806 Subsurface Geologic Methods 



Assume that the same 160 acres overlies gas instead of oil. In gas 

 valuation, the formula begins with 43,560 as the number of cubic feet in 

 one acre (one always deals with cubic feet in gas — mcf, thousand cubic 

 feet — or mmcf, million cubic feet) . In order to obtain cubic feet of 

 reservoir space it would be necessary to multiply by porosity and con- 

 nate water factors — in this case again refer to figure 413 and find that 

 for 22 percent porosity and 34 percent connate water, there are 6,512 cubic 

 feet of gas space in each acre foot. Again, using 5,576 acre feet of reser- 

 voir, one finds that there are 36,310,912 cubic feet of storage space in the 

 reservoir. At atmospheric pressure and temperature, this reservoir would 

 contain 36,310,912 cubic feet of air, and would hold that much gas. 



But this reservoir is not at atmospheric conditions; it is under very 

 high pressure and is always much hotter than the atmosphere. According 

 to Boyle's Law, as the pressure is increased, the amount of gas a reservoir 

 contains .also increases. If the pressure is doubled (temperature remain- 

 ing the same) there is twice as much gas — so that each time an atmosphere 

 of pressure (14.7 lbs.) is added, a reservoir full of gas, or 36,310,912 

 cubic feet, is added. 



If bottom-hole pressures have been taken on the gas wells, exactly 

 the pressure existing in the reservoir will be known. Or, if the well-head 

 pressures, 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 46-| lbs. 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, 5,000 X 0.465 gives 2,325 lbs. 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 2,325 lbs. amounts to 158 atmospheres; (2,325 

 -i- 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 pres- 

 sures. 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 addi- 

 tive multiplier down to depths of around 10,000 feet, and with normal 

 pressures is greatest around 4,000 feet. 



