Mobilizing Bitumen Under Reservoir Conditions

ABSTRACT An experimental and theoretical study was conducted of bitumen/heavy oil mobilization under cold conditions, using a solvent, carbon dioxide, and surfactant, to investigate ways of mobilizing bitumen or heavy oil ahead of a displacing thermal front. A rectangular cross-section model was used to carry out experiments simulating a bottom water layer underneath an oil zone. Results showed a strong dependence on solvent slug size, as well as the thickness and permeability of the water zone. Of the methods used, a combination of surfactant and carbon dioxide showed the best performance. Water zone thickness is important, because too thin a water zone leads to high injection pressures, while a thick water zone causes dissipation of the solvent. An optimal thickness seems to be about one-fifth of the oil zone thickness. A miscible displacement/leaching simulator was extended to include a number of additional effects, notably bitumen transport in the water layer. The efficiency of the leaching process was found to depend on the flow rate, solvent slug size, location of the injection/production points, and the relative thickness of the bottom water zone. A good match of the experimental results was obtained. It was found that a thin bottom water zone is helpful for obtaining initial injectivity, yet not detrimental to bitumen recovery. As in the experiments, a bitumen-to-water zone thickness ratio of 5.0 gave optimal results. The concentration profiles showed that the bottom water zone served as a transport layer for the mobilized bitumen. Upon the injection of a large solvent volume, the leaching process became very inefficient, shown by a sharp drop in the bitumen concentration in the effluent. As much as 85% of the bitumen and 68% of the solvent were recovered in the process.