In-Situ Leaching of South Texas Uranium Ores: Part 3-Post-Leach Assessment of Recovery and Sweep Efficiency

Abstract

Summary This paper reports results of a post-leach study following nearly 3 years' uranium in-situ leaching in a south Texas ore body. Wells drilled in strategic locations were logged [including delayed fission neutron (DFN) and gamma ray logs] and cored and the recovered core chemically analyzed. Overall uranium recovery for the total 11.9-m [39-ft] ore zone was about 70%, estimated from core and log data. Although ammonia analyses gave evidence of some leachate contact at some time over the whole interval, uranium recovery was not uniform over the total 11.9 m [39 ft]. The upper half of the ore zone appears to have been leached exhaustively, showing not only complete uranium depletion but zero reducing capacity. In the lower half of the interval, however, there was evidence of severe reservoir stratification. There were two narrow, visually oxidized bands from which uranium had been leached. These were interposed between three zones containing strong uranium shows, which had been totally or partially bypassed by the leaching fluid. Lithological cross sections suggested that, in the bypassed zones, a higher degree of sand compaction or the presence of clayballs or clay streaks caused lower permeability, thereby presence of clayballs or clay streaks caused lower permeability, thereby severely limiting the accessibility of the uranium mineral to the injected fluids. Introduction In the past few years in-situ leaching has become an important alternative to open-pit and shaft-mining recovery of uranium. In-situ leaching has a high potential for recovering reserves not presently minable by conventional techniques and with minimal disturbance of the surface environment. Water requirements are much less than in a conventional mine, and undesirable tailings ponds are not needed. Many leaching studies have been reported for south Texas, including case histories of field operations and commercial-size projects. Great interest in this technology is indicated by the more than 12 pilot and commercial in-situ projects in operation in south Texas in 1981. Reservoir engineering aspects of in-situ uranium leaching are similar in some respects to oilfield waterflooding operations. Factors affecting in-situ leaching performance include areal sweep efficiency, vertical stratification, and permeability/porosity variations. Unlike waterflooding, however, leachate/ore contact is fundamentally the most important aspect of in-situ uranium leaching and in many instances the most difficult to achieve. Since most uranium exists in a chemically reduced form, contact between uranium mineral and fluid-containing oxidant is required to form a soluble product. Most leachate/ore contacting problems result from reservoir stratification, specifically where there are adjacent channels of widely varying permeabilities. When uranium occurs in the lower-permeability layers, contact with leachate is sometimes difficult or impossible. The contacting problems encountered with in-situ leaching contrast with uranium ore processing mills, where a high surface area is exposed by mechanical means. Very little documentation is available for post-leach analyses of in-situ uranium leaching projects, partly because of the large expenditure of time and money necessary to complete a post-leach study and the many technical problems associated with such a project. An important problem associated with any post-leach analysis is obtaining a "control" sample before the injection of leachate. A representative control sample is particularly difficult to obtain if horizontal correlation is not good -- particularly difficult to obtain if horizontal correlation is not good -- i.e., if the formation is laterally heterogeneous in mineralogy or permeability, or if there is cross bedding (non-horizontal layering). The permeability, or if there is cross bedding (non-horizontal layering). The later validity of the post-leach analysis will depend largely on how much preleach information was obtained, and how accurately the data describe the preleach information was obtained, and how accurately the data describe the formation. This is especially dependent on the type of logs available and the number and location of the determinations. The uranium ore body in which the present coring was performed lies in the Oligocene-Miocene Catahoula formation of south Texas. These deposits are commonly found in ancient fluvial sediments along the flanks of major fluvial channel-fill and occasional crevasse-splay facies. They frequently show the characteristic C-shape roll-type geometry, with mineralization at the interface of oxidized sediments and reduced pyritic sands. JPT p. 1018