Water flooding as an oil recovery technique has been in use since 1890 when operators in the US realised that water entering the productive reservoir formation was stimulating production. In some cases, water is supplied from an adjacent connected aquifer to push the oil towards the producing wells. In situations where there is no aquifer support, water must be pumped into the reservoir through dedicated injection wells. The water phase replaces the oil and gas in the reservoir and thereby serves to maintain pressure. Recovery factors from water flooding vary from 1-2% in heavy oil reservoirs up to 50% in light oil reservoirs with typically values around 30-35%, much lower than the microscopic sweep efficiency of 70-80%.
The reason for sub-optimal recovery factors is related to the macroscopic sweep, which in turn is a reflection of reservoir heterogeneity and fluid mobility ratios. Fluid mobility ratio may be controlled to some extent by adding viscosifying agents to the injection phase, such as polymers or foams, but the presence of large permeability variations requires a different approach to improve macroscopic sweep. An extreme case is a direct high-permeability conduit, either natural or induced, between an injector and one or more producers, which requires complete or at least partial plugging. This process is known as conformance control.
Conformance control generally requires a combination of mechanical and chemical solutions. The role of the mechanical part is to ensure that the chemicals reach the part of the reservoir, which they are intended to plug. Although commercial chemicals already exist for plugging high-permeability zones, the chemical mixture has to be tailored to a particular application, depending on salinity, temperature, pore size etc. The main challenge lies in conveying the chemicals to the right place in the reservoir. Failure to achieve proper delivery may result in plugging of the near well-bore, with reduced production or injection as a result.
Zhu et al. (Journal of Canadian Petroleum Technology, 1993, 32/9, p 37-43 teaches a salting-out concept in which a non-electrolyte (alcohol) is added to water to reduce the solubility of electrolytes (salt). A preflush of highly salt-concentrated brine was followed by a flush with ethanol. The brine and ethanol both flow through the water flooded high-permeability zones and will mix there. The ethanol leads to the salt precipitating, which serves to plug these high-permeability zones. In homogenous and heterogeneous sandpacks the reduction of brine permeability was 70% and 50%, respectively, leading to an incremental oil recovery of 15% of oil initially in place (after water flooding). However, there is a concern that the ethanol and brine may mix not only in the desired permeability zone, leading to salt precipitation in an incorrect location. Moreover, the ethanol may mix with other salt bearing solutions which may lead to salt precipitation again occurring in an incorrect location.
It is amongst the objects of the present invention to obviate and/or mitigate at least on of the aforementioned disadvantages.