A Simplified Method of Pressure Buildup Analysis for a Stabilized Well

Abstract

This method of pressure buildup analysis employs a graphical procedure termed "negative superposition". The technique estimates the effect of one reservoir disturbance when the effects of two or more disturbances are being superimposed upon each other. A special advantage of this method is that it permits the use of much simpler mathematical expressions in the analysis. Introduction This paper presents a new buildup analysis method for a well that has produced long enough to be in pseudosteady-state, or stabilized flow. (These two pseudosteady-state, or stabilized flow. (These two terms are used interchangeably here.) The method, which is equally applicable to infinite-acting wells, is based on the change in pressure caused of oil by the negative rate effect due to shutting the well in. This effect is separated graphically from the continued pressure decline tendency caused by production previous to shutting the well in. production previous to shutting the well in. Although a Homer-type analysis can be used for most infinite-acting well it is inadequate if the pressure buildup has been taken from drillstem tests when pressure buildup has been taken from drillstem tests when the pressure is increasing at the time of shut-in. A later paper is planned for dealing with this problem. Many different techniques have been published to analyze pseudosteady-state pressure buildup. including the Muskat, Miller-Dyes-Hutchinson, and Homer methods. The method of this paper, which for simplicity will be referred to as the "Slider method" has two distinct advantages over those previously published: previously published: 1. When the drainage area approximates radial flow, such as for a well in the center of a square or hexagon, a complete analysis can be accomplished without prior knowledge of the porosity or effective compressibility. 2. The Slider method gives a straight-line pressure plot for much longer periods than do the other methods. plot for much longer periods than do the other methods. This is not an insignificant effect. The plot may be straight for as much as 40 times as long as it is for some of the other methods. There does not seem to be much to say about the first point. To do a complete buildup analysis by other methods it is necessary to know at least the porosity and effective compressibility for the drainage porosity and effective compressibility for the drainage area. The Slider method uses the change in pressure with time before shut-in (which is a function of the porosity and compressibility) to avoid the independent porosity and compressibility) to avoid the independent evaluation of the porosity and compressibility. This is no small advantage since the compressibility of a reservoir below the bubble point is very sensitive to the gas saturation, which in turn is difficult to determine with sufficient accuracy. Ramey and Cobb compared the straight-line portion obtained by various plotting techniques as applied to a square drainage area with the well in the center. Part of their results the time limits on the Part of their results the time limits on the various methods are shown in Table 1, along with the time limit using the Slider plot. Since pseudosteady state begins at about tDA = 0.1 for the pseudosteady state begins at about tDA = 0.1 for the square drainage, smaller producing times were of no significance. Note that the proposed techniques will give an accurate straight-line plot for a period as much as 40 times as long as the other two plots. JPT P. 1155