6.1 Introduction

This chapter discusses the most frequently used pressure buildup test. It is the simplest test that can be run on a gas well. If the effects of wellbore storage can be determined, much useful information can be obtained. This information includes permeability k, apparent skin factor s ?, and average reservoir pressure p _R. The test consists of flowing the well at a constant rate q _sc for a period of time t _p, shutting the well in (at ?t = 0), and measuring wellbore pressure increase with shut-in time ?t. Horner developed the test, and this method of analysis is generally considered the best. Other conventional methods of analysis include the Horner plot, the Miller Dyes Hutchinson2 plot (often abbreviated as the MDH plot), and the Muskat plot. Horner1 showed that a plot of the shut-in pressure p _ws versus log ( t _P + ?t)/ ? t should result in a straight line for an infinite-acting reservoir. In the buildup tests, t refers to the drawdown period prior to a buildup and ?t refers to the shut-in or buildup time. Matthews, Brons, and Hazebroek,3 abbreviated as MBH, extended the application of the Horner plot to finite reservoirs.

A buildup test is always preceded by a drawdown, and the buildup data are directly affected by this drawdown. Usually, the drawdown starts from a stabilized reservoir condition represented by the stabilized reservoir pressure,