In the case of a porous reservoir containing liquid and gas, the formation energy is the elastic energy of the liquid, the gas, and the porous reservoir, which are under stress because of formation pressure and rock pressure. Petroleum always contains a large number of gases in a dissolved state that are liberated from it when the pressure is lower than the saturation pressure. The gases dissolved in petroleum usually account for a significant part of the formation energy. When liquid or gas is extracted, there is a decrease in the formation pressure. The volume of pore space of the bed decreases, and the liberated energy is used to move the liquids (petroleum and water) and/or gas through the pores of the bed to the bottoms of the boreholes and then up the holes—that is, the energy is made use of in the exploitation of the oil field.
The exploitation of an oil bed is governed by the type of formation energy expended in the bed to move liquids and gases toward the bottoms of the boreholes. The formation energy expended in the exploitation of an oil, oil-gas, or gas field can be replenished by a natural influx of water if the oil- and gas-bearing bed extends as far as a basin of water, a runoff point for surface waters, or a similar feature. The replenishment can also result from the artificial introduction of supplementary energy into the bed through the injection of water and/or gas.
The formation energy balance, which is the ratio of energy expended for extraction and energy introduced into the bed from outside, is one of the most important indexes of the exploitation of a deposit and is characterized primarily by the value of the formation pressure.
IU. P. BORISOV