Spent shale grouting of abandoned in-situ oil shale retorts

In vertical modified in-situ (VMIS) oil shale retorting, the resource is processed in the ground. Large chambers of rubblized oil shale are formed by mining out about 20 to 40 percent of the in-place shale and blasting the balance into the created void. The mined-out material is brought to the surface and oil is recovered from it by surface retorting. The in-place material is pyrolyzed to recover oil, leaving large numbers of abandoned retort chambers underground. This type of oil shale processing may result in a number of environmental problems including in-situ leaching of the abandoned retorts, low resource recovery (large pillars are required to support the overburden), and subsidence. These problems may be mitigated by filling abandoned retorts with a grout prepared from spent shale produced during surface retorting of the mined shale. This would fill the void space created by mining, thus improving retort structural strength and stiffness, and reduce retort permeability to groundwater flow. If sufficient strength could be developed, it may be possible to design retorts so that the pillars could be retorted and resource recovery improved. This paper summarizes the results of laboratory investigations to produce a grout from spent shale. Spent shale grouts may be variously produced by modifying surface retorting conditions and/or by adding chemicals such as limestone or gypsum to the spent shale. A number of processes have been proposed and investigated including heating in steam at 700°C, limestone addition to surface spent shale followed by calcining at 1000OC, and limestone addition to raw shale followed by calcining at 1400-1500°C. This paper reviews these data in the framework of required grout characteristics for effective environmental control and technical and economic feasibility.