Volumetric
The volumetric method entails determining physical size of the reservoir, the pore volume within the rock matrix, and the fluid content within the void space. This provides an estimate of the hydrocarbon in place, from which the ultimate recovery can be estimated by using an appropriate recovery factor. Each of the factors used in the calculation have inherent uncertainties that, when combined, cause significant uncertainties in the reserves estimate.
The estimated ultimate recovery (EUR) of an oil reservoir, STB, is given by:
EUR = N(t) RF
Where N(t) is the oil in place at time t, STB, and RF is the recovery factor,fraction. The volumetric method for calculating the amount of oil in place (N) is given by the following equation:
Where:
N(t) = oil in place at time t, STB
Vb = bulk reservoir volume, RB = 7758 A h
7758 = RB/acre-ft
A = reservoir area, acres
H = average reservoir thickness, ft
φ = average reservoir porosity, fraction
So(t) = average oil saturation, fraction
Bo(p) = oil formation volume factor at reservoir pressure p, RB/STB
Similarly, for a gas reservoir, the volumetric method is given by:
EUR = G(t) RF
Where G(t) is the gas in place at time t, SCF, and RF is the recovery factor, fraction. The volumetric method for calculating the amount of gas in place (G) is given by the following equation:
Where:
G(t) = gas in place at time t, SCF
Vb = bulk reservoir volume,
CF = 43560 A h 43560 = CF/acre-ft
A = reservoir area, acres
h = average reservoir thickness, ft
φ = average reservoir porosity, fraction
Sg(t) = average gas saturation, fraction
Bg(p) = gas formation volume factor at reservoir pressure p, CF/SCF
Note that the reservoir area (A) and the recovery factor (RF) are often subject to large errors. They are usually determined from analogy or correlations. The following examples should clarify the errors that creep in during the calculations of oil and gas reserves.
Centre for Petroleum Studies 