The Uncertainty in the Answer
Learning objectives
- See the deterministic STOIIP as the P50 of a distribution
- Run a Monte Carlo over the volumetric inputs to get P90, P50, and P10
- Read the probability of commercial success against an economic limit
- Watch uncertainty and the recovery factor move the chance of success
One Number Hides a Distribution
The last two sections produced a single oil-in-place of about 151 MMstb and turned it into a go. But that number is not the truth, it is the middle of a range. Every input that fed it, the drainage area, the net pay, the average porosity, and the average saturation, is an estimate with its own uncertainty, and because the volumetric equation multiplies them the uncertainties compound.
P90, P50, P10
Draw one value from each input range, compute the oil in place, and repeat thousands of times, and the single number becomes a histogram. The petroleum convention reports it as three points: the P90, the proven low that is exceeded nine times in ten; the P50, the middle, where the deterministic answer sits; and the P10, the upside. The deterministic 151 was always just the P50 of this spread.
The Probability of Commercial Success
Now overlay the economic limit. At a given recovery factor the field must hold more than some threshold of oil in place to clear the minimum recoverable volume, and the fraction of the Monte Carlo above that line is the probability of commercial success. Widen the input uncertainty and the spread fans out; drop the recovery factor and the limit slides right, eating into the probability. A prospect is sanctioned on the P90 and the chance of success, never on a single number, and that is the honest end of every petrophysical evaluation.
References
- Rose, P. R. (2001). Risk Analysis and Management of Petroleum Exploration Ventures. AAPG.
- Cosentino, L. (2001). Integrated Reservoir Studies. Editions Technip.