Evaluation of Hydro-Mechanical Inelastic Fracture Propagation in Weak Sandstone Reservoirs

Abstract:

Although the hydro-mechanical (H-M) coupled processes have been studied extensively for hydraulic fracturing of georeservoirs, not enough focus has been given to the role of plasticity. Typical interpretation of the fracture propagation in georeservoirs includes Linear Elastic Fracture Mechanics (LEFM), which assumes that rock behaves elastically prior and during the fracture propagation. Quasi-brittle materials, such as is weak sandstone modeled in this study, exhibit certain level of plasticity under high in-situ stresses. This study uses the J-integral to evaluate an inelastic fracture propagation from a wellbore in two dimensional model. Discrete Element Method (DEM) is used because it explicitly models micro-cracks and stress-strain redistribution via flat-joint contact bond breaking. Hydro-mechanical coupling in DEM uses a scheme of interconnected fluid flow paths associated with DEM particle contacts and fluid flow reservoirs, which has been previously introduced and validated in DEM by different authors. DEM and flat-joint model have shown to accurately capture quasi-brittle rock behavior. The J-integral was measured for five different cases of far-field confinement stresses, ranging from s_h,min=5-25 MPa and s_h,min=10-35 MPa. Results indicate significant plastic portion of the J-integral, which increases with increasing confinement stresses. A plastic portion of the J-integral is approximately 5-6 times larger than the elastic portion for lower confinement, and up to 82 times larger for maximum considered confinement. However, a significant decrease in length of process zone is observed as the confining stresses increase. To conclude, the process zone size may not be uniquely related to the fracture propagation criteria. The apparent fracture toughness, K_J, calculated form the J-integral and using Youngs’ modulus (questionable for the local stress state), shows up to seven-fold increase in the fracture toughness at highest confinements, compared to elastic case. It can be concluded that at higher confinement stresses, inelastic fracture mechanics is more appropriate tool than LEFM to be used for fracture propagation criteria. Fig. 1 shows a typical model output (a), and the J-integral elastic and plastic portions obtained at different confining stresses (b).