Preparation and Injection of Grout From Spent Shale for Stabilization of Abandoned In-Situ Oil Shale Retorts
Authors: Mallon Richard G. 
Published: December 2007

Summary
A procedure has been developed for stabilizing abandoned in-situ oil shale retorts by means of high-temperature treatment of surface-processed shale, followed by grinding, mixing with water, and injection underground. Laboratory results indicate that abandoned in-situ retorts can be made strong enough to prevent subsidence and sufficiently impermeable to reduce ground-water flow greatly. The underground injection also reduces the amount of spent-shale disposal on the surface.

Introduction
For several years. Lawrence Livermore Natl. Laboratory has been developing procedures for in-situ processing of oil shale. In the proposed process described here, a large underground retort is prepared. Typically, each horizontal dimension of the retort is 20 to 100 m, and the height is 100 to 300 m. The retort height may be limited to avoid intersection with major aquifers in a particular location. The retort is prepared by removing a small portion of shale to the surface, probably about 20%, and rubbling the remainder. The size of oil-shale particles in the retort varies from a minimum equivalent to dust to a maximum of about 1 m. Ideally, the mining and rubbling are done in such a way as to provide uniform size distribution and void fraction within the retort. After preparation, the retort is ignited at the top, and a combustion zone is forced downward by flow of oxygen containing gas. Oil is released ahead of (below) the combustion zone. Some of the oil collects at the bottom of the retort and is pumped to the surface; the remainder is carried out of the retort by the gas stream.The processing of oil shale by this procedure may result in some problems in the area after processing is complete.1. Compaction of the processed shale and collapse of the retort root could lead to subsidence at the surface above the retort.2. Disturbance of normal groundwater flow could result in water contamination. This could be caused by leaching of material from the processed shale by groundwater as the water flows through the abandoned retort, or it could be caused by transfer through the retort of saline water from one aquifer to pure water in a different aquifer.3. Toxic gases could be released from the retort to the surface or to adjacent underground mining operations.4. Disposal of the oil shale taken to the surface to provide the void space for gas flow in the retort would be required.A possible solution to these problems lies in making use of the shale taken to the surface. A portion of this shale can be processed to form a grout that then can be returned underground to fill the void space. After bring crushed, the shale is subjected to three heat-treatment steps. First, the shale is retorted in an oxygen-free atmosphere. Second, the carbonaceous residue, or char, is burned off the shale. Third, a higher temperature and different gas are used to cause a reaction between the carbonate and silicate minerals. The products of these reactions have properties similar to those of ordinary cement. A requirement of the process is that the first two steps, retorting and burning, Occur at a low enough temperature that most of the carbonate minerals do not decompose. At least one available process, the Lurgi-Ruhrgas process, satisfies this requirement. After heat treatment, the shale is ground to fine powder, water is added, and the grout is injected underground to fill the void space in a processed in-situ retort.To solve the problems listed, the grout must have certain characteristics. First, the freshly mixed grout must be sufficiently mobile to be pumped readily and to flow easily into the void space underground.
JPT
P. 1609^