Active Oil and Reservoir Energy

Abstract

In 1929, Coleman, Wilde, and Moore undertook an investigation of thetheoretical decline in reservoir pressure as related to the production of oiland gas. The most important part of this work was presented in an equationdefining the relationship between the reservoir pressure, the quantities of oiland gas produced the oil and gas content of the reservoir, and the propertiesof the reservoir fluids. With this equation, it appeared that, with sufficientdata, it would be possible to calculate the oil content of the reservoir andpredict quantitatively the effect of gas-oil ratio on the decline of reservoirpressure. Needless to say, such an expression would be highly important inevaluating fields and in arriving at a choice of production methods to beused. Unfortunately, at the time these investigators published their work, there werenot sufficient data available to permit application of the equation. Thetechnique of measuring reservoir pressures had not been developed, gasmeasurements were uncommon, oil and water production records were in most casesinadequate, and little was known of the properties of the complex hydrocarbonmixtures making up the oil and gas in the reservoirs. For these reasons, littleadditional work of a similar nature could be done. However, during the pastfour years many data have been systematically accumulated on reservoirpressures and the properties of oil and gas, and it is now possible to usethese data in furthering the studies on the behavior of oil and gasreservoirs. It is the purpose here: first, to present the derivation of a modified form ofthe equation given by Coleman, Wilde and Moore, which will permit thecalculation of what later will be defined as the "active oil" contentof a reservoir; second, to derive formulas for calculating the reservoir-energychanges that occur during the course of production, and finally, to discuss thelimitations and application of the equations developed.