SAGD is a commercially proven technology for Athabasca reservoirs. It produces high oil rates and high ultimate recoveries. Injecting a solvent with the steam can reduce required steam rates and/or improve oil rates and recovery. Oil viscosity is reduced both thermally and by solvent dissolution.
A scaled model experiment was performed at the Alberta Research Council to examine the effectiveness of the Expanding Solvent-SAGD (ES-SAGD) process at a low pressure (1,500 kPa), which was still high enough to allow sufficient drive to transport the produced fluids to the surface at typical Athabasca formation depths. A lower SAGD pressure typically results in reduced oil production as a result of the correspondingly lower steam temperature. However, a lower operating pressure can also result in a reduced SOR because of lower steam density (lower mass of steam required to fill a specific volume of the steam chamber) and also a higher heat of vaporization, which allows more heat to be transferred to the reservoir when a given mass of steam is condensed. At lower pressures, the steam saturation temperature changes more quickly with saturation pressure than it does at higher pressures. Thus, at low pressure, a small reduction in pressure can lead to steam flashing in the producer and create unstable conditions.
This experiment was history matched and then a parametric investigation was performed based on field scale numerical simulations using CMG STARS. The same solvent (gas condensate) and solvent concentration (9.3 volume%) were used in the lab and field scale simulations. The condensate had many components, which were represented in the numerical simulations by four pseudo-solvent components.
2-D and 3-D field scale simulations examined the effect of: operating pressure, injection rate, sub-cool, oil and gas phase diffusion and dispersion, live oil versus dead oil performance, use of pressure drawdown when oil rates have declined, and compared low pressure ES-SAGD to low pressure SAGD.
The simulations indicated that the effects of production pressure, sub-cool, and solvent concentration must be considered simultaneously as they impact each other. At 1,500 kPaa production pressure and 10 °C sub-cool, co-injection of solvent with steam increased the average oil rate by 15% while reducing the SOR as compared to SAGD at the same operating pressure.