How Wellbore Dynamics Affect Pressure Transient Analysis
Authors: Mattar L. , Santo M. 
Published: March 2010

There are numerous technical papers on pressure transient analysis which address both simple and complex reservoir-related phenomena. These papers all make the implicit assumption that reservoir pressure can be measured directly. It is a fact of life, however, that the pressure recorder is located in a wellbore, not in the reservoir. The wellbore is an intrinsic link between the reservoir and the recorders. Although the pressures recorded in the wellbore are normally representative of the pressures in the reservoir, they can also be affected by a number of wellbore related phenomena.

The many wellbore related phenomena that can have a Significant effect on the measured pressure have not been addressed in the literature, except for wellbore storage and the classical phase redistribution hump.

This paper presents several examples of tests that have been affected by wellbore dynamics, and shows that these could easily have been misinterpreted as complex. reservoir phenomena (dual porosity, etc.) instead of wellbore effects. These effects are often accentuated by the (semilog) derivative which is traditionally used to diagnose reservoir characteristics. The fact that these are wellbore and not reservoir effects must be recognized to prevent misdiagnosis. Often this can only be done from examination of the test data other than the recorded pressure-time trace.

Introduction

There have been more than 1000 papers published on Pressure Transient Analysis. These publications address in great detail such fundamental topics as homogeneous infinite reservoirs, fractures, convolution, dual porosity, multi-layers, or such esoteric concepts as super-bilinear-equivalent-pseudo-time or integral type curves. With the progress made in both analytical and numerical solutions to reservoir problems, and with the ready availability of powerful computers, virtually every imaginable reservoir description can be modelled and the solution presented in graphical form (type curve).

In contrast, there are very few papers that deal with wellbore effects. The only two classical wellbore topics that have been studied in any detail, are the concepts of wellbore Storage(l) and Phase Redistribution(2). Even then, the treatment has been more theoretical than practical. In this presentation the authors will address some of the many issues under the over-all umbrella of "Wellbore Dynamics". These reflect the experience with "practical" welltest interpretation, which stems from years of specialization in this field, (more than 10 000 tests have been analyzed in detail by the authors and their colleagues).

Wellbore Dynamics

The topics which will be addressed in this paper will obviously deal with wellbore phenomena. However, not all of these wellbore phenomena will be studied. For example, we will not be reviewing the effects of temperature on wellbore fluids or pressure recorders; nor will we address such topics as gas/oil solution/ liberation or retrograde condensation. Moreover, some of the ideas discussed do not take place "in" the wellbore, but in the immediate vicinity of the wellbore, and often, their effects are inseparable from wellbore effects. The "Wellbore Dynamics" that we shall illustrate will be grouped into the following topics: Liquid Influx/Efflux, Phase Redistribution, Wellbore (and near wellbore) Clean-up, Differences between Drawdown and Buildup, Plugging, Recorder Effects, Mysterious Effects.