Short-Time Well Test Data Interpretation in the Presence of Skin Effect and Wellbore Storage
Authors: Ramey H.J. 
Published: December 2007

The log-log type-curve described here shows clearly the presence and duration of wellbore storage as well as the presence and duration of wellbore storage as well as the presence of linear flow due to fracturing. It can be used to presence of linear flow due to fracturing. It can be used to obtain quantitatively the information normally obtained from pressure buildup analyses and to identify the proper straight pressure buildup analyses and to identify the proper straight line in pseudo-radial flow for a fractured well.

Introduction

Specifications for modem well testing (drawdown or buildup) are usually written in such a way that a well will be tested for a period of time long enough to reach and define a proper "straight line" when test data are plotted in conventional manners. Pressure data obtained before the straight line is reached are not often analyzed, despite the fact that a number of publications have advanced methods for doing so. publications have advanced methods for doing so. One reason for this situation is that many factors are known to affect the short-time data. "Short-time data" signify data obtained before a conventional straight line is reached. Some of the factors are the effects of wellbore storage, perforations, partial penetration, and well stimulation such as fracturing or acidizing.

Although the effects of such factors are generally known, the duration and importance have not been clearly defined in all cases particularly when these effects are combined in a well test. However, recent studies have revealed a great deal of potentially useful information concerning the analysis of short-time well test data. Our purpose here is to illustrate the interpretation of short-time well test data through presentation of field examples. Factors to be considered presentation of field examples. Factors to be considered will include wellbore storage, well damage, and fractured wells.

Wellbore Storage and Skin Effect

The effect of wellbore storage or unloading was originally considered by van Everdingen and Hurst. These studies called attention to the fact that the storage or unloading of fluid contained within the wellbore could cause a significant difference between the surface production rate and the sand-face flow rate in a well immediately following sudden changes in production rate. Gladfelter et al. presented a method production rate. Gladfelter et al. presented a method for correcting pressure buildup data for the changing sand-face flow rate. In another publication a method was presented for estimating the duration of the storage effect, and the Gladfelter et al. correction was generalized to apply to drawdown data.

Fundamentally, wellbore storage can occur in several ways. Fluid can be stored by compression of the fluid in a completely filled wellbore, or by movement of a gas-liquid interface. Russell presented a method for analysis of the latter case, pointing out that the virtue of the method was that it was not necessary to know the sand-face flow rate as was the case in the Gladfelter et al. method. One problem with Russell's method was that only a portion of the short-time data was used, yet no criterion for selection of the data was presented. presented. Recently, Agarwal et al. re-examined this problem and presented dimensionless pressure-dimensionless time plots for the case of a well in an infinitely large reservoir, producing at constant surface production rate, and having wellbore storage and a skin effect. Fig. 1 presents a portion of their results. The usual definitions of dimensionless groups were employed.

JPT

P. 97