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SPE 135804

Direct Method of Estimating Average Reservoir Pressurefor Flowing Oil and Gas WellsRam G. Agarwal, SPE, Petrotel Inc.

Copyright 2010, Society of Petroleum Engineers

This paper was prepared for presentation at the SPE Annual Technical Conference and Exhibition held in Florence, Italy, 1922 September 2010.

This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewedby the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, ormembers. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print isrestricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract

A knowledge of the average reservoir pressure ( p ) and its changes as a function of time or cumulative production is essential to

determine the oil-in-place (OIP) or original gas-in-place (OGIP), to estimate reserves and to track and optimize reservoir

performance. The common practice of determining p in moderate permeability reservoirs has been to run pressure buildup tests.

In the current economic environment, buildup tests are almost non-existent except for very expensive exploratory wells. Moreover,

time required for a pressure buildup test to reach p in low permeability reservoirs is prohibitively long. Fortunately, flowing

pressures and rate data are continually collected from oil and gas wells. Data quality and quantity is usually good especially from

wells installed with permanent pressure gauges. Such data for gas wells is currently being analyzed by assuming OGIP and

estimating p required for calculating pseudo time. This is done in an iterative manner for using advanced decline curve analysis

methods.

The purpose of this paper is to discuss a new findingthat will enable direct estimation of p using flowing pressures and rate dataobtained from oil and gas wells during the pseudo steady-state flow period. In theory, pseudo steady-state flow requires that a well

is produced at a constant rate. However, this limitation can be easily removed based on the work published in the SPE literature bythis author and others whereby variable rate data can be converted to constant rate production data.

The significance of the subject paper is that it will permit: a) direct determination of p using flowing wellbore pressures and rate

data thus facilitating estimates of OGIP and OIP, b) estimation and/or validation of the value of the initial reservoir pressure ( )ip ,which is normally suspect, and finally, c) enhancement or possible elimination of the current iterative process used for determining

OGIP by advanced decline curve analysis methods.

Introduction

Estimation of oil-in-place, gas-in-place, and reserves is an integral part of reservoir development and management. This requires

knowledge of initial reservoir pressure and average reservoir pressure. Initial reservoir pressure should be readily available fromnewly drilled wells. Unfortunately, a true value of initial reservoir pressure is rarely available. For new wells, initial reservoir

pressure is often obtained from pressure buildup tests. The quality of such estimation depends on wellbore storage, the reservoir

permeability and the duration of the well test. The same limitations apply to the estimation of average reservoir pressure. Theproblem is not too serious in case of moderate to high permeability reservoirs but it becomes practically impossible to estimate

reservoir pressure in low permeability reservoirs. Moreover, average reservoir pressure values are needed as a function of

production time or cumulative production. In the current economic environment, pressure buildup tests are rarely conducted. Thisposes a challenging situation for reservoir engineers to estimate OIP, OGIP, and reserves.