Immiscible CO2 injection is a potentially viable method of enhanced oil recovery (EOR) for medium oil reservoirs in southwestern Saskatchewan. The relatively high reservoir pressures could result in a large extent of CO2 dissolution, significant oil viscosity reduction, and oil swelling. Laboratory corefloods were used to compare the performance of different modes of CO2 injection into a medium-gravity oil system. The following methods were compared: injection of a single CO2 slug chased by water, simultaneous injection of water/CO2, and different water-alternating-gas (WAG) cycles.
The results indicate that both a single CO2 slug and the first WAG cycle in a series produced oil very efficiently, possibly due to good gas/oil contact at the relatively high residual oil saturation at this stage. The simultaneous injection of water/CO2 was not as effective as a single CO2 slug, possibly because the co-injected water shielded the oil from being contacted by the gas. Among the four runs, a coreflood with four WAG cycles recovered the most incremental oil – 20.58% initial oil in place (IOIP) – while the co-injection process produced the least (8.91% IOIP).
This experimental study suggests that the immiscible CO2 EOR process is viable for medium oil reservoirs with relatively high pressures, and that proper process application is important for maximizing additional oil production.
Miscible/immiscible CO2 flooding is an increasingly popular enhanced oil recovery (EOR) technique. So far, most of the CO2 miscible/immiscible processes have been applied in light oil reservoirs, and there has been little application in medium oil reservoirs. The in-place resources of medium oils, defined as 20–30 °API, in Saskatchewan are estimated at up to 1,338?106 m3 (8.42 billion barrels).1 Most of these reservoirs are characterized by thin pay (two to ten meters in thickness) and shaly sand, and they have been under production for over four to five decades. The estimated average recovery by conventional waterflood, as listed in Table 1, is only about 23% initial oil in place (IOIP) because this method has generally exhibited early water breakthrough and high water cuts due to the unfavourably high water-to-oil mobility ratio. By adapting miscible/immiscible CO2 flooding technologies that have matured in light oil applications, the immiscible CO2 process is potentially viable for recovering additional oil from these medium oil reservoirs if we can improve its displacement and sweep efficiency.
Current EOR technologies for mature oil reservoirs are outlined in Figure 1. The nature of the medium reservoirs in Saskatchewan means that the commonly used EOR methods are very limited as a means of efficiently recovering the remaining trapped oil. For a number of medium reservoirs in Saskatchewan, miscibility between the oil and injected gas, such as CO2, is unlikely due to reservoir fluid properties and formation limitations (mainly that the reservoir pressure is lower than needed to attain miscibility). Thermal recovery techniques are not suitable for these thin reservoirs because of the heat loss to the overburden and underburden.
