The Petroleum Resource Management System categorizes petroleum resources into three major categories as shown in Figure 1.
- Prospective Resources are those petroleum resources that may be recovered from undiscovered accumulations in future projects;
- Contingent Resources, are quantities of petroleum anticipated to be commercially recoverable from known accumulations from projects that are not yet mature enough to be considered commercial; and
- Reserves are quantities of petroleum accumulations that are anticipated to be commercially recoverable from developed or soon to be developed projects.
Two Methods for Calculating Resources
Volumetric estimates of petroleum resources and reserves are subject to substantial uncertainty because they rely on subsurface data collected from widely spaced wells. For this reason the SPE Guidelines recommends that estimates be provided at three levels of certainty – Low, Medium (Best) and High (1C, 2C,3C or 1P, 2P,3P).
The SPE also allows an organization to make its estimates using either deterministic or stochastic estimating processes. Here we will show you how they may be calculated using the stochastic process and then, a little later, the deterministic process.
Both estimates rely on the volumetric equation for calculating stock tank oil originally in-place (STOOIP), in barrels, is as follows:
- STOOIP = 7758 (bbls/acre-ft) x Oil Reservoir Area (acres) x Average Net Pay Thickness (ft) x Porosity (fraction) x Oil Saturation (fraction) x Oil Shrinkage (1/Bo)
The Recoverable Oil Volume is equal to the STOOIP multiplied by the recovery factor, which depends on the reservoir properties and the recovery mechanism (e.g. natural drive, pressure maintenance, enhanced recovery methods). The Estimated Ultimate Recovery (EUR) is the volume of oil that may ultimately be recovered using the enhanced recovery mechanism that may be applied at some time in the future.
- Recoverable Oil (bbls) = STOOIP x Recovery Factor
- EUR (bbls) = STOOIP x Ultimate Recovery Factor
In the deterministic process the measured Low, Best and High values or estimates are used for each variable (e.g. area, average thickness, porosity etc) in order to calculate the three deterministic estimates for Prospective Resources.