Only 5 - 15% of the oil in Lloydminster heavy oil reservoirs is recovered during cold heavy production with sand (CHOPS). Solvent injection processes are being explored as a means of recovering the heavy oil remaining in the reservoir after CHOPS has been completed. Solvent retention becomes a main concern for the process economics. This paper evaluates the relative importance of different solvent retention mechanisms and how solvent can be recovered from the reservoir so that solvent injection processes (e.g. cyclic solvent injection (CSI), Vapor Extraction Process (VAPEX), thermal solvent, and steam-solvent) can be economically viable. In particular, CSI is a promising post-CHOPS follow-up process.
Sources of solvent loss / retention include:
Solvent trapped in the reservoir due to surface and interfacial forces including adsorption and capillary pressure
Solvent vapor (free gas or trapped bubbles) in porous media
Dissolution in un-recovered oil
Dissolution in formation water or thief water zones
Other possible sources such as:
Lack of confinement of injected fluids; especially important for post-CHOPS reservoirs
Three experiments were performed to estimate solvent losses due to different retention mechanisms. In these experiments, gaseous propane was injected into a sand pack to a pressure of 750 kPag and then depressurized (at an ambient temperature of 21 °C) in 100 kPa steps to 50 kPag. The propane produced at each depressurization step was measured. The sand packs used in the experiments were:
Sand pack initially saturated with water ("wet pack")
Dry sand pack
Sand pack initially flooded with water and then with dead Husky Edam oil
The experimental results showed that there was significant propane retention in unproduced oil and as solvent gas. However, little propane adsorption occurred on the sand as it had a small surface area due to an insignificant amount of clays being present in the test packs. Solvent adsorption in a reservoir can be significant if there is a considerable amount of clays with a large surface area. It may be particularly high when shale is present.
Propane concentrations in the produced water from the wet pack were similar to literature values. Solvent loss in water was small due to the low pressures involved in the test. However, solvent loss in water will be significant if the solvent dissolves into the formation water at high temperature (for liquid solvent) and/or high pressure especially if there is a significant water source to sweep away the dissolved solvent or the water causes hydrate formation.