a measure of the completeness of extraction of oil from a bed. Oil recovery also refers to the degree of depletion of an oil bed. The coefficient is defined as the ratio of the quantity of oil extracted to the quantity originally contained in the bed under similar conditions and is expressed numerically in fractions of a unit or in percentages.
In the extraction of oil through displacement by various agents, the recovery factor is determined by the completeness of displacement of oil by water, gas, solvents, or other agents (called the displacement factor) and by the degree of conformance of the entire bed—that is, the totality of oil-saturated beds, intercalated beds, and lenses making up the object of exploitation (the conformance factor). In this case the recovery factor is expressed as the product of the displacement and conformance factors. The conformance factor depends to a great extent on the nonuniformity of the bed and the ratio of the viscosity of the oil to that of the displacing agent. The more heterogeneous the structure and physical properties of the bed and the greater the ratio of the oil viscosity to the agent viscosity, the lower the value of the conformance factor. The conformance factor is close to 1 in oil field recovery operations using depletion (drive) or predominantly gravity drainage. The recovery factor depends on the properties of the oil under reservoir conditions, the physical and chemical characteristics of the bed and the natural or artificial recovery conditions, and the system of development of the deposit.
For water drive, the most common and efficient oil recovery method, a distinction is made among waterless production, which characterizes the completeness of oil extraction at the start of water contamination of the output (that is, the appearance of water together with the oil); current production, which determines the completeness of oil extraction at any given moment in the exploitation of an oil-bearing bed; and final production, which is determined by the maximum water contamination of the output, which halts the operation of individual wells, the bed, or the oil field as a whole. The maximum water contamination is determined by economic indicators and depends on the properties of the oil and the depth of the bed. With water drive, the final production ranges from 0.25 to 0.7, depending on the viscosity of the oil, the heterogeneity of the bed, and other factors (low values occur in the recovery of oil pools having high viscosity under reservoir conditions). The maximum, or limiting, production that can be attained by the use of a given method is also distinguished. For example, the maximum production with water flooding can be reached by prolonged washing of the bed with water until the output is completely contaminated with water.
As the petroleum industry develops and as methods for working oil-bearing beds appear and are improved, the recovery factor has risen from 0.3 to 0.5 and, for certain oil fields, to 0.7. The average recovery factor for individual petroleum-extracting countries is increasing relatively slowly (Figure 1) because of the exploitation of oil fields in which the properties of the oil and reservoirs have deteriorated.
(See also: Production.)
A. G. KOVALEV