Propagation of In Situ Horizontal Fractures in Shale Due to Steam Injection
Authors: Wong R.C.K. , Chau K.T. 
Published: March 2010

Thermal recovery processes, such as cyclic steam stimulation and steam assisted gravity drainage, involve the injection of large volumes of steam into oil sand formations. Dilatation of oil sand formations due to steam injection induces stresses and deformations in the shale overburden. Natural horizontal fissures or extensive fractures 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 not only cause casing impairment problems, but could also cause an 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 a 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(1). 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(2, 3).

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(4). Surface heave of up to 20 cm has been recorded(5). Overlying the oil sand formation are low-permeability clay shales forming an impermeable barrier against the migration of fluid. Natural horizontal fissures or fractures are commonly found in these shale formations(6, 7). These fractures have been formed or been subjected to a large shearing process during glaciation. Therefore, there is no tensile strength or small residual strength remaining along these fractures. One of the geomechanical-related problems is the question of whether these pre-existing fractures will propagate under steam injection. Intercepting these fractures with subsurface well casings could lead to casing impairment or rupture (see Figure 1).

The main objective of the present paper is to evaluate the potential of initiation and propagation of in situ fractures in shales induced by cyclic steam stimulation in an oil sand reservoir using a simple analytical model(8). This model analytically considers the stress intensity factors at a crack parallel to the free surface of a half-plane under the action of a centre of dilatation. It is proposed that the introduction of steam in the oil sand formation can be modelled by introducing a centre of dilatation (dipole) in a two-dimensional homogenous isotropic elastic half-plane (see Figure 1).