GRC Transactions, Vol. 40, 2016 353 Poroelastic Analysis of Natural Fracture Propagation and Coalescence Amirhossein Kamali and Ahmad Ghassemi Reservoir Geomechanics and Seismicity Group, The University of Oklahoma, Norman OK Keywords Poroelasticity, wing crack, displacement discontinuity, natural fractures, coupled hydro-mechanical ABSTRACT Propagation of natural fractures under a compressive in-situ stress state often involves mode I and mode II propaga- tion. Shear slip on the fracture surfaces initiates the so-called wing cracks which kink at approximately 70 degrees from the crack tip. To date, the literature is mostly concerned with the propagation of dry cracks under increasing compressive stresses. This, however, is not a true representation of the actual in-situ stress path since the far-field stresses remain un- changed in most petroleum, and geothermal operations. Instead, injection pressure and the associated thermo-poroelastic stresses are the main driving forces for propagation. Hence, a coupled poroelastic hydro-mechanical model is proposed to study the reservoir stimulation mechanism in naturally fractured reservoirs. The model is used to study wing crack growth considering pore pressure and poroelastic stresses. The results indicate that the injection pressure required for wing crack propagation is higher in a poroelastic rock than in an elastic one due to the back stresses caused by the pore fluid diffusion into the matrix. 1. Introduction EGS design in several field projects (e.g. Newberry, Soultz) relies on the concept of permeability enhancement by slip on the natural fractures. This concept suggests that the shear slip of the natural fractures due to water injection yields shear failure of critically-stressed fractures. Based on a number of field observations and experiments, Jung (2013) argues that the tensile fracturing and not the commonly accepted shear fracturing is the main stimulation mechanism. In this paper we explore the concept of wing crack propagation and show that this stimulation mechanism is indeed an essential part of the geothermal reservoir stimulation. Extensive research has been done on the propagation of wing cracks including the early works of Bombolakis (1973), Hoek and Bieniawksi (1984), Horii and Nemat-Nasser (1986). Both of Bombolakis (1973), Nemat-Nasser and Horii (1983), and Horii and Nemat-Nasser (1986) conducted experiments on open flaws in C.R.39 photoloelastic plates. Wing crack propagation has also been investigated numerically and analytically (see Shen and Stephansson, 1994; Bobet and Eisntein, 1998; Rao et al. 2003). It is our objective in this study to analyze wing crack propagation due to water injection in a poroelastic medium. A coupled hydro-mechanical model is developed by considering the poroelastic stresses due to pore fluid diffusion. Several sensitivity analyses are carried out to investigate the importance of some poroelastic parameters in wing crack propagation. Moreover, a comparative study is presented to highlight the differences between the elastic and the po- roelastic model.
Poroelastic Analysis of Natural Fracture Propagation and Coalescence
May 29, 2023
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