Formation of in situ oil shale retort in plural steps

Abstract

A subterranean formation containing oil shale is prepared for in situ retorting by forming a fragmented permeable mass of formation particles containing oil shale in an in situ retort site. The retort is formed by excavating a lower level drift adjacent to a lower portion of the retort site and excavating an undercut within the retort site below a zone of unfragmented formation remaining within the retort site above the undercut. The bottom of the undercut slopes downwardly toward the lower level drift which opens into one side of the undercut, the slope being generally at the natural angle of slide of oil shale particles. The remaining zone of unfragmented formation is blasted downwardly toward the undercut in a series of lifts in sequence progressing upwardly in the retort site. The mass of formation particles formed during such blasting in lifts tends to slope downwardly toward the side of the retort adjacent the lower level drift. Formation particles are withdrawn from the fragmented mass between lifts through the lower level drift to provide void space toward which each lift is blasted. Such withdrawal of formation particles can create relatively higher permeability in the fragmented mass along the side above the lower level drift and relatively lower permeability in the fragmented mass along the opposite side of the retort. During retorting operations, to compensate for such permeability gradient, oxygen supplying gas is introduced into the upper low permeability region of the fragmented mass, and off gas is withdrawn through the lower level drift at the lower high permeability region for producing a generally diagonal gas flow pattern through the retort.