Steam-assisted gravity drainage (SAGD) is a promising approach for recovering heavy and viscous oil resources. In SAGD, two closely-spaced horizontal wells, one above the other, form a steam-injector and producer pair. The reservoir oil is heated by the injected steam and drains to the producer under the effect of gravity. The success of steam-assisted gravity drainage has been demonstrated by both field and laboratory studies mostly based on homogeneous reservoirs and reservoir models. A comprehen- sive understanding of the effects of reservoir heterogeneities on SAGD performance, however, is required for wider and more successful implementation. This dissertation presents an investigation of the effects of reservoir heterogeneities on SAGD. In addi- tion, two potential methods, hydraulic fracturing and mobility control using foamed steam, are proposed and reported here to enhance SAGD performance, especially for heterogeneous reservoirs.
Reservoir simulations of SAGD are conducted with a number of realizations of Athabasca-type oilsand reservoirs that contain randomly-distributed shales geosta- tistically generated with a stochastic model. We interpret the complex effects of reservoir heterogeneities by identifying two flow regions, the near well region (NWR) and the above well region (AWR). Our simulations indicate that the drainage flow of hot fluids within the NWR, characterized by short flow length, is very sensitive to the presence of shale, whereas the expansion of the steam chamber in the AWR, characterized by long flow length, is affected adversely only when the AWR contains
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long, continuous shale or a high fraction of shale. Vertical hydraulic fractures are found to improve steam chamber development considerably for reservoirs with poor vertical communication. For the synthetic reservoir under study, an increase in the oil production rate by a factor of two and considerable improvement of energy efficiency with the cumulative oil-steam ratio lifted from 0.2 to 0.3 bbl oil/bbl CWE steam are achieved by adding a vertical fracture.
The new concept of foam-assisted SAGD (FA-SAGD) is evaluated numerically with a foam simulator that incorporates the physical mechanisms of foam generation, destruction, and transport. To reduce computational costs, we develop a simplified foam model based on the assumption of local equilibrium of foam generation and co- alescence at field scale. Foam displacements in a linear sandstone core are measured using pressure transducers, X-ray Computed Tomography (CT), and a visualization cell to quantify foam bubble texture. The local equilibrium approximation is vali- dated, and good agreement between the experimental results and the predictions of the simplified model is found, with a minor mismatch in the entrance region. For the scenario under study, numerical simulation of the FA-SAGD process shows consider- able improvement in the process efficiency over the conventional SAGD process. Live steam production is reduced by a factor of 5 for FA-SAGD compared to conventional SAGD. Consequently, cumulative oil production is increased by about 30% when pro- duction versus the volume of steam injected is compared for cases with and without foam.