Importance

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Abstract

An in situ oil shale retort is formed in a retort site in a subterranean formation containing oil shale. The in situ retort contains a fragmented permeable mass of formation particles containing oil shale within top, bottom and generally vertically extending side boundaries of unfragmented formation. Formation is excavated to form at least one void in the subterranean formation while leaving zones of unfragmented formation above and below such a void and at least one support pillar of unfragmented formation in the void. Such a pillar of unfragmented formation extends between the zones of unfragmented formation above and below the void and is explosively expanded by use of techniques which minimize shock damage to the zones of unfragmented formation. The techniques, which can be used individually or in various combinations, include placing explosive charges into the top, bottom and middle portions of the pillar. Explosive charges in the middle portion are detonated first, thereby explosively expanding at least a first segment of the middle portion toward the void. Such a first segment is substantially an entire horizontal cross section of the pillar. Thereafter, all of the remaining explosive charges in the pillar are detonated. If desired, the explosive charges in the middle portion can be more energetic than the charges in the top and bottom portions. Prior to explosively expanding the pillar, a fracture is formed between the top and bottom portions of the pillar and overlying and underlying zones of unfragmented formation to decouple the pillar from the formation. Additionally, when the roof of the void is the top boundary of the retort being formed, a plurality of rock bolts are placed into the zone of unfragmented formation overlying the void at angles toward such a support pillar so that at least a portion of such rock bolts converge in the formation directly above the pillar.

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First Claim