Reservoirs containing only free gas are termed gas reservoirs. Such a reservoir contains a mixture of hydrocarbons, which exists wholly in the gaseous state. The mixture may be a dry, wet, or condensategas, depending on the composition of the gas, along with the pressure and temperature at which the accumulation exists. Gas reservoirs may have water influx from a contiguous water-bearing portion of the formation or may be volumetric (i.e., have no water influx).
Gas formation volume factor: It is factor Bg is defined is defined as the actual volume occupied by n moles of gas at a specified pressure and temperature, divided by the volume occupied by the same amount of gas at standard conditions. Applying the real gas equation-of-state to both conditions gives:
The gas expansion factor is simply the reciprocal of Bg,
Bg = gas formation volume factor, ft3/scf .
This chapter presents two approaches for estimating initial gas in place G, gas reserves, and the gas recovery for volumetric and water-drive mechanisms: