Establishing a combustion zone below a sill pillar in an in situ oil

Abstract

An in situ oil shale retort is formed in a subterranean formation containing oil shale. The retort contains a fragmented permeable mass of particles containing oil shale. An open base of operation is excavated in the formation above the retort site, and an access drift is excavated to the bottom of the retort site. Formation is explosively expanded to form the fragmented mass between the access drift and an elevation spaced below the bottom of the base of operation, leaving a horizontal sill pillar of unfragmented formation between the top of the fragmented mass and the bottom of the base of operation. The sill pillar provides a safe base of operation above the fragmented mass after it is formed. The fragmented mass is formed by, among other steps, drilling blasting holes from the base of operation down through the sill pillar and detonating explosive in the holes to form the fragmented mass of particles in the retort below the sill pillar. The fragmented mass is ignited through at least a first one of such blasting holes to establish a combustion zone in the top of the fragmented mass. The combustion zone is then advanced across the top of the fragmented mass by generating a gas pressure differential between the first blasting hole and at least a second blasting hole to draw gas down through the first blasting hole, across a top portion of the fragmented mass, and up through a second blasting hole so that gas flow across an upper portion of the fragmented mass spreads the combustion zone across the fragmented mass.