In the previous sections, the initial gas in place was calculated on a unit basis of 1 ac-ft of bulk productive rock, given information on the porosity and connate water. To calculate the initial gas in place on any particular portion of a reservoir, it is necessary to know, in addition, the bulk volume of that portion of the reservoir. If the porosity, connate water, and/or bulk volumes are not known with any reasonable precision, the methods described cannot be used. In this case, the material balance method may be used to calculate the initial gas in place; however, this method is applicable only to the reservoir as a whole because of the migration of gas from one portion of the reservoir to another in both volumetric and water-drive reservoirs.
The general material balance equation for a gas reservoir is derived:
Equation (3.10) could have been derived by applying the law of conservation of mass to the reservoir and associated production.
For most gas reservoirs, the gas compressibility term is much greater than the formation and water compressibilities, and the second term on the left-hand side of Eq. (3.10) becomes negligible:
When reservoir pressures are abnormally high, this term is not negligible and should not be ignored.
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