Analysis of Short-Time Pressure Data Dominated by Wellbore Storage and Skin

Abstract

This paper suggests a method for significantly improving type-curve analysis. The method also provides quantitative information when available data are inadequate for type-curve or semilog analysis. Field examples are presented to demonstrate the usefulness of the method. presented to demonstrate the usefulness of the method. Introduction The log-log type curve method of analyzing pressure data at an unfractured well dominated by wellbore storage and skin was presented in 1970. Since then, two other type curves also have been presented for analyzing these effects. The essential differences and similarities among the three type curves have been discussed. If used propers, these type curves should provide identical results within limits of engineering accuracy. The principal problem with the type-curve approach is that the shapes of the various curves are so similar that in some instances we cannot match data or obtain quantitative information with as much precision as with conventional semilogarithmic methods. This has resulted in a loss of confidence in the use of wellbore-storage type curves to analyze data quantitatively. This paper discusses an alternative method of analysis. A new correlation is proposed whereby the permeability-thickness product, skin factor, etc., may be permeability-thickness product, skin factor, etc., may be determined when pressure data are affected by wellbore storage, and testing has not been conducted long enough for semilog analysis. This method may be used when other techniques do not provide satisfactory answers. In addition, the method can be used with the type-curve procedure to provide a greater degree of confidence in procedure to provide a greater degree of confidence in test interpretation. Note that this procedure is not intended to replace any of the approaches presented in the literature. If wellbore storage dominates pressure data and testing has been conducted long enough, two semilog straight lines normally are obtained. If boundary effects do not interfere, then the second straight line is the correct one. (If testing is not performed long enough, the first straight line may be chosen as the proper straight line erroneously.) Examination of the theoretical buildup lots obtained using the principle of superposition with dimensionless-pressure/dimensionless-time data indicates that the slope the first straight line can be correlated with the slope of the correct semilog straight line. Here, we present a correlation of the ratio of the slopes of the two straight lines as a function of the dimensionless wellbore-storage constant and skin factor. This correlation is independent of the producing time, provided the sandface production rate before shut-in is provided the sandface production rate before shut-in is constant - that is, wellbore storage effects do not influence the flowing pressure at the instant of shut-in. To our best knowledge, this correlation has not been presented before. presented before. Theory and Assumptions The basic assumptions of the problem considered here are identical to Those examined by Agarwal et al. The problem is the classic one of the flow of a slightly compressible fluid in a uniform, homogeneous, isotropic porous medium where gravitational forces are neglected. porous medium where gravitational forces are neglected. The well is assumed to be located either in an infinite system or at the center of a closed-square drainage region. P. 623