Characteristics of the Fracture Process Zone for Reservoir Rock with Various Heterogeneity

Abstract

Hydraulic fracturing for oil-gas and geothermal reservoir stimulation is closely related to the propagation of Mode I crack. Nonlinear deformation due to rock heterogeneity occurs at such crack tips, which causes the fracture process zone (FPZ) to form before the crack propagates unsteadily. However, the relationship between the FPZ characteristics and rock heterogeneity still remains elusive. We used three rock types common in reservoirs for experimental investigation, and each of them includes two subtypes with different heterogeneity due to grain size or microstructural characteristics. Drawing on the experiment results, we calculated the FPZ size (represented by the radius of an assumed circular FPZ) in each cracked chevron-notched Brazilian disk, and we reproduced the formation process of the FPZ in marble using the discrete element method. We showed that strong heterogeneity is favorable to large FPZ size, can enhance the ability of crack generation and make crack morphology complex. Coupling the Weibull distribution with fracture mechanics, the dependence of the FPZ size on heterogeneity degree can be theoretically explained, which suggests that the inherent heterogeneity of rocks sets the physical foundation for formation of FPZs. These findings can improve our recognition of propagation mechanisms of Mode I cracking and provide useful guidelines for evaluating reservoir fracability.

An Evaluation Index of Fracability for Reservoir Rocks Based on Fracture Process Zone

“…Firstly, this index has a firm physical basis derived from the FPZ size, representing the nonlinearity of deformation due to micro-crack generation before the macroscopic propagation of the crack. Additionally, the formation of the FPZ is the inherent mechanical behavior of heterogeneous rock materials, and the FPZ size highly depends on the degree of heterogeneity as the previous [ 27 ] and present experiment results revealed. From the aspect of practicability, the crack tolerance value can be determined conveniently in the laboratory because BD and CCNBD tests are very common rock mechanical tests, and the small-size specimens they use can be easily obtained from cores.…”

“…Such a model takes the mineral grain texture into account ( Figure 8 a), making a simulation more vivid. The modeling method for GBM of marble refers to [ 27 ]. Soft-bonded [ 30 ] and smooth-joint models [ 31 ] were employed to express bonded and unbonded behaviors characteristic of intra-grain and inter-grain contacts ( Figure 8 b–d), respectively.…”

“…Before conducting the simulation of the CCNBD tests, the microscale parameters of the particles and contacts required iteratively calibrating through trial and error referring to the BD test results and the previous work [ 27 , 33 , 34 ]. The GBMs of the BD specimens were positioned between two stiff walls representing the loading end and platform of a compression machine, and the walls moved toward each other at the same constant velocity to result in a quasi-static loading rate.…”