Casing Impairment Induced by Shear Slip Along a Weak Layer in Shale Due to Fluid (Steam) Injection
Authors: Wong R.C.K. , Chau K.T. 
Published: March 2010

Abstract
Thermal recovery processes such as cyclic steam stimulation and steam assisted gravity drainage involve large volume fluid and steam injection into oil sand reservoirs. Thermal expansion and dilatation of oil sand formation causes horizontal and vertical displacements in the overburden. These deformations may not exert detrimental effects on casings and facilities if the deformation properties of the overburden are homogeneous without drastic local variation along depth. However, if there exists some natural (clay seams) or induced weak layers (swollen and softened shale layer), shear slip could develop along these discontinuities, thereby impairing the nearby casings and facilities. This paper develops a semi-empirical analytical method for estimating the relative shear slip along a weak layer in shale due to fluid and steam injection. The magnitude of slip depends on the volume and distribution of fluid and steam injection, and the extent and location of the weak layer. Calculated results demonstrate that the shear slip along the weak layer could impose concerns regarding the steaming strategy and environmental protection.

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 in Canada may contain about 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, a large volume of steam at high temperatures and pressures (300 °C and 10 MPa) is injected into the oil sand formation at depths of 400 ? 450 m through wells placed in rows(2, 3).
Steam injection produces dilatation of an oil sand reservoir due to an increase in pore pressure and thermal expansion of the reservoir. Because oil sands are dense, uncemented sands, the resulting deformation induced during the recovery process could be excessive and detrimental to surface and subsurface facilities(4). Surface heave of up to 30 cm has been recorded(5) in some facilities. Overlaying the oil sand formation are low-permeability clay shales forming essentially an impermeable barrier to upward migration of the injected fluid. Weak layers such as clay seams and natural horizontal fissures are commonly found in these shale formations(6, 7). These weak layers retain no tensile strength or low residual strength. Wong and Chau(8) investigated the potential of propagation of these weak fractures in shale due to steam injection. They found that the propensity for occurrence of tensile and shear fracturing in discontinuous shale is low because the high overburden stresses are sufficient to limit the fracture initiation process. However, one of the potential geomechanics-related problems is if shear slip occurs along these weak layers due to fluid injection into the oil sand reservoir leading to casing impairment or rupture.
The main objectives of this study are to:investigate if such a slip mechanism is admissible in natural weak layers confined by the competent overburden shale;