Recovering Oil by Retorting a Nuclear Chimney in Oil Shale*

Abstract

A method is proposed for recovering oil by retorting oil shale shattered by deep underground nuclear explosions. Because most of the oil will be obtained from chunks of oil shale with maximum dimensions exceeding 1 ft, retorting times of up to several weeks are needed for complete recovery. A study of the heat economy of the retorting process in an underground nuclear chimney suggests that the nuclear detonation itself and the subsequent controlled combustion of residual carbon in retorted oil shale will provide ample energy. The proposed method is shown to be thermally efficient. A 5-ft diameter, 12-ft high aboveground batch retort has been constructed for the experimental retorting of mixtures of oil shale particle sizes. Low rates of air and recycle gas, low-retorting temperatures and slow combustion front advance have characterized its operation. In spite of imperfect mist-separating equipment, recoveries of about 60 percent of Fischer assay have been obtained. The particle size distribution of 30 gal/ton oil shale has not been appreciably altered by the retorting. The operating conditions of a nuclear chimney retort will be defined by the recycle gas-to-air ratio and the overall rate of gas injection. Determination of optimum operating conditions will be complicated by the large number of factors involved. However, the proposed technique appears to be a promising one for recovering the oil from thick, deep, oil shale deposits.

Introduction

The possibility of using nuclear explosions to produce bulk permeability in large masses of oil shale removes a major technical obstacle to oil recovery by in-situ retorting. This article presupposes the feasibility of making a nuclear chimney in oil shale, and is directed at the techniques of retorting the shale in such a chimney.

DEFINITIONS

(Many petroleum engineers are unfamiliar with the subject of this article. Following are definitions of terms that may be new to the reader. For a more comprehensive discussion of underground nuclear explosion technology, see Refs. 1 through 5.) Retorting of oil shale - Conversion by heating of kerogen, the waxy organic matter contained in oil shale, to a liquid very similar to petroleum; vapors and gases are also produced in retorting. In-situ retorting of oil shale - Retorting of oil shale without first mining it; ordinarily this is impractical due to low heat conductivity and low bulk permeability of oil shale deposits. Oil shale grade - Volume of organic liquid produced by retorting oil shale, divided by the weight of the raw (unretorted) sample. The modified Fischer assay is the standard procedure for measuring grade, which is commonly expressed in gallons per ton. Bulk permeability or fracture permeability - Average permeability of a large mass of material including the effect of fractures (for example, as opposed to the permeability that would be measured on a core sample). Underground nuclear explosion - A nuclear explosion buried so deeply in the ground that there is no dynamic venting of gases created by the detonation. Nuclear breaking - Fracturing and fragmenting of large volumes of rock by the explosive force of underground nuclear detonations, and the concomitant increase in bulk permeability of this rock. Introduction of such permeability into oil shale deposits may make in-situ retorting practical. Nuclear chimney - The roughly cylindrical region produced by an underground nuclear explosion that is filled with rock fragments. A fractured (but not fragmented) region surrounds the chimney (Fig. 1). Chimney collapse - The process by which a nuclear chimney is formed. Energy yield - A measure of energy released in a nuclear explosion (usually expressed in kilotons, equivalent to thousands of tons of TNT). One kiloton is more precisely defined as 10(12) calories.

JPT

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