| 摘要 |
A subterranean formation containing oil shale is prepared for in situ retorting by initially excavating a lower level drift adjacent a lower portion of an in situ oil shale retort site and excavating an air level void above the retort site. An undercut is excavated below a zone of unfragmented formation remaining within the retort site above the lower drift. The undercut tapers downwardly and inwardly to an opening in the lower drift for forming a draw point for withdrawing fragmented formation particles from the retort site. Formation within the remaining zone of unfragmented formation is explosively expanded downwardly in lifts for forming a fragmented permeable mass of formation particles containing oil shale within the retort site. A plurality of vertical blasting holes drilled in the remaining zone of unfragmented formation are loaded with explosive from the air level void prior to blasting each lift. After each lift is blasted, formation particles are withdrawn through the draw point for providing a void space with a selected void volume toward which the next lift is blasted. In one embodiment, such a selected void volume can be sufficient to provide substantially free expansion of formation within each lift. The blasting holes provide a means for access from the air level void to each new free face and to the void space below each free face for determining the void volume prior to blasting each lift. The steps of alternately blasting each lift and then withdrawing formation particles through the draw point for forming each new void space are repeated until a fragmented mass of a desired height is formed.
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