Cryogenic liquid nitrogen stimulation-induced cracking in gas-enriched shale reservoirs: A numerical investigation.

• The trans -scale cracking process of the shale reservoir stimulated by LN 2 is modeled. • The mechanism of cryogenic stimulation-induced multiple fractures is revealed. • Fracture initiation pressure and fracture morphology under different factors are analyzed. • Preferable candidates for LN 2 stimulation in shale reservoirs are recommended. • The stimulation effects of three fracturing fluids, LN 2 , H 2 O and S-CO 2 , are compared. Cryogenic shock-induced brittle cracking has promising application prospects in reservoir stimulation. In this paper, a fully coupled thermo-hydro-mechanical (T-H-M) model combined with the statistical meso -damage theory is proposed to investigate the trans -scale progressive cracking process of shale gas reservoirs stimulated by liquid nitrogen (LN 2). In this theoretical framework, shale meso -heterogeneity in both thermo-mechanical and hydraulic properties is characterized by two independent statistical distribution functions; temperature-dependent rock properties and temperature- and pressure-dependent fluid thermodynamic properties are described using some experimental and theoretical formulas. The numerical model is implemented in a finite element context and subsequently validated against existing classical coupling results. Numerical results demonstrate that LN 2 fracturing accompanied by the unique cryogenic shock effect leads to a complex cracking process involving multiple-point fracture initiation, asymmetric macrofracture propagation, and primary fracture bifurcation. Sensitivity analysis indicates that approximately equidistant or equilength radial fractures may be generated at a lower stress difference, lower permeability, or higher elastic modulus; both fracture initiation pressure and induced fracture morphology are more sensitive to injected fluid temperature relative to initial reservoir temperature; a larger thermal expansion coefficient is beneficial for inducing more fractures, but not applicable to the thermal conductivity. Preferable candidates for LN 2 stimulation to generate multiple fractures in shale reservoirs are recommended. It is also found that compared with H 2 O and supercritical CO 2 , fracturing using LN 2 can generate fractures with lower initiation pressure and greater complexity. Preliminary results confirm the potential of using LN 2 fracturing to improve the stimulation effect of shale reservoirs. [ABSTRACT FROM AUTHOR]