All forms of material balance equations are simply a different way of expressing the following fundamental relationship:
Barrels of original oil in place =
In the midst of some initial period the productive life of a reservoir (usually in original condition) and a subsequent period, the production cause the decline of reservoir pressure unless the invasion of water to keep it. If the pressure declines certain changes occur in the physical characteristics of oil and gas remaining in the reservoir in the subsequent period. A study and evaluation of these various changes in the characteristics of the reservoir fluid in relation to the volume removed as production allows calculation of the original oil on the surface.
The reserve estimate should apply a recovery factor. Changes in the properties of fluids in the reservoir should be recognized and listed. These radios require high production, significant time intervals of several years, or perhaps both. The engineers only use a slide rule or calculator mechanics often face a prodigious task in the discovery and identification of minor changes in the properties of fluids in the reservoir and applying it mathematically in the equation of material balance. The problem is more complicated when the reservoirs caused by combinations of drive mechanisms: solution gas drive, gas cap expansion, or pushing water.
Consequently the engineers have to resort to electronic analysis. These mechanisms intrinsically naive discover and apply many of the changes under the reservoir fluid properties that may be overlooked and neglected in ordinary computations. So the solution resulting from the material balance equation can be done very quickly and well before the period elapsed from the reservoir productive life in a way possible.
Space does not permit derivations of the basic equation of balance of matter or description of all the factors that are involved. This discussion does nothing but give the exploration geologist or engineer an overview of the basic concept. Several excellent texts covering the subject in detail. (Pirson, 1958, Muskat, 1949 Calhoun, 1953).