ABSTRACT: Stimulation mechanisms in unconventional geothermal and petroleum reservoirs are poorly understood. Permeability enhancement via shear slip is commonly accepted as the main stimulation mechanism. On the surface, this appears to exclude the propagation in tensile and shear mode of the natural fractures that experience slip. The misconception, has led to some even claim the discovery of a new stimulation mechanism via the formation of wing cracks. But, wing cracks are an integral part of the shear slip stimulation mechanism because shear slip increases the stress-intensity at the fracture tips, potentially leading to fracture propagation. This is particularly the case when natural fractures are subjected to direct fluid injection. On the other hand, natural fractures subjected to pore pressure through diffusion in the rock matrix could respond differently during stimulation. In an effort to better understand the impact of pore pressure and poroelastic stresses on the stimulation of naturally fractured reservoirs, a poroelastic displacement discontinuity model is developed and used in this study to illustrate various stimulation mechanisms. Mohr-Coulomb contact elements are used to represent pre-existing natural fractures. Our results indicate that natural fracture propagation is less likely to occur when water is injected in the rock matrix outside the natural fractures. Moreover, the orientation of natural fractures with respect to the wellbore is found to significantly impact their response. We also observe local destabilization and non-uniform distribution of shear deformation which is usually neglected or greatly simplified in other models.
Unconventional resources are produced by reservoir stimulation by water injection. The pore pressure change due to injection/production can trigger shearing on critically-stressed natural fractures. Permeability enhancement through shear slip is a commonly accepted stimulation mechanism (Pine and Batchelor, 1984; Murphy et al. 1999). At times, this has been misunderstood to exclude the possibility of slip induced fracture propagation leading to the suggestion that a new and different stimulation mechanism is operating. In fact, some have even claimed the discovery of a new stimulation mechanism via the formation of wing cracks (McClure and Horne, 2013). But, the formation of wing cracks is an integral part of shear slip stimulation mechanism when fracture shear slip increases the stress-intensity at the fracture tips which can cause fracture propagation. It was this understanding that motivated the development of mixed mode fracture propagation models for geothermal reservoir development (Huang et al., 2013; Min et al., 2010). This is particularly the case when natural fractures are directly subjected to fluid injection at pressure below the minimum in-situ stress. Several studies have addressed the possibility of natural fracture propagation and coalescence as a stimulation mechanism (Min et al., 2010; Huang et al., 2013; Jung, 2013; Kamali and Ghassemi, 2016a; Kamali and Ghassemi, 2016b). Jung treated the subject analytically and provided field evidence for wing crack formation but did not consider the possibility of propagation in the “shear” mode. The wing crack is a well-established concept (Hoek and Bieniawksi, 1984; Horri and Nemat-Nasser, 1986; Shen and Stephansson, 1994; Rao et al., 2003), describing the extension in tension or shear mode of mechanically-closed cracks under applied compressive stresses. Kamali and Ghassemi (2016) have explicitly simulated the phenomenon during injection and have shown shear slip occurs at injection pressures below the minimum in-situ stress and triggers the out-ofplane wing cracks (Min et al., 2010; Huang et al., 2013; Kamali and Ghassemi, 2016a). Further propagation and coalescence might be achieved by maintaining the injection at pressures slightly higher than the minimum in-situ stress.