The method described here for determining the static pressure of a well from the buildup test data differs from the commonly used methods in that it does not require the knowledge of the porosity, permeability, viscosity, compressibility, and the drainage area. It permeability, viscosity, compressibility, and the drainage area. It does, however, require the value of the pressure at time zero.

Introduction

In a buildup analysis, the buildup pressure, Psw, is

plotted vs on a semilog plot. A straight line plotted vs on a semilog plot. A straight line is obtained from whose slope kh/ is calculated.

Extension of the straight line to = 1 gives a

p*. If the boundary effect has not been felt during p*. If the boundary effect has not been felt during the production period prior to the buildup test, then p* ps, the static pressure of the well. On the other p* ps, the static pressure of the well. On the other hand, if the well has been producing for a considerable period of time so that the boundary influences the period of time so that the boundary influences the behavior of the system, then the p* value must be corrected to obtain ps. A method is derived that relates p* to ps. The method differs from the commonly used ones in that it does not require the knowledge of porosity, permeability, viscosity, compressibility, and drainage area. However, it does require the value of pressure at time zero. The derivation follows.

Derivation and Analysis

From a Horner-type plot for a hounded reservoir

p ws = p* - ln
(1)

The average static pressure ps is related to the extrapolated pressure p* by the Matthews-Brons-Hazebroek pressure correction function pressure correction function

= p DMBH (t DA)
(2)

From the material balance equation,

= 4 t DA,
(3)

where pi is the initial pressure prior to withdrawing a volume qtp of fluid. Also, pi can be considered any accurage stastic pressure with a corresponding production time. production time. Eq. 3 is written as

2.303 (pi - ps)= t DA,
(4)
4 m

where m, the negative of the slope of the buildup, is2.303 q equal to .4 kh

Substituting Eq. 4 in Eq. 2 gives

2.303 (pi - ps)= p DMBHm 2.303 4 m
(5)

JPT

P. 621

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