Question

When calculating OOIP, what is the mathematical effect on the final volume if the net-to-gross ratio is applied before calculating the drainage area volume instead of after?

Accepted Answer

The mathematical effect on the final volume is zero, meaning the result remains identical regardless of whether the net-to-gross ratio is applied before or after calculating the drainage area volume. This occurs because the standard calculation of Original Oil in Place (OOIP) is a multiplication of several independent variables: area, net pay thickness, porosity, the net-to-gross ratio, and oil saturation, all divided by the formation volume factor. Because multiplication follows the associative property, the order in which these factors are multiplied does not change the final product. For example, if you calculate the drainage area volume first by multiplying area by the total thickness and then apply the net-to-gross ratio, you are effectively calculating the net rock volume. If you apply the net-to-gross ratio to the thickness first to find the net pay and then multiply by the area, you are also calculating the net rock volume. Net-to-gross is a ratio representing the fraction of the total rock volume that is capable of storing and producing hydrocarbons, while the drainage area is the total spatial extent of the reservoir being evaluated. Since both approaches mathematically isolate the total net volume of the reservoir before applying variables like porosity and saturation, the final volumetric estimation of oil in place is mathematically invariant.

Home → All Courses → Engineering and Technology Courses → Petroleum Geology, Hydrocarbon Exploration and Production, and Reservoir Evaluation → Flashcard

When calculating OOIP, what is the mathematical effect on the final volume if the net-to-gross ratio is applied before calculating the drainage area volume instead of after?