Three of the key tasks performed by reservoir engineers working on a natural gas accumulation are the estimation of the Original-Gas-in-Place, OGIP, in a gas accumulation, the estimation of the stabilized production rate from a vertical production well, and the forecasting of future performance of a gas reservoir or well.
These tasks have analogs with crude oil reservoirs, but there are significant differences between the methods used to analyze gas reservoirs due to the highly compressible nature of natural gas
Two common methods for calculating the OGIP in a gas accumulation are:
The Volumetric Method
The Material Balance Method
There are four drive mechanisms in a gas reservoir:
Gas expansion (most significant drive mechanism in conventional gas reservoirs)
Gas desorption (may only be present in certain unconventional gas reservoirs)
Rock and fluid expansion (expansion of the reservoir rock and interstitial water – typically only significant in over-pressured gas reservoirs)
Natural aquifer drive (or water encroachment)
The major flow regimes experienced by vertical gas production wells are identical to those experienced by vertical oil production wells. These flow regimes are:
Well dominated flow
Transient flow
Late transient flow
Boundary dominated flow
The stabilized production rates from a gas production well occur during the boundary dominated flow regime. These stabilized production rates are governed by the inflow performance relationships.
The stabilized inflow performance of gas wells and the transient performance of gas wells can be estimated with three formulations. These formulations are based on the manner in which the term, , is integrated. These three formulations are:
Pressure Formulation: valid for
Pressure-Squared Formulation: valid for
Pseudo-Pressure Formulation: valid for all pressures
The Pseudo-Pressure Formulation is the most rigorous formulation within any pressure range of interest (even if another formulation is valid in that pressure range)
Two common methods for making future reservoir or well forecasts for gas accumulations are:
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