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Mr. Max Ramey June U, 2003 Page 10 Cavern Width Depth of yield into the roof and walls increases as cavern width increases. As mentioned previously, a mature cavern of 100 ft radius is expected to cause up to 89 tt of yielding into the roof' and up to 34 ft of yielding into the wall. Even at a relatively low pressure of I00 psi, modeling results suggest that with typical crown pillar thicknesses, cavern widths significantly in excess of maturity should be stable. Crown Thickness While temperature, pressure, and diameter affect the extent of yielding around a cavity, crown pillar thickness controls the depth of yielding a cavity can undergo and still maintain stability. Figures 4 and 5 show the spatial distribution and frequency distribution of crown pillar thickness for the existing wells. The crown pillar thickness should be considered in determining if a well should be developed to maturity or if the well should be exposed to low pressures. The crown pillar thickness for Well 29-24 is 269 ft. For an intact crown pillar of at least 100 ft, the results of the thermal-mechanical analysis indicate that Well 29-24 could be expanded to over 300 ft in diameter, even with the cavern pressure reduced to 100 psi. In the thermal-mechanical model, the depth of the cavern was less than that at Well 29-24. The effect of the increased depth at Well 29-24 will be to increase the n2 srlu stress in proportion to the increased depth. The yield in the roof above die cavern is a result of the combination of the effects of the in siiai su~ess and the thermally induced stress, with the thermally induced stress dominating. A small increase in depth is not expected to change the extent of the yield in the roof. However, prior [o expansion of any cavern beyond 200 ft in diameter or prior the depressurization of a particular well, site-specilic conditions should be considered. Pressure Cycling Pressure cycling contributes to increases in yielding as depicted in Figure 13. Most cycling-induced yielding ocew-s during the first one or two cycles, with diminishing affects thereafter. If the rock mass strength diminishes with cycling then the depth of yield will increase slightly. Yielding into the roof after the first depressurization cycle jumps fiom 82 to 90 ft after an additional cycle for the base temperature case (350°F and 200 ft diameter). Subsequent cycling (ten cycles) causes yielding to increase to 105 and 113 ft for fixed temperature and fixed temperature with the reduced strength. The reduced su~ength scenario is valid after multiple cycles, but is not realistic for three or less cycles. The estimates of yield for the fixed temperahu~e case would be high if the cavern cools during cycling. Similar results are seen for the hot temperature case where yield distances jump from 82 to 90 ft and from 90 to 97 ft for free and fixed temperature situations, respectively, for the second pressure cycle. A yielding depth of Ili ft is shown to occur after ten cycles in the hot Agapito Associates, Inc.