Steam stimulation is one of the viable methods in extracting heavy oil from oil sand reservoirs. In this thermal process, the injection well is subjected to high temperature heating. Heat is conducted from the injection well through metal casing, grout cement annulus, and surrounding geological formation. In lowpermeability and water saturated formations such as clay shale, high fluid pore pressure can be induced in the formation due to heating. Previous studies indicate that tensile fracturing could occur if the rate of increasing pore pressure is higher than that of overburden stresses. Development of such tensile fracturing disrupts the hydraulic integrity of the shale formation, thereby resulting in casing impairment and environmental concern. This study investigates possible steaming strategies at early cycles to reduce the risk of causing tensile fracturing around the thermal well. Finite element methods were used to simulate the thermalhydraulic- geomechnical process around the injection well. Results based on parametric studies are presented along with practical implications.
When fluid-saturated porous medium is heated, both the pore fluid and the solid matrix expand. Due to the higher thermal expansion of fluid (Butler 1986), excess pore pressure is induced in this process, which may lead to the development of effective tensile stress and fracturing or failure of structure.
Several elaborate mathematical equations for thermoporoelasticity (e.g., Booker and Savvidou 1984; Wang and Papamichos 1994; Wong and Samieh 1997) have been developed since Schiffman's work (1971). For coupled thermalhydraulic- mechanical process, there are two characteristic time scales related to the time response of rock media. One is related to the thermal energy or temperature diffusion. Another is related to the pore pressure buildup, which may be induced by thermal effects.
When the time-scale of pore pressure diffusion is much greater than that of thermal diffusion, the time response of rock is mainly controlled by its permeability and bulk compressibility.
For one-dimensional diffusion, the pore pressure response is governed by: (equations (1)) (Available in full paper)
where k is the rock permeability, μ is porous fluid viscosity, and p C is bulk rock compressibility. Thus the time-scale (figure) (Available in full paper) of pore pressure diffusion in porous rock is estimated as: (equations (2a)) (Available in full paper)
where h is the length of grid. The time scale for one-dimensional heat conduction is similar to that in Eq. (2a), except that the hydraulic diffusivity term (Dp) is replaced by the thermal diffusivity term (Dt): (equations (2b)) (Available in full paper)
where k, c are average thermal conductivity and capacity of the system, respectively.
For low-permeability shale, the hydraulic diffusivity is much lower than the thermal diffusivity (Table 1). The pore pressure buildup could be excessive causing shear failure or hydraulic fracturing in shale.
In this paper, we firstly recast the mathematical framework for coupled thermal-hydraulic-geomechnical process, and attempt to reach a comprehensive understanding of its response, especially the excess pore pressure and effective stress of structure shale around thermal well under steam injection. Secondly, finite element methods for coupled governing equations are developed.
