Experimental Study of Water and CO2 Flooding in the Tight Main Pay Zone and Vuggy Residual Oil Zone of a Carbonate Reservoir

摘要

In this paper, water and CO2 flooding processes in the upper tight main pay zone (MPZ) and lower vuggy residual oil zone (ROZ) of a carbonate light oil reservoir were experimentally studied and compared. First, some carbonate reservoir rock samples from these two zones in the Steelman oilfield (Canada) were characterized using thin-section analysis and X-ray diffraction. Second, the minimum miscibility pressure (MMP) between the light crude oil and CO2 at the actual reservoir temperature was determined. Third, a total of eight coreflood tests were conducted with the tight dolomite or vuggy limestone core plugs to examine the effects of the production pressure, CO2 injection timing, and rock properties on the water- and CO2-based oil recovery processes in the two different zones. It was found that, for the CO2 secondary flooding in the non-waterflooded tight core plugs (i.e., the MPZ), the oil recovery factor (RF) was not dramatically reduced when the production pressure was lower than the MMP. For the CO2 flooding in the waterflooded vuggy core plugs (i.e., the ROZ), however, much less residual oil was recovered in the immiscible case than that in the miscible case, especially after CO2 breakthrough. In comparison to waterflooding or miscible CO2 secondary flooding alone, miscible CO2 tertiary flooding after waterflooding was found to be the most effective method to recover the light crude oil from the tight core plugs and especially from the vuggy core plugs. Moreover, the petrographic properties of these two types of core plugs had rather different effects on the miscible CO2 secondary flooding, whereas the miscible CO2 tertiary flooding had similar production trends.