Volumes: From Pay to Barrels

The Number Becomes a Decision

The interpretation ends in a number, and the number ends in a decision. The net pay from the last section becomes a hydrocarbon pore thickness, the volumetric equation turns it into the stock-tank oil in place, a recovery factor turns that into recoverable reserves, and an economic threshold turns the reserves into a go or a no-go.

Every Bar a Fraction of the Last

The cascade is sobering. The gross rock of the oil column is billions of barrels of volume, but most of it is grain. Take the porosity and only the pore space is left; take the oil fraction and only the hydrocarbon pore is left; divide by the formation volume factor for the shrinkage at surface and you have the oil in place; multiply by a recovery factor of a third or less and you have what you can actually produce. The last sliver, a fraction of a fraction, is what pays for the well.

The volumetric equation behind the bars is the standard one, $N = 7758,A,h,\phi,(1-S_w),/,B_o$, with $A$ in acres, $h$ the hydrocarbon pore thickness in feet, and $B_o$ the formation volume factor that shrinks reservoir barrels to stock-tank barrels at the surface.

Why It All Mattered

Slide the drainage area and the recovery factor and watch the decision flip from go to no-go. That flip is why every cutoff, every saturation exponent, and every $R_w$ in the course mattered: a few saturation points or a few feet of net pay move the recoverable reserves across the economic line. The same logs, read carelessly, are a field or a dry hole.

References

• Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG.
• Cosentino, L. (2001). Integrated Reservoir Studies. Editions Technip.