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Mr. Max Ramey .tune 17, 2003 Pa;~e 2 numerical model of a solution cavity. The model is the same model used in the .lone 2002 study on the effects of elevated temperature on cavern stability with modifications to incorporate cavern pressure reductions and cycling, cavern diameter expansion up to 300 tt. and improvements in model resolution in the crown pillar area, This modeling approach was used initially (AAI 19982) in the design of the well field for development of the commercial mine plan for the Yankee Gulch lease. This report is presented in sections with the basic model inputs described, then the model results followed by interpretation of the model results. Conclusions from this analytical study are presented at the end of the report. DESCRIPTION OF THERMAL-MECHANICAL MODEL The cavern stability model used in this study is based on asingle-cavity axisynuneu'ic thermal-mechanical model described in earlier studies (AAI 19982, AAl 2002). The thermal- mechanical software FLAG (Itasca 2001') was applied in this study as in the 2002 study. Model Geometry The model considers the solution cavity [o be a hot, pressurized cylinder surrounded by a rock mass. Borehole casing and injection tubulars are not explicitly modeled. The progression of mining is accounted for in the model by successive extraction of concenu~ic rings of nahcolite- rich ore. The radial growth rate is assumed to be 0.10 ft/day. Maintaining consistency with the previous study, cavern height and depth are assumed constant at 525 ft and 1665 ft, respectively. These height and depth values reflect Well 20-11 while the average height and depth of the current production wells are 520 ft and 1798 ft, respectively. Table 1 lists cavern statistics of height, effective radius as of April 2003, depth to the top of the cavern, thickness of the dissolution surface to the cavern roof, and the estimated tons produced from each cavern. Figures ? 3 and 4 show cavern height, depth and crown pillar thickness contours in the region around the existing production wells. The cavern locations with the cavern diameters are shown in Figure 2 through 4. The thiclmess between the dissolution surface and the top of a mine cavity' (crown pillar thickness) is an important factor in stability evaluation- Considering the Z6 current production wells, the average crown pillar thickness is approximately 2~1 ft. A histogram showing the distribution of crown pillar thicknesses is shown in Figure 5. The two wells consu'ucted for the Pilot Tests, wells 20-2 and 20-14, have crown pillar thicknesses of 149 and 146 ft, respectively. Agapito Associates, Inc. (2002), "Mechanical Stabilip~ of 420°F Solution Test Cavity," Letter report prepared for Ma.e Ramey, American Soda. L.L.P.. Rit7e. CO. Jw~e 1 T° Agapito Associates, Inc. (1993), "An Evaluation of Stability and Potential [miraunental Impact of the Commercial Solution Mine Plan, Yankee Gulch Lease, Rio Blanco County, CO'~ Confidential report prepared by Agapito Associates, Inc. and American Soda, L.L.P., prepared for American Soda, L.L.P., Glemvood Springs, CO, July. ' Itasca Consulting Group (2001), "FLAG Fast Lagrangian Analysis of Continua, Version 4.0;' Minneapolis, MN. Agapito Associates, Inc.