Abstract

There are numerous technical papers on pressure transient analysis whichaddress both simple and complex RESERVOIR-related phenomena. It is a fact oflife, however, that the pressure recorder is located in a WELLBORE, not in the RESERVOIR, but connected to a reservoir. The WELLBORE is an intrinsic linkbetween the RESERVOIR and the RECORDERS. The WELLBORE can, and often does, interact dynamically with the RESERVOIR, and this effect is readily observed inwell testing.

The many WELLBORE related phenomena that can have a significant effect on themeasured pressure have not been addressed in the literature, except forwellbore storage and the classical phase redistribution hump.

This paper presents several examples of tests that have been affected bywellbore dynamics, and shows that these could easily have been misinterpretedas complex RESERVOIR phenomena (dual porosity, etc...) instead of WELLBOREeffects, These effects are often highlighted by the (semilog) derivative whichis traditionally used to diagnose RESERVOIR characteristics. The fact thatthese are WELLBORE and ~ RESERVOIR effects must be established to preventmisdiagnosis. 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 1,000 papers published on Pressure Transient Analysis. These publications address in great detail such fundamental topics ashomogeneous Infinite reservoirs, fractures, convolution, dual porosity, multi-Iayers, or such esoteric concepts as super-bilinear equivalent-pseudo-time or Integral type curves. With the progress made in both analyticaland numerical solutions to reservoir problems, and with readily availablepowerful computers, virtually every imaginable reservoir description can bemodelled and the solution presented in graphical form (Type Curve).

In contrast, there are very few papers that deal with wellbore effects. Theonly two classical wellbore topics that have been studied in any detail, arethe concepts of Wellbore Storage(l) and Phase Redistribution(2). Even then, the treatment has been moretheoretical than practical. 10 our presentation we will address some of themany issues under the overall umbrella of "Wellbore Dynamics". These reflectour experience with "practical" welltest interpretation, which stems from yearsof specialization in this field. (more than 10,000 tests have been analyzed indetail by the authors and their colleagues).

Wellbore Dynamics

The topics which will he addressed in this paper will obviously deal withwellbore phenomena However, not all of these wellbore phenomena will bestudied. For example, we will not be reviewing the effects of temperature onwellbore fluids or pressure recorders; or will we address such topics asgas/oil solution/liberation or retrograde condensation. Moreover, some o(theideas discussed do not take place "in" the wellbore, but in the immediatevicinity of the wellbore, and often, their effects are inseparable fromwellbore effects. The "Wellbore Dynamics" that we shall illustrate will begrouped into the following topics:

  • Liquid Influx/Efflux

  • Phase Redistribution

  • Wellbore (and near-wellbore) Clean-up

  • Differences between Drawdown and Buildup

  • Plugging

  • Recorder

  • Mysterious Effects

  • Many (not all) of these "wellbore" transients occur at very early time.

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