced cracking has attracted extensive attention in improving reservoir permeability. In this paper, a thermo-elastic coupling model incorporating the strain-based elastic-brittle damage theory is used to analyze the cracking behaviors of the coal reservoir subjected to cryogenic liquid nitrogen shock. The evolution of temperature and thermally-induced stress with damage is analyzed. The effect of different factors including in-situ stress difference, elastic modulus, thermal expansion coefficient, thermal conductivity and quenching temperature on the induced crack morphology is investigated. It is found that the failure mechanism of the coal rock during cryogenic shock is mainly dominated by elastic brittle tensile damage. The induced fracture morphology is more sensitive to elastic modulus and thermal expansion coefficient relative to in-situ stress difference and thermal conductivity. The increases in elastic modulus and thermal expansion coefficient will bring about more fractures with greater complexity. The higher in-situ stress difference or lower thermal conductivity can generate more short thermal fractures. The critical quenching temperature for inducing thermal cracks around the wellbore is between −110 °C and −105 °C. The results of this study can provide some guidelines for cryogenic fracturing in coal reservoirs. INTRODUCTION Coalbed methane (CBM) reservoirs have the characteristics of low permeability and low porosity. At present, hydraulic fracturing is still the most widely used stimulation technology to improve the overall permeability of unconventional oil and gas reservoirs, including CBM, shale gas and tight gas (Montgomery et al., 2010; Kumari et al., 2018; Ma et al., 2021). However, hydraulic fracturing will bring about a series of thorny problems, such as high fracture initiation pressure in hard formation, pore plugging and water locking effect in water sensitive formation, serious waste of water resources, treatment of flowback fluid and environmental pollution. Compared with hydraulic fracturing, cryogenic liquid nitrogen (LN2) waterless fracturing can overcome these negative problems (Gregory et al., 2011; Rozell et al., 2012, Hung et al., 2020). The representative advantage of LN2 fracturing is that the low temperature LN2 with a boiling point of −195.8 °C produces a huge temperature difference in the rock (Jacobsen et al., 1986), resulting in tens of MPa of thermal stress. In addition, hundreds of times of the gas-liquid ratio of nitrogen also plays an important role in rock cracking. However, because of its high risk of vaporization, it is necessary to control the pressure in the wellbore through the safety valve, thus weakening the effect of nitrogen expansion-induced fracturing (Cha et al., 2018).