Steam-stimulation processes are the viable insitu thermal recovery techniques for heavy oil and oil sand reservoirs. These thermal recovery processes introduce many complex geomechanical problems in oil sand and overburden formations. This paper addresses the geomechanical response of Colorado shale near a cased wellbore due to heating. The temperature, pore pressure and stresses response in Colorado shale near a cased wellbore due to heating are analyzed using a coupled thermal-mechanical-hydraulic solution. The possibility of failure or fracturing of the shale due to heating is assessed.
Thermal recovery processes such as cyclic steam stimulation and steam assisted gravity drainage are the viable techniques for oil recovery from oil sand reservoirs. These thermal recovery processes cause many complex geomechanical problems in the oil sand layer and the geologic overburden strata because elevated temperature and pressure are involved in steam injection.
The study in this paper focuses on the effect of heating on Colorado shale near a cased well. Analytical solutions developed by Booker and Savvidou1 are used to analyze the induced pore pressure, temperature, and stresses distributions in Colorado shale near a heated wellbore. These results will be used to investigate the potential occurrence of any tensile fracture in the heated shale. Conclusions will be drawn, along with the limitations of the analysis and recommendations for further studies.
Figure I provides a schematic sketch of the problem. Steam is injected into a cased well such that the inside temperature is elevated to a constant temperature. Heat is continually conducted away from the well into its metal casing, cement annulus and shale formation. Heating of shale causes thermal expansion of the shale structural matrix and pore fluid. Expansion of structural matrix induces total stress change. Thermal expansion of pore fluid increases the pore pressure, thereby generating pore pressure gradient and pore fluid flow. Hence, the heating process results in thermal-hydraulic- mechanical coupled process. Calculation of the temperature, pore pressure and stresses changes near the heated well will require solution to this complex coupled problem.
For a saturated thermo-elastic geological medium, the equilibrium equations are:
Equation (1.a) (Available in full paper)
Equation (1.b) (Available in full paper)
Equation (1.c) (Available in full paper)
where σxx, σyy, ..... σyz are the increase of total stress components (compressive stresses and strains are considered to be positive).
In the present study, the stiffness of the solids and pore fluid are assumed to be infinite in comparison to the shale skeleton stiffness. Thus, volume changes are due to changes in temperature and effective stresses. For an isotropic thermo-linear elastic material, the stress-strain relations are given by:
Equation (2.a) (Available in full paper)
Equation (2.b) (Available in full paper)
Equation (2.c) (Available in full paper)
Equation (2.d) (Available in full paper)
Equation (2.e) (Available in full paper)
Equation (2.f) (Available in full paper)