Propagation of In Situ Horizontal Fractures in Shale Due to Steam Injection

Abstract Thermal recovery processes such as cyclic steam stimulation and steam assisted gravity drainage involve steam injection of large volume into oil sand formation. Dilatation of oil sand formation due to steam injection induces stresses and deformations in the shale overburden. Natural horizontal fissures or fractures of large extent with no tensile and cohesive strength are commonly found in the shale formation. New fractures will be initiated at these in-situ horizontal fracture tips if the stresses and deformation induced by the steam injection are excessive. Reactivation and propagation of these in-situ fractures in the shale formation could cause not only casing impairment problem but also environmental hazard. This paper presents an analytical model to investigate the propensity for fracture propagation in shale due to steam injection under tensile (mode I) and shear (model II) fracturing. Practical numerical examples are presented along with risk assessment and implications. Introduction Heavy oil and bitumen are major energy resources in Canada, and have become a major focus for the oil and gas industry. The heavy oil deposits of Canada contain 30% of the world's recoverable oil (ERCB 1991). The challenge in developing this resource is the difficulty in recovering these highly viscous substances. Steam stimulation is one of the viable thermal recovery methods to extract bitumen from the oil sand ores buried in deep overburden. In this steam-based recovery method, large volumes of steam at high temperatures and pressures (300 °C and 10 MPa) are injected into the oil sand formation at depths of 450–500 m through wells in rows (Butler 1991). Steam injection produces stress and thermal dilatation of oil sand. Because oil sands are dense uncemented sands, the resulting deformation induced during the recovery processes could be excessive and detrimental to surface and subsurface facilities (Morgenstern et al. 1988). Surface heave of up to 20 cm was recorded (Wang and Kry 1997). Overlain the oil sand formation are lowpermeability clay shales forming an impermeable barrier to migration of fluid. Natural horizontal fissures or fractures are commonly found in these shale formations (Williams and Burk 1970; Wong 1998). These fractures have been formed or subjected to large shearing process during glaciation. Therefore, there is no tensile strength or small residual strength remained along these fractures. One of the geomechanics-related problems is if these preexisting fractures would propagate under steam injection. Interception of these fractures with subsurface well casing could lead to casing impairment or rupture (see Fig. 1). The main objective of the present paper is to evaluate the potential of initiation and propagation of fractures in shales induced by cyclic steam stimulation in oil sand reservoir using a simple analytical model (Chau et al. 2000). This model considers analytically the stress intensity factors at a crack parallel to the free surface of a half-plane under the action of a center of dilatation. It is proposed that the introduction of steam to the oil sand formation can be modelled by introducing a centre of dilatation (dipole) in a two-dimensional homogenous isotropic elastic half-plane (see Fig. 1).

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