Abstract

The pressures used in constructing the traditional material balance (p/z) plotmust be fully built-up reservoir pressures, usually obtained by shutting-in thewell The procedure described in this presentation does NOT require the well tobe shut-in. Instead, it utilizes information obtained from standard goodoperating practices (production and flowing pressure) to quantify thegas-in-place, without having to shut-in the well.

The classical pseudo-steady state analysis and its shortcomings are discussed.In addition, a new procedure known as the "flowing" Material Balance isintroduced. This procedure consists of p/z plot of the "flowing" pressure (asopposed to the "shut-in average" reservoir ressure) versus cumulativeproduction. A straight line can be drawn through the data and then, a parallelline, drawn through the initial reservoir pressure, gives the initialgas-in-place, ariations of this method, using wellhead pressures (tubing andcasing) re discussed, along with field examples. The method is a very PRACTICALand powerful tool for the early quantification of reserves.

Introduction

The determination of gas reserves is a fundamental calculation in reservoirengineering. This information is of critical importance in determiningproduction strategy, design of facilities, contracts and the value of thereserves. Reserves can be estimated in three ways - Volumetric, Production Decline and Material Balance. The Production Decline gives an estimate ofrecoverable gas, whereas the ther two give an estimate of gas-in-place, In thereservoir engineering literature, the word "reserve refers" to the rawgas-in-place whereas in the commercial world, the word "reserve" often meansthe recoverable sales gas. In this paper, the word "reserve" is used to denotethe raw gas-in-place.

Volumetrically determined reserves can be very imprecise, because they dependon the areal extent of the pool, which is often unknown Production Declinemethods give the "recoverable" gas volumes under the existing operatingconditions. A change in these operations, for example a lowering of thecompressor suction pressure can change the magnitude of the recoverable gas.The original gas-in-place is difficult to ascertain. The Material Balancemethod (given well-behaved data) gives the most accurate estimate of originalgas-in-place. Once this value is known, it can be used to forecast therecoverable gas under various operating scenarios.

Classical Material Balance

The classical material balance expresses a relationship between the averagepressure in the reservoir and the amount of gas produced. When there has beenno production, the pressure equals the initial reservoir pressure, when all thegas has been produced, the pressure in the reservoir is zero. This assumes thatthe reservoir acts like a tank and there is no external pressure maintenance.The relationship between pressure and cumulative production is approximatelylinear. If the compressibility factor z, is taken into account, then thematerial balance plot of p/z versus cumulative production, Q, is a straightline going from the initial pressure p/z to the initial gas-in-place, IGIP.Deviations from this straight line can be caused by external recharge or offsetdrainage. For the purposes of this resentation, we are only consideringreservoirs which have straight line material balance plot of p/z versus Q.

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