GENERAL49575 - Laserfiche WebLink

Home Browse Search

CRAPTERTWO Proposed Action and Altennawes would dissolve the mineral nahcolite from the nahcolite/oil shale matrix. The rate and efficiency at which nahcolite is dissolved in the solution mining well aze dependent on many factors, which include but aze not limited to: • injection fluid and average cavity temperature • injection fluid volumetric and mass flow rate • purity of injection fluid • solubility of nahcolite • quantity of nahcolite in the cavity • stage of cavity development. The solution mining cavities would grow in diameter as the wells operate over time. The predicted final configuration of a single solution mining cavity is roughly a cylinder with an average diameter of approximately 200 feet and a height of approximately 600 fc:et. With final well spacing at 300 feet, this would leave 100-foot-wide pillaz between wells to minimize potential subsidence. Upward growth of the cavity would be controlled by using an inert gas at the top of the cavity to protect the cavity roof from the active mining process. The cavity would not be a void because only a portion of the volume would have been mined since the insoluble oil shale matrix would remain after solution mining, At the Piceance Site, the proposed mining interval contains 70 to 75 percent oil shale and other residual rock. When totally mined, the leached area within each cavity would have had 25 to 30 percent of its original volume, i.e., that part occupied by nahcolite, removed by solution mining. The solubility of nahcolite in water generally increases as temperature increases but is also dependent on other factors. Pressure also affects solubility, but the effects aze minor compared to those of temperature. For American Soda's purposes, the primary function of pressure is to keep the sodium bicarbonate from decomposing into sodium carbonate and to effectively purge the production fluid from the cavity. The ultimate size of a cavity is dependent on several factors and complicated processes. Basically, the growth of a cavity is slow until the surrounding rock is heated to higher temperatures. When higher temperatures aze reached, the volumetric growth rate increases because the surface azea of the cavity is increasing and the solubility of nahcolite has increased. Although the volumetric growth rate increases, the growth rate in terms of diameter is expected to be relatively constant over the life of the cavity. As the mining process continues, the cavity would reach a sufficiently lazge size that it would become difficult to maintain th~~ higher temperatures and the efficiency of the mining would decline. American Soda's modeling of the cavity growth process indicates that a cavity diameter of 200 feet is the maximum size that could reasonably be expected. Because of the many variables involved and because this is a new application of the solution mining process, the ultimate cavity size is not fully known. It is unlikely that the cavity size would exceed 200 feet, but it could well be less than 200 feet. A conservative estimate of ultimate cavity size was used in planning for commercial operations because the ultimate cavity size has not been demonstrated. The well spacing and development 2-6 1