Mapping Probability of Success and Risk of Failure

Michael E. Hohn

Before drilling, a company should know both the volume of hydrocarbon expected, and the range of likely outcomes. Cumulative production of gas can be estimated from wells near a site under consideration, but the actual value cannot be known until a well has been drilled. How much the predicted and observed values differ depends on well control and homogeneity of the reservoir.

Contour maps of exceeding probability are used to set confidence intervals around predictions of gas volumes. A new variable is created by assigning a value of one to wells that exceeded a selected production threshold and zero to those that did not. Likelihood of exceeding this threshold for a site equals the weighted average of this variable in nearby wells.

Ordinary contour maps of 10-year cumulative gas production from 400 wells producing from Devonian shales in southwestern West Virginia showed a complex pattern of highs and lows, whereas smoothed maps of kriged estimates revealed only major trends. Mapping the probability of exceeding 100 mmcf showed northeast-southwest and northwest-southeast trends, as well as many local trends in other directions. Mapping probability of exceeding 200 mmcf uncovered a single trend of favorable areas from northeast to southwest.

Drawn as histograms, site-specific probabilities for a number of cut-offs show that an appreciable probability exists for obtaining production within the range of 0-350 mmcf across most of the study area. Isolated areas of very high production display a marked departure from the typical log-normal distribution, probably because natural fractures enhanced production.

AAPG Search and Discovery Article #91031©1988 AAPG Eastern Section, Charleston, West Virginia, 13-16 September 1988.