Quantitative analysis of the economically recoverable resource

The objective of this study is to obtain estimates of the economically recoverable gas in the Appalachian Basin. The estimates were obtained in terms of a probability distribution, which quantifies the inherent uncertainty associated with estimates where geologic and production uncertainties prevail. It is established that well productivity on a county and regional basis is lognormally distributed, and the total recoverable gas is Normally distributed. The expected (mean), total economically recoverable gas is 20.2 trillion cubic feet (TCF) with a standard deviation of 1.6 TCF, conditional on the use of shooting technology on 160-acre well-spacing. From properties of the Normal distribution, it is seen that a 95 percent probability exists for the total recoverable gas to lie between 17.06 and 23.34 TCF. The estimates are sensitive to well spacings and the technology applied to a particular geologic environment. It is observed that with smaller well spacings - for example, at 80 acres - the estimate is substantially increased, and that advanced technology, such as foam fracturing, has the potential of significantly increasing gas recovery. However, the threshold and optimum conditions governing advanced exploitation technology, based on well spacing and other parameters, were not analyzed in this study. Their technological impact on gas recovery is mentioned in the text where relevant; and on the basis of a rough projection an additional 10 TCF could be expected with the use of foam fracturing on wells with initial open flows lower than 300 MCFD. From the exploration point of view, the lognormal distribution of well productivity suggests that even in smaller areas, such as a county basis, intense exploration might be appropriate. This is evident from the small tail probabilities of the lognormal distribution, which represent the small number of wells with relatively very high productivity.