Optimum timing of CO2-cyclic solvent injection (CO2-CSI) in the heavy oil reservoirs

A number of laboratory tests and some field-pilot tests have shown that CO2-based cyclic solvent injection (CO2-CSI) has a great commercial potential to enhance heavy oil recovery after the primary production. CO2-CSI timing post the primary production is an important and practical operating consideration in the oilfield applications. When to start CO2-CSI may considerably affect the primary production and CO2-CSI production as well as the total oil recovery factor (RF). In this paper, a series of eight experimental tests were conducted in a 2-D rectangular visual sandpacked physical model to study the primary production and CO2-CSI in the heavy oil reservoirs. In each test, five major production data were measured by using a differential fluids production (DFP) method to characterize the primary production and CO2-CSI production: production pressure (Ppro); total isothermal compressibility (ct); total oil and gas formation volume factor (Bt); instantaneous gas−oil ratio (iRg); and cumulative free gas−oil ratio (cRfg), in addition to the total oil RF. It was found from the five measured production data that the primary production with a lower ending production pressure (Pe) had three distinct fluids production trends: single-phase flow; foamy-oil flow; and foamy-oil flow + free-gas flow. In addition, the early cycles of CO2-CSI had three possible fluids production trends: free-gas flow; free-gas flow + foamy-oil flow; and free-gas flow again. It was also found that a higher total oil RF of the primary production and CO2-CSI was achieved if the primary production was terminated at a lower ending production pressure (Pe). This was because the remaining live oil in the sandpacked model had a relatively lower solution gas (CH4) concentration when the primary production was terminated at a lower production pressure and more CO2 could be injected and dissolved into the oil. Together with proper production history matching and scaling-up criteria, the optimum CO2-CSI timing with the highest total oil RF identified in this study helps to design and optimize the field applications of CO2-CSI post the primary production in the heavy oil reservoirs.