Calculating Unit Recovery from Volumetric Gas Reservoirs

In many gas reservoirs, particularly during the development period, the bulk volume is not known. In this case, it is better to place the reservoir calculations on a unit basis, usually 1 ac-ft of bulk reservoir rock. This one unit, or 1 ac-ft, of bulk reservoir rock contains

Connate water: 43,560 × φ × S_w ft³

Reservoir gas volume: 43,560 × φ × (1 – S_w) ft³

Reservoir pore volume: 43,560 × φ ft³

The initial standard cubic feet of gas in place in the unit is

G is in standard cubic feet (SCF) when the gas volume factor B_gi is in cubic feet per standard cubic foot (see Eq. [2.16]). The standard conditions are those used in the calculation of the gas volume factor, and they may be changed to any other standard by means of the ideal gas law. The porosity, φ, is expressed as a fraction of the bulk volume and the initial connate water, S_wi, as a fraction of the pore volume. For a reservoir under volumetric control, there is no change in the interstitial water, so the reservoir gas volume remains the same. If B_ga is the gas volume factor at the abandonment pressure, then the standard cubic feet of gas remaining at abandonment is

Unit recovery is the difference between the initial gas in place and that remaining at abandonment pressure (i.e., that produced at abandonment pressure), or

The unit recovery is also called the initial unit reserve, which is generally lower than the initial unit in-place gas. The remaining reserve at any stage of depletion is the difference between this initial reserve and the unit production at that stage of depletion. The fractional recovery or recovery factor expressed in a percentage of the initial in-place gas is

or

Experience with volumetric gas reservoirs indicates that the recovery factor will range from 80% to 90%. Some gas pipeline companies use an abandonment pressure of 100 psi per 1000 ft of depth.

The gas volume factor in the Bell Gas Field at initial reservoir pressure is 0.00533 ft³/SCF, and at 500 psia, it is 0.03623 ft³/SCF. The initial unit reserve or unit recovery based on volumetric performance at an abandonment pressure of 500 psia is

These recovery calculations are valid provided the unit neither drains nor is drained by adjacent units.