The paper was presented at the SPE/DOE Unconventional Gas Recovery Symposium of the Society of Petroleum Engineers held in Pittsburgh, PA, May 16-18, 1982. The material is subject to correction by the author. Permission to copy is restricted to an abstract of not more than 300 words. Write: 6200 N. Central Expwy., Dallas, TX 75206.

Abstract

Vast quantities of natural gas exist as solid hydrates in the world's permafrost and deep sea environments. Russian experts estimate potential permafrost and deep sea environments. Russian experts estimate potential resources in excess of 10 7 TCF natural gas. Geophysical inference and direct sampling seem to substantiate the existence of hydrates where thermodynamic conditions are favorable. Their ubiquity provides an interesting link to another unconventional hypothesis of unconventional gas supply, namely the abiogenic origin of deep earth gas. A method to evaluate the abiogenic hypothesis is to compare the isotopic, trace element and compositional signature of hydrate samples from deep lake sediments in basins over crystalline basement rocks (Lake Superior) and over sedimentary rocks in connection with a producing petroliferous province (Lake Michigan). Because of the complex relations between in situ dissociation and production of the free gas phase actual extraction methods will be technically complicated. A method to circulate warm surface water through a set of injection and withdrawal wells in deep sea hydrate deposits promises some advantages over conventional offshore drilling practices. promises some advantages over conventional offshore drilling practices. Even if only a fraction of the claimed hydrate resources are accessible and producible the absolute amount is still very large compared to all other producible the absolute amount is still very large compared to all other conventional and unconventional gas resources.

UNCONVENTIONAL GAS SOURCES AND ENERGY SUPPLY

In its quest to augment available fossil energy reserves in general and to lessen energy dependency from outside sources the U.S. oil and gas industry is intensifying and extending its exploration and development efforts for new sources of oil and gas. This activity concentrates first on the finding of new reservoirs of conventional hydrocarbons and developing new, more efficient production and recovery techniques. At the same time this search is extended to so-called unconventional supplies more specifically to unconventional gas. These gas sources are: (1) tight gas formations (especially Western Sands) ; (2) mineable and unmineable methane rich coal beds; (3) gas bearing Devonian shales (especially in the Appalachian basin); (4) geothermal-geopressured dissolved methane in formation waters (Gulf Coast area); (5) methane hydrate sediments. Of these the first four have received most of the official attention in various studies of the National Petroleum Council. Petroleum Council. In the conventional analysis of unconventional gas the greatest potential for future supply is assigned to tight gas reservoirs, furnishing potential for future supply is assigned to tight gas reservoirs, furnishing 30 to 50% of the demand in the next century. Baker has based his definition of unconventional gas on a purely economic context. "It is located in a reservoir with properties that prevent its recovery by conventional practice at current prices." Their geologic setting remains quite ordinary. In the case of the tight sands a distribution plot of permeability of U.S. reservoirs versus percent reservoir volume on a permeability of U.S. reservoirs versus percent reservoir volume on a log-probability plot produces a smooth curve. Conventional gas reservoirs are those with permeabilities of one millidarcy and more, unconventional gas reservoirs have less than one millidarcy permeability. This definition may be useful, but it is nevertheless arbitrary and masks the fact that the transition from conventional to unconventional gas is gradual for this particular example. particular example. This can be more clearly demonstrated with a modified McKelvey diagram for the definition of reserves and resources applied to natural gas as shown in Figure 1. It bases its classification of resources as proven, probable, and speculative on the basis of certainty of identification and probable, and speculative on the basis of certainty of identification and location, and as feasible, marginal, and submarginal on the state of technological feasibility of extraction and economic feasibility. In the upper left hand corner we have a field of proven reserves--essentially congruent with conventional sources. As one moves outwards from this field reserves become less certain as conditional hypothetical and finally speculative resources.

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