Heavy oils with about 500 cp to 50,000 cP viscosity in unconsolidated sand reservoirs are commercially produced without heat injection, by a production method known as the Cold Heavy Oil Production with Sand (CHOPS). CHOPS reservoirs are generally about a kilometer wide and may range from 3 to 5 m thick sand blankets to 35 m thick sand channels which are between 0.5 to 5 Darcy permeability, 28 to 32% porosity, and have about 88% oil and 12% water saturations. CHOPS production is always coupled with the production of the dissolved gas, mostly CH4 and minor amounts of H2S and CO2, and sand production which causes formation of wormholes in the reservoir. Formation of wormholes in the initial phase of the reservoir life promotes oil migration and thus increases oil production; however, continued wormhole formation in long term operations reduces the oil production rate and harms the sweeping efficiency of the original oil in place, typically to less than 10%. Because of low sweeping efficiency, several Post-CHOPS process concepts have been investigated to recover the remaining oil in the reservoir.
The objective of the present study is to increase the production and sweeping efficiencies of CHOPS and Post-CHOPS processes by altering the wettability of oil and reservoir sand as well as the wettability of oil and connate water and/or water injected into the reservoir. In this study, an oil-water emulsion sample produced by CHOPS process was centrifuged to separate the oil and water phases to avoid using chemical demulsifiers. The separated oil and water phases were physically and chemically characterized and the oil-water interfacial tension was determined using pendant drop method at a temperature range between 20 to 50 oC. The effects of solvent, surfactant, and pH on oil-water interfacial tension were independently investigated by either diluting the oil with pentane as a solvent additive (5% to 15% by mass of oil dosages), emulsifying biodiesel (fatty acids methyl esters, chemical formula: CnHm-COOCH3; m<2n+2) as a surfactant additive into the water phase (0.05% to 0.3% by mass of oil dosages) and altering the pH of the water with using NaOH and HCl.
Results of these studies suggest that bitumen mobility and sweeping efficiency of CHOPS and Post-CHOPS processes could be improved by reducing the oil-water interfacial tension. The present experimental research demonstrates that oil-water interfacial tension could be reduced by either a basic or a nonionic surfactant additive. Implementation of these findings to CHOPS and Post CHOPS processes needs further understanding of the CHOPS reservoir characteristics. Laboratory test results are encouraging enough for further laboratory and field trials to increase the production and oil sweeping efficiencies of the CHOPS reservoirs.