During the primary stage, oil with higher viscosity reaches an economic limit of oil production rate earlier compared to oil with lower viscosity. High viscosity makes oil less mobile to water. High viscosity oil is still flowing into wellbore initially until certain lower reservoir pressure reached. In Figure 1, the lower reservoir pressure at where the pressure is no longer strong enough to drive oil is indicated by sudden water cut (WC) pick-up at reservoir pressure of 917 psi. Some reservoirs contain oil with such high viscosity that oil recovery is only 9% when the economic limit of oil production reached. We can easily observe 98% of WC from many producers. Much of the oil reserve is still left in the reservoir. Producing wells with high WC will continuously decrease reservoir pressure which is required to drive oil. Some oil and gas companies shut-in production well with high WC and do remedial action to stop water production. Injecting water back into the same reservoir is mandatory, usually peripherally.
Peripheral water flooding will require voluminous water to increase reservoir pressure. The water will not drive oil efficiently since most of the water will escape into aquifer instead of pushing the oil into producers.
By assuming the reservoir isotropic, patterned water flooding will increase areal sweep efficiency significantly. The pattern might 5 spots, 7 spots, or line drive. A reservoir simulation will help to assess the best pattern for flooding.
In a heterogenic reservoir, the water flooding performance could be very low with early water breakthrough and bypassed oil. Water will preferentially flow through larger pore spaces or micro fractures especially in carbonate reservoir leaving oil in smaller pore spaces. By changing the interfacial tension of oil to rock, some oil will easier to release from small pore spaces. Surfactants can help to improve the performance of water flooding.
Water is a good pusher to displace oil horizontally but poor vertically due to lower vertical permeability. Gas is more superior in vertical displacement aided by gravity but poor for horizontal displacement due to low viscosity of the gas such that gas will finger through oil. Water can control this fingering so that flooding combination of water and gas will increase oil recovery. Gas will displace oil from small pore spaces to larger pore spaces where water can easily push it into producers. A problem arises over certain distance where gas and water are segregated gravitily. Gas will build layers at the upper of the reservoir and fingering into producers. On the other hand, water escapes to the bottom of the reservoir and channeling through micro fractures. It is obvious that the mixed flow of water and gas is more effective and less fluid loss.
The patent “improving vertical sweep” resolves the segregation problems by mixing water and gas in the reservoir. Water is injected at the upper part of the reservoir and gas is at the bottom of the reservoir preferably slightly below water oil contact (WOC) as depicted in Figure 2. The mixing ratio of water to gas is simulated such that gas mobility is as small as possible. Water injection at the upper of the reservoir can be stopped for a while for allowing water and gas mixtures. Water and gas injection rate, duration of water injection interruption will determine the area of mixed flow zone. The bigger the mixed flow zone, the better is the sweep efficiency horizontally and vertically. A higher gas injection rate will build a bigger mixed flow zone. It is desirable to inject gas with pressure close to reservoir fracturing pressure.
Horizontal wells can deliver fluid with a higher rate into the reservoir if compared to vertical wells. In a thin reservoir, vertical wells will have a low maximum injection rate to avoid fracturing. Denser well spacing will be required to achieve higher oil recovery. For a thin reservoir, horizontal wells are more desirable to deliver a high gas rate and a bigger mixed flow zone so that denser well spacing is not required. Figure 3 shows the comparison of the area influenced by a vertical well and by a horizontal well. For the same reservoir, horizontal well can influence 7700 acres while vertical well only influences 80 acres.
As for general purposes, the gas can be replaced by liquid with density and viscosity smaller than water. A surfactant that decrease the interfacial tension (IFT) Oil-Water, Oil-Rock, Water-Rock would benefit sweep efficiency.