Water- and CO2-Based Oil Recovery Processes in the Tight Main Pay Zone and Vuggy Residual Oil Zone

Date
2015-04
Authors
Gong, Yanbin
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Faculty of Graduate Studies and Research, University of Regina
Abstract

In this thesis, water- and CO2-based flooding processes in the upper tight main pay zone (MPZ) and in the lower vuggy residual oil zone (ROZ) of a carbonate light oil reservoir were studied and compared. In particular, the oil recovery performance of miscible CO2 simultaneous water-and-gas (CO2-SWAG) injection in the tight Bakken formation was experimentally examined. More specifically, first, several carbonate reservoir rock samples from the MPZ and ROZ in the Steelman oilfield (Canada) were characterized by using thin-section analysis and X-ray diffraction (XRD). Second, the minimum miscibility pressure (MMP) between the light crude oil and CO2 at the actual reservoir temperature was determined by applying the vanishing interfacial tension (VIT) technique. Third, a total of ten coreflood tests were conducted with the tight dolostone or vuggy limestone core plugs to evaluate the effects of the production pressure, rock properties, and CO2 injection timing on the water- and CO2-based oil recovery processes in the two different zones. Then the effective viscosities of high-salinity water and supercritical CO2 mixtures with twelve different water volume fractions were measured at the actual reservoir conditions by using a capillary viscometer. Finally, six more coreflood tests were carried out with different CO2-EOR schemes and different injected water‒gas ratios (WGRs) in volume to determine the optimum miscible CO2-SWAG injection in the tight Bakken formation. The MMP between the light crude oil and CO2 at the actual reservoir temperature of Tres = 51.1 °C was determined to be 10.8 MPa. The coreflood test results showed that for the CO2 secondary flooding in the non-waterflooded tight MPZ, the oil recovery factor (RF) increased almost linearly with the production pressure, whether it was lower or higher than the MMP. For the CO2 flooding in the waterflooded vuggy ROZ, however, much less residual oil was recovered in the immiscible case than that in the miscible case, especially after CO2 breakthrough (BT). 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 in the two zones. In comparison with waterflooding or miscible CO2 secondary flooding alone, waterflooding and miscible CO2 tertiary flooding together were the most effective method to recover the light crude oil from the upper tight MPZ and lower vuggy ROZ of the carbonate reservoir. In addition, it was found that the measured effective viscosity of the saline water‒CO2 mixture was increased with the water volume fraction and can be well modelled by using the Arrhenius equation. In the tight Bakken formation, the miscible CO2-SWAG injection with an injected WGR of 1:3 in volume achieved the highest oil RF, in comparison with miscible CO2 secondary flooding, waterflooding together with miscible CO2 tertiary flooding, and miscible CO2 water-alternating-gas (CO2-WAG) injection. The WGR also showed strong effects on the fluid production trends and the oil RF of the miscible CO2- SWAG injection. A water bank might be formed ahead of the water‒CO2 mixture in the miscible CO2-SWAG injection with a higher injected WGR of 1:1 or 3:1. Furthermore, the calculated mobility ratios of the injected fluids to the light crude oil indicated that in comparison with water or CO2 alone, the water‒CO2 mixture had the lowest mobility in the tight reservoir core plugs. Hence, the highest oil RF of the optimum CO2-SWAG injection with the injected WGR of 1:3 was attributed to a substantially weakened waterblocking effect and a well-controlled CO2 mobility.

Description
A Thesis Submitted to the Faculty of Graduate Studies and Research In Partial Fulfillment of the Requirements for the Degree of Master of Applied Science in Petroleum Systems Engineering, University of Regina. xix, 121 p.
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