Valuation and Subsurface Geology 807 



Gas reserves are figured to atmospheric pressure of 14.7 lbs.; but 

 sometimes the gas is already dedicated under contract of sale at a higher 

 pressure, such as 16.4 lbs. In such a case, the 16.4 is divided into the 

 reservoir pressure, and a smaller quotient results. It is obvious that it is 

 to the advantage of the gas pipe line to buy the gas on as high a pressure 

 base as possible — on a 29.4-lb. base only half as many cubic feet are paid 

 for as on a 14.7-lb. Gas pipe lines nearly always sell gas on a 14.7-lb. 

 basis. 



In addition to correcting for pressure and supercompressibility, cor- 

 rection must be made for the reservoir temperature. Charles' Law states 

 that as the temperature is increased, volume decreases in proportion to 

 absolute temperatures. If bottom-hole temperature is known, this reduc- 

 tion in volume can be figured by direct application of Charles' Law to 

 reduce the gas to the volume it would have at 60° F., which is the standard 

 temperature. If reservoir temperature is not known, it can be estimated 

 from temperature-gradient curves for the area — such temperature gradients 

 vary grossly from one province to another, but in any particular area are 

 quite regular and are well known. For the example lease, from curves 

 available on this area, it is found that the reservoir temperature should be 

 170° F., and by application of Charles' Law, 0.825 is arrived at as the 

 temperature-correction factor. 



Only the recovery factor [F in the formula) remains to complete the 

 calculation. Many gas fields have been produced to exhaustion — some 

 in recent enough times for complete production figures to be available. 

 Recoveries run very high, and with proper location of wells, one could 

 recover about all the gas in a water-drive reservoir. But, because of im- 

 proper structural location of wells, lenticularity, and other irregularities, 

 recoveries seldom run over 90 percent. On the Gulf Coast in water-drive 

 fields, the factor of 85 percent is commonly used at this time. 



In confined reservoirs where only gas expansion drive is available, it 

 is usual to fix some abandonment pressure. Trunk-line carriers do not 

 desire gas at a pressure of less than 500 to 750 pounds per square inch — 

 as the saying goes, "it will not buck the line" at lower pressures. Gas at 

 less than line pressure must be compressed before it can be sold, and com- 

 pressing requires equipment and costs money. Depending on the outlet to 

 which the gas is going, an abandonment pressure of 500 lbs. or more Ts 

 allowed for in fields where the pressure will decline. If one neglects 

 supercompressibility change with pressure, the recovery factor is equal to 

 the original-minus-abandonment-pressure difference divided by the origi- 

 nal pressure. 



For the example lease, a water-drive recovery factor of 85 percent is 

 used. The calculation therefore is 36,310,912 X 158 X 1.225 X .825 

 X .85, a total of 4,928,368,853 feet of recoverable gas. As gas is sold in 

 one-thousand-cubic-foot units, this amoilnt would be written as 4,928,368 

 mcf — and as reserves are usually estimated in millions of cubic feet, this 

 amount would be written as 4,928 mmcf. From production data it is 



