It is generally accepted that solvent-steam injection in heavy oil/bitumen reservoirs outperforms steam only injection in terms of oil recovery rate, ultimate oil recovery and steam oil ratio. Important parameters in SA-SAGD design are solvent selection, injection strategy and solvent retention in situ. The role of geomechanics in optimum application of SA-SAGD, however, remains largely unexplored.
Recent studies suggest that solvent transport, solvent dilution effect and the temperature distribution around the edge of the steam chamber have major control over SA-SAGD performance. Solvent-steam injection significantly alters the temperature distribution around the chamber edge compared to steam only processes (SAGD). The temperature redistribution, in addition to the altered pore pressure, produces geomechanical effects such as porosity and permeability alteration, relative permeability changes and residual saturation alterations which in turn, influence solvent-steam-oil phase behavior and solvent transport and retention, among others. These chains of events affect the steam chamber growth and the solvent distribution, which affect optimum solvent selection and solvent-steam injection scenarios. These geomechanical considerations are of particular interest for bitumen deposits, both oil sands and carbonates, where chemical, thermal and fluid pressures can impose significant volume changes within the reservoir especially in shallower, lower confining stress settings.
The role of geomechanics in SA-SAGD was explored numerically using a sequentially coupled modeling approach with STARS (CMG) and FLAC (Itasca). A 2D homogenous oil sands reservoir geomodel at a shallow depth was considered for the purpose of this paper. Studies were conducted at two scales: 1) edge of steam chamber and 2) reservoir scale including underburden and overburden. The results of these numerical studies revealed geomechanics considerations directly affect the optimum solvent type selection and injection strategy during high pressure SA-SAGD processes. These studies provide valuable direction for further detailed mechanistic studies (both numerically and experimentally) and provide valuable input to the challenges of optimizing SA-SAGD processes in oil sands.