Jetting out weak areas for forming an in situ oil shale retort

Abstract

An in situ oil shale retort is formed in a subterranean formation containing oil shale. A void can be formed in formation within the retort site by directing fluid under pressure against a zone of relatively weakened formation, such as tuffs, gravel beds, or fractured oil shale, to erode such weakened formation into particle form, leaving a void space adjacent a remaining zone of unfragmented formation within the retort site. The void space can be formed by drilling a bore hole into the zone of weakened formation, placing a jet nozzle in the bore hole, and forcing a fluid such as water through the nozzle against the weakened formation for eroding it to form the void space. Eroded formation particles are passed to the bottom of the bore hole. Such water jetting techniques can be used to form voids in zones of weakened formation interspersed throughout the retort site. Remaining formation within the retort site is explosively expanded toward such a void space for forming a fragmented permeable mass of formation particles containing oil shale in an in situ oil shale retort. The amount of eroded formation particles jetted from the retort site can be measured prior to explosive expansion for providing a selected void fraction in the resulting fragmented mass. Explosive also can be placed in voids excavated by such jetting for such explosive expansion.