The efficiency with which oil is displaced by water during waterflooding of reservoirs varies from less than 10 per cent to as much as 80 per cent. Both the properties of the fluids and the properties of the rock-pore system contribute to this variability. The total recovery efficiency is the product of the volume fraction of the reservoir contacted or swept by water, which is related to macroscopic properties of the reservoir and to the pore-by-pore displacement efficiency in the swept regions, which is related to the microscopic properties of the reservoir rocks. Trapping on the microscopic scale in swept regions can be estimated from tests on individual samples of core, and it is these tests that are discussed here. Oil displacement efficiency is commonly estimated from relative-permeability tests, but these are difficult and time-consuming to perform and few tests are normally available for a large, heterogeneous reservoir. There is a need for simpler test methods to measure displacement efficiency so that chances of properly modelling a reservoir can be improved. Mercury capillary-pressure tests may be useful in this regard because they can be performed simply, rapidly and inexpensively. Although they provide a measure of displacement efficiency, they do not provide the information on multiphase flow rates obtained from relative-permeability tests. The major objective of this study is to compare mercury recovery efficiency, measured by capillary-pressure tests, to oil recovery efficiency obtained from relative-permeability tests. Furthermore, the results of both these types of tests are related to other petrophysical data such as porosity and gas permeability, as well as to petrographic observations and direct observations of pore structure made from resin casts. For a diverse group of sandstone reservoir rocks there is a statistically significant correlation between mercury recovery efficiencies and oil recovery efficiencies. Rock-pore properties that are significantly related to high recovery efficiency are high porosity, small pore-to-throat size ratio, small mean particle size and low percentage of carbonate. Although permeability is regarded as a property of fundamental importance in modelling the performance of a reservoir, there is no correlation between any measure of permeability, absolute or relative, and mercury or oil recovery efficiency. Porosity is a more sensitive indicator of nonwetting-phase recovery efficiency than is permeability.
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Wardlaw et al. (1979) studied this question.