fluid levels, and the weight of the gas are known, the bottom-hole pressure can 

 be computed. Often the original, bottom-hole pressure can be estimated from 

 the depth of the formation. Normal pressure in post-Eocene sediments on the 

 Gulf Coast increases 461/2 pounds per square inch with each 100 feet of depth. 

 There are a few low-pressure and a few high-pressure reservoirs, but not many 

 which vary from the 0.465 rule — which incidentally, is the weight of a column 

 of sea water. When depth in feet is multiplied by 0.465, the approximate 

 bottom-hole pressure is obtained. In this case, 5000 X 0.465 gives 2325 

 pounds as the bottom-hole pressure. This 0.465 gradient applies well along the 

 Gulf Coast on reservoirs of Oligocene and younger age — in other areas factors 

 as high as 0.760 and as low as 0.420 are used. These values have been published 

 and are usually well-known to experienced geologists. 



A reservoir pressure of 2325 pounds amounts to 158 atmospheres (2325 

 -f- 14.7) ; so there is 158 times as much gas in the reservoir as it would hold at 

 atmospheric pressure. This statement, however, is not quite true because Boyle's 

 Law does not work exactly at higher pressures — it was worked out at lower 

 pressures and later research has indicated variance at high pressures. This 

 amount is known as supercompressibility, and the factor is called the Z factor. 

 If the composition or density of the gas is known, one can figure the Z factor 

 for different pressures and temperatures. In the present case, the Z factor could 

 be estimated for normal Gulf Coast gas at that depth to give a multiplier of 1.225. 

 The Z factor results in an additive multiplier down to depths of around 10,000 

 feet, and with normal pressures, is greatest around 4000 feet. 



Gas reserves are figured to atmospheric pressure of 14.7 pounds; but some- 

 times the gas is already dedicated under contract of sale at a higher pressure, 

 such as 16.4 pounds. In such a case, the 16.4 is divided into the reservoir pres- 

 sure, 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-pound base only half as many cubic feet are paid for as on a 14.7-pound 

 base. Gas pipe lines nearly always sell gas on a 14.7-pound base. 



In addition to correcting for pressure and supercompressibility, correlation 

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

 temperature is increased, volume decreases in proportion to absolute temper- 

 atures. If bottom-hole temperature is known, this reduction in volume can be 

 figured by direct application of Charles' Law to reduce the gas to the volume it 

 would have at 60F, 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 170F, and by application of Charles' Law, 0.825 is arrived at as 

 the temperature-correction factor. 



808 



