In–Situ Dilation Affects Solvent–Assisted Steam–Assisted–Gravity–Drainage Performance: The Case of a Shallow Athabasca–Type Oil–Sands Reservoir

Summary 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 (SOR). Important parameters in the design of solvent-assisted steam-assisted-gravity-drainage (SA-SAGD) are solvent selection, injection strategy, and solvent retention in situ. The role of geomechanics in optimal 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. In SA-SAGD, elevated temperature and solvent concentrations within a few meters of the steam-chamber edge reduce the virgin-oil viscosity, causing oil drainage. However, this is the same region where geomechanically induced volume changes alter porosity, permeability, and relative permeability profiles. These alterations could improve the convective-heat transfer and solvent dispersion into the cold bitumen zone, and could also enhance the drainage rate. Consequently, the solvent/oil-phase behavior, steam-chamber growth, and solvent retention and distribution will be affected. This chain of events could have an effect on optimal 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 by use of a sequentially coupled modeling approach with STARS (CMG 2015a) and FLAC (Itasca 2016). A 2D shallow-depth homogeneous-oil-sands-reservoir geomodel, with properties similar to the Underground Test Facility (UTF) Phase A project, was constructed (Edmunds et al. 1994). Studies were conducted at two scales: the edge of the steam chamber and the reservoir scale including underburden and overburden. The results of these numerical studies revealed that geomechanics considerations directly affect the optimal solvent-type selection and injection strategy during a high-pressure SA-SAGD process. 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.