Subsidence control at boundaries of an in situ oil shale retort

Abstract

An array of in situ oil shale retorts is formed in a development region in a subterranean formation containing oil shale. At least one void is excavated in each retort site, and remaining formation within each retort site is explosively expanded toward the void for forming a fragmented permeable mass of formation particles containing oil shale in each in situ retort. Overburden loads over an area of the development region are carried largely by the fragmented masses and partly by unfragmented partitions between retorts. Subsidence of overburden following explosive expansion is controlled at the boundary of the development region to avoid an abrupt change in the overburden load supported largely by the fragmented masses inside the boundary and the overburden load supported by unfragmented formation outside the boundary. Subsidence can be controlled by providing a gradual transition in overburden load support inside the boundary by progressively decreasing the proportion of horizontal area of formation explosively expanded to form the fragmented masses as the fragmented masses approach the boundary. This can be achieved by progressively reducing the area of the fragmented masses, or progressively spacing the fragmented masses farther apart, or both.