Abstract

Steam-assisted gravity drainage (SAGD) is one successful thermal recovery technique applied in the Albertan oil sand reservoirs. When considering in situ production from bitumen reservoirs, bitumen viscosity must be reduced to achieve the mobility required to flow toward the production well. Steam injection is currently the most promising thermal recovery method. While steam flooding techniques have proved to be commercially viable methods to extract bitumen from bitumen reservoirs, caprock integrity and the risk of losing steam containment can be a challenging operational problem. Since permeability is low in Albertan thermal project caprock formations the water trapped in pores undergo large pressure increases during heating. In addition, water undergoes a large volume increase as it flashes to steam and the resultant pore pressure causing profound effective stress reduction. Once this condition is established, pore pressure increases can lead to shear failure of the caprock, and to subsequent caprock integrity failure or casing failure. It is typically believed that low permeability caprocks impede the transmission of pore pressure from the reservoir, making them more resistant to shear failure (Collins, 2005, 2007). In cases of "thermo-hydro-mechanical pressurization", low permeability caprocks are not always more resistant. As the steam chamber rises into the caprock, the heated pore fluids may flash to steam. Consequently, there is a vapour region between the steam chamber interface penetrated into the caprock and water region within the caprock which is still at subcritical state.

This study develops the fluid mass and thermal energy conservation equations and presents analytical solution to evaluate the thermo-hydro-mechanical pressurization in low permeability caprocks and flow of steam and water after initiation of steam injection in SAGD process. Both short-term and long-term response are calculated. The evaluated thermal pressurization is compared for shallow and deep oil sand reservoirs, for similar transport properties. These results showed that the thermo-hydro-mechanical pressurization is larger in deep reservoirs; SAGD application can cause high pore-pressure and potential shear with the caprock resulting steam releases to surface and casing failures.

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