Abstract
The equilibrium free-gas: liquid ratio (FGLR) method is a new method for calculating gas production rates within a material balance model. The method calculates the reservoir FGLR at which there would be no overall movement of the gas-liquid contact. It is then assumed that the reservoir is tending towards such an equilibrium position i.e. the reservoir FGLR converges towards the equilibrium FGLR. Given an estimate of the rate of convergence and a forecast of liquid production (from decline analysis, for example), these assumptions are sufficient to give an approximate forecast of a reservoir or well gas production profile.
The method can be used to forecast associated gas production from oil-fields with significant gas-caps, especially thin oil-rims, for which wells are usually drilled close to the original gas-oil contact. It gives a gas production forecast broadly comparable to a decline-curve oil production forecast. Unlike the Muskat method, it does not depend on unmeasurable reservoir-scale relative permeabilities.
The method has been been mathematically derived from some basic assumptions about average reservoir behaviour. The validity of the assumptions and the method have been checked by comparison with historical production data from eight oil-rim reservoirs. The method has been successfully in use for reserves evaluation for a group of producing Nigerian oil-rim fields for the past four years. The evidence suggests that the method is a useful and robust approximation, but further validation would be very useful.
If further validation confirms its general validity, then the equilibrium free-gas:liquid ratio method would become a useful part of material balance theory and calculations for oil-fields with associated gas production.
