Abstract
Cryogenic fracturing using liquid nitrogen (LN2) is a waterless fracturing technology that is under extensive research nowadays given the high demand of environmental-friendly development of unconventional oil and gas resources. Excluding the exerted hydraulic pressure, the initiation and propagation of cryogenic fractures in rock materials largely depend on thermal stress generated by LN2 and the rock properties. So far, there are quite a few experimental studies elaborating on the cryogenic fracturing process of coal rocks; however, modeling studies on the topic are still very preliminary with no damage quantification. This study first set up a coupled thermo-mechanical (TM) modeling procedure by solving the heat conduction and rock mechanics equations in a serial manner and incorporating the maximum principal stress failure criterion. Then the numerical stability of this modeling procedure was examined through time step and grid resolution tests, and the model accuracy was corroborated by analytical solutions. Furthermore, the damage criterion was quantitatively validated against our experimental observations. Using this thermo-mechanical damage (TMD) modeling procedure, a systematic examination of the effects of key factors on cryogenic circulation treatment of coal rock borehole was carried out using realistic parameters. Trends and patterns of the cryogenic damage upon key factors, including realistic temperature boundary with the Leidenfrost effect, Young's modulus, thermal expansion coefficient, Poisson's ratio, specific heat capacity, density, thermal conductivity, in-situ stresses, and initial temperature of the coal rocks have been revealed and discussed. The modeling results herein are of great significance to understand the mechanisms of cryogenic fracturing of coal rocks, and this TMD modeling procedure could be further expanded to evaluate and optimize cryogenic treatment process of coal seams in field pilot tests.
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