The Volumetric Equation

Part 13, Chapter 13: Net Pay, Cutoffs, and Volumetrics

Learning objectives

Write the volumetric STOIIP equation and name each term
Read the cascade from bulk rock to stock-tank oil
Explain the formation volume factor as the reservoir-to-surface shrinkage
Separate oil in place from recoverable reserves

Everything in One Line

Every curve the course produced meets in the volumetric equation. The stock-tank oil in place is

N = \frac{7758\,A\,h\,\phi\,(1-S_w)}{B_o},

with the area $A$ in acres, the net thickness $h$ in feet, and the constant 7758 turning acre-feet into barrels. Porosity, saturation, and net thickness are exactly the answers the chapters before delivered.

The Cascade

Read left to right the equation whittles the rock down. The bulk rock volume becomes the pore volume when multiplied by porosity, the pore volume becomes the hydrocarbon pore volume when multiplied by the oil fraction $(1-S_w)$ , and that reservoir oil becomes stock-tank oil when divided by the formation volume factor $B_o$ . $B_o$ is greater than one because a barrel in the reservoir, with gas dissolved in it under pressure, shrinks as that gas comes out of solution on the way to the surface.

In Place Is Not Recoverable

The number is the oil in place, not what will be produced. Recoverable reserves are the STOIIP times a recovery factor, anywhere from ten to sixty percent depending on the drive mechanism and the development, the subject of the reservoir-engineering course downstream. The volumetric equation is the petrophysicist's handoff: every term in it was measured from the logs, and the single number it returns is what the asset team carries forward.

References

Tiab, D. and Donaldson, E. C. (2015). Petrophysics, 4th ed. Gulf Professional Publishing.
Cosentino, L. (2001). Integrated Reservoir Studies. Editions Technip.