On Subcool Control in the SAGD Producers—Part III: Efficiency of Subcool Trapping in the Nsolv Process

Steam-assisted gravity drainage (SAGD) is currently the preferred thermal-recovery method used to produce bitumen from Athabasca deposits in Alberta, Canada. SAGD is, however, an energy-intensive process with large amounts of greenhouse-gas (GHG) emissions and required water treatment. One option to reduce emissions and water usage is with solvent-based techniques, such as the Nsolv™ process. Suncor and Nsolv have been working together on a bitumen-extraction solvent-technology (BEST) field demonstration at Suncor's Dover test site. The solvent-injection–produced-oil ratio (SvOR) is among the key performance indicators (KPIs) of the BEST facility. Solvent breakthrough caused by inefficiency of thermodynamic trapping, such as subcool trapping, contributes to SvOR and affects the facility's economic and artificial-lift efficiencies. Subcool is the temperature difference between the injected butane (at saturated condition) and produced fluids (mixture of butane condensate, upgraded bitumen, and formation water). One of the unknowns in this process is the efficiency of thermodynamic trapping. On the basis of field results, it is shown that, like the SAGD process, liquid-pool depletion presents a critical control on the performance of the recovery process. Although the produced fluids are depleted from the liquid pool at the base of the chamber, because liquid-butane viscosity dependency on temperature is not strong and its viscosity changes slightly with temperature, thermodynamic trapping (or subcool trapping) is not efficient. Stability analysis of BEST data suggests that vapor breakthrough is part of the process but can be minimized by operating at temperatures greater than the second subcool limit. The second subcool limit (or Nsolv optimal reservoir subcool) will vary over time, and it is shown to occur at slightly less than 4°C after liquid-pool development.