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This report develops a method of analyzing pressure buildup data in a finite-acting reservoir and demonstrates the method with exemplifying calculations. It is believed that this method is, in most cases, superior to methods currently in use. Many engineers currently attempt to apply the Horner method of analysis, which is based entirely upon infinite-acting reservoir equations, to wells which have obviously been affected by the drainage limits of the reservoir and thus are finite-acting. Furthermore, the methods presently available for buildup analysis in a finite-acting reservoir are not as simple as the Horner method to apply and require a knowledge of the reservoir size, effective compressibility, effective formation thickness, porosity and fluid viscosity if the skin factor and average pressure are to be evaluated. In contrast, the method presented herein is very similar to the Horner method in that it requires similar pressure plots and uses similar equations; also, the formation capacity, the average drainage area pressure, and the skin factor can all be determined with a knowledge of only the fluid viscosity and the [re/rw] ratio. Even the Horner solution does not provide the skin factor with such limited data.
A solution based on such limited data is made possible by using the change in reservoir pressure prior to shut-in to determine the product of effective compressibility, porosity and effective formation thickness.
The application of pseudo-steady-state flow to pressure-buildup analysis is made Practical by an analytic derivation of the time when pseudo-steady-state flow begins. The development of these concepts, with a review of the basic pseudo-steady-state technology, is presented herein.
Reservoir fluid flow may be generally classified as steady-state, unsteady-state and pseudo-steady-state. This last classification, with which this report is concerned, has also been referred to as "stabilized") or as "steadystate in a bounded drainage area". This type of reservoir flow occurs much more frequently than steady-state flow or unsteady-state flow with an expanding drainage radius.