The "Flowing" Gas Material Balance
Authors: Mattar L. , McNeil R. 
Published: March 2010

Abstract
The traditional material balance (p/z) plot for gas pools requires fully built-up reservoir pressures, obtained by shuttingin the wells. The procedure described in this presentation does not require shut-in of wells. Instead, it utilizes information normally obtained but not usually used by reservoir engineers to quantify the original gas-in-place-the daily gas production rates and flowing pressures.
The classical pseudo-steady state analysis and its shortcomings are discussed. In addition, a new procedure called the "flowing" material balance is introduced. This procedure consists of a p/z plot of the "flowing" pressure (as opposed to the average "shut-in" reservoir pressure) versus cumulative production. A straight line drawn through the flowing pressure data and then, a parallel line, drawn through the initial reservoir pressure will give the original gas-in-place. A variation of this method, using wellhead pressures (tubing and casing) is discussed and a field example is included. The method is a very practical and powerful tool for the early quantification of reserves.

Introduction
The determination of gas reserves is a fundamental calculation in reservoir engineering. This information is crucial for the development of a production strategy, design of facilities, contracts and valuation of the reserves. Classically, reserves are estimated in three ways: volumetric, material balance and production decline. The volumetric and material balance methods estimate original gas-in-place whereas production decline yields an estimate of recoverable gas.
Volumetrically determined reserves can be very imprecise, because the method depends upon detailed knowledge of many reservoir characteristics that are often unknown such as the areal extent of the pool. The material balance method uses actual reservoir performance data and therefore is generally accepted as the most accurate procedure for estimating original gas-in-place. Once determined, the original gas-in-place can be used to reliably forecast the recoverable raw gas reserves under various operating scenarios. The production decline method also uses reservoir performance data but the result is an estimate of recoverable raw gas reserves under the existing operating conditions. A change in these operations, for example a lowering of the compressor suction pressure, can change the deliverability and the recoverable raw gas reserves. The original gas-in-place is therefore difficult to ascertain from production decline.
It has been understood for many years that estimates of original gas-in-place are theoretically possible using measured gas volumes and flowing pressures. The goal of this paper is to take the flowing material balance from a theoretical possibility to a practical reality.

Classical Material Balance
The classical material balance expresses a relationship between the average pressure in the reservoir and the amount of gas produced. When there has been no production, the pressure equals the initial reservoir pressure; when all the gas has been produced, the pressure in the reservoir is zero. In the case where the reservoir acts like a tank and there is no external pressure maintenance, the relationship between pressure and cumulative production is approximately linear.