In situ oil shale retort having horizontally offset pillars

Abstract

An in situ oil shale retort is formed in a subterranean formation containing oil shale. Formation is excavated from within a retort site for forming a plurality of vertically spaced apart voids extending horizontally across different levels of the retort site, leaving a separate zone of unfragmented formation between each pair of adjacent voids, and leaving one or more pillars within each void for providing temporary roof support for unfragmented formation above each void. A plurality of horizontally spaced apart vertical blast holes are drilled in each zone of unfragmented formation below the voids. A pillar within a first void is offset horizontally from at least a portion of a pillar in a second void excavated directly below the first void. This provides an access region in the first void above at least a portion of the pillar in the second void so that vertical blast holes can be drilled into a zone of unfragmented formation below the pillar in the second void from the access region in the first void. Explosive is placed in the blast holes and such explosive in detonated for explosively expanding the zones of unfragmented formation toward the voids for forming a fragmented permeable mass of formation particles containing oil shale in an in situ oil shale retort. By horizontally offsetting pillars in adjacent voids, a combination of upwardly drilled and downwardly drilled blast holes can be used to place explosive throughout a zone of unfragmented formation between such voids.