Estimation of Natural Gas Reserves
Authors: Dranchuck P.M. 
Published: December 2007

In order that a natural gas reservoir may be properly exploited, the amount of gas contained in the reservoir must be known. Since this quantity, commonly called the reserve, cannot be measured directly during the life of the reservoir, it must be estimated. This paper, which restricts itself to dry gas reservoirs, presents a detailed review of the various reserve estimation methods currently available. Specifically, it reviews the volumetric and pressure decline methods and presents a unified material balance approach designed to deal with reservoirs which exhibit water production and encroachment simultaneously. The latter is capable of dealing with reservoir aquifer systems where the rate of influx is constant, or the rate of influx is a function of time and the influx, either radial or linear, is from aquifers which are either finite or infinite.

Introduction

As soon as an accumulation of a natural resource is discovered it is necessary to decide whether its exploration is economically feasible. One of the important factors entering into such a decision is the size of the accumulation. In the particular case where petroleum accumulations are involved it has become common practice to refer to the amount of petroleum contained in the accumulation as the reserve.

Although the reserve serves as the basis for the evaluation of assets and the making of exploitation policy decisions for all petroleum reservoirs, an error in the determination of a gas reserve is often mare critical than a similar error in the determination of an oil reserve. This stems from the fact that the sale and distribution of natural gas is such as to require producers and distributors to enter into long-term contractual agreements. However, the reserve can be determined with certainty only when the accumulation has been depleted, which time such knowledge is of historical interest only. Throughout the producing life of an accumulation, the reserve can be estimated with an accuracy which usually varies directly as the degree of depletion. Thus when an accumulation is first discovered and an accurate reserve estimate is most desired, it is least possible to obtain it. Consequently, the estimation of gas reserves has long been considered a problem of major significance.

In recent years the demand for natural gas has increased to the point where a more sophisticated engineering approach to natural gas reservoirs is desirable. Therefore, it seems appropriate at this time to review the various reserve estimation techniques available and to examine them critically with a view towards improvement wherever possible. Since the major portion of existing reserves is of the dry-gas type this paper will be restricted to reservoirs of that type.

THE VOLUMETRIC METHOD

One of the oldest known methods of reserve estimation has been referred to as the pore space method or the volumetric method. It consists of determining the total pore volume available to the gas and, from a knowledge of the gas composition and the reservoir pressure and temperature, calculating the amount of gas present. This method may be described in detail by means of a mathematical model. Therefore let us consider a differential segment of a reservoir rock, as indicated in Fig. 1. It may be noted that:

dV = dxdydh = dAdh..................(1)

= an incremental volume of the porous medium.