Mechanism of fracture expansion in deep shale by supercritical CO2 fracturing

AO Xiang; LAI Liangfeng; LU Zhaohui; TANG Hongkun; XIANG Zuping

doi:10.13225/j.cnki.jccs.CQ25.0890

Abstract：Deep shale reservoirs are subjected to high-temperature and high-stress conditions,under which the mechanical properties of shale gradually transform from elastic-brittle to ductile-plastic behavior,making it difficult to form complex fracture networks.Although supercritical CO2 can facilitate the development of complex fractures in shale reservoirs,its compressibility and high diffusivity render existing fracture propagation criteria inapplicable,resulting in significant dif-ferences compared to hydraulic fracturing mechanisms.In this study,the critical energy release rate under elastic-plastic shear conditions was derived based on the G criterion.By integrating the phase-field method,fluid-solid coupling prin-ciples,and the Drucker-Prager yield criterion,an elastic-plastic phase-field evolution model capable of describing the frac-ture propagation process involving CO2 phase transitions was established.The research reveals that the compressibility and high diffusivity of CO2 play key roles in fracture propagation.The energy accumulated by CO2 is released during frac-ture growth,inducing phase transitions whose energy provides an additional driving force for continuous fracture exten-sion.Meanwhile,the high diffusivity of CO2 increases pore pressure in the shale,which on one hand reduces the effective stress and lowers the breakdown pressure,and on the other hand decreases the critical tensile fracture energy,promoting fracture branching and more complex fracture patterns.The study further clarifies the influence of confining pressure on fracture propagation.Under low confining pressure,the phase transition of supercritical CO2 instantaneously releases a large amount of energy across a wider range,far exceeding the critical energy release rate and making multi-branch frac-tures more likely.As confining pressure increases,the energy released by phase transitions during CO2 fracturing de-creases nonlinearly from 1.74×108 J/m3 to 1.80×107 J/m3,and the affected area becomes progressively constrained.Simul-taneously,the increase in the critical energy release rate makes fracture extension more difficult,resulting in simpler frac-ture geometries.The contribution of phase-transition energy to fracture propagation decreases from 34.55%to 1.78%.