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Mr. Max Ramey <br />,June ] 7, 2003 <br />Page 4 <br />solution mining when the pressure at the surface is maintained at 300 psi or greater in the <br />production string. The cavern pressure was varied in the model at different cavern diameters to <br />assess the effect of lower pressures and pressure cycling on the stability of the cavern roof and <br />walls. The pressure in the cavern would be reduced by up to 400 to 500 psi after closure of the <br />cavern as the cavern would be seated with the thud pressure approximatirn+ the head of the <br />groundwater in the overlying aquifers. <br />The minimum radial pressure in the cavern would be from the residual oil shale <br />remaining in the cavern after mining is complete, assuming that all the fluid was removed and <br />[he gas depressurized. This material provides some internal pressure within the cavity. Figure 6 <br />shows the predicted radial pressures along the height of a mined cavity of 100, 20Q and 300 ft in <br />diameter. These pressures are calculated by the ,tanssen (Woodcock and Mason 1987) formula <br />for radial pressure on the vertical wall of a cylinder filled with material The assumptions for <br />this estimate are listed on the figure. The stresses in the residual oil sliale could be higher than <br />that depicted in Figure 6, depending on the rn sitar stress and the destressing associated with <br />nahcolite dissolution and the thermally induced yielding of the oil shale with solution mining. <br />With no fluid or gas pressures, the internal radial pressure in a Z00-tt-diameter cavern is low at <br />the top of the cavern and increases to 154 psi at the bottom of a 525-tt-high cavern. <br />Cavern Temperature <br />Heating of the cavity by hot injection fluid is considered in the model by fixing the <br />temperatures along the borehole and the cavity a[ a value equal to the average of the injection <br />and extraction fluid temperatures. Recent operating experience shows a surface injectiun <br />temperature of 400°F and a production temperature of 275°F to 300°F. l he average surface <br />temperatw'e is 337.5°F to 350°F. 'there is a IO°F to l5°F temperature drop in both the injection <br />and production strings. When this is take^ into account, the mid-cavern temperature is in tlie <br />range of 340°F to 322.5°F. This study uses 350°F for the base case, and 420°F for a hot case. <br />The ho[ case corresponds to the average cavern temperature with a surface injection temperature <br />of 500°F, a production temperature of 375°F, and a 15°F temperature loss in the injection and <br />production strings. <br />Thermal-Mechanical Properties <br />The thermal-mechanical and geometric model inputs are detailed in Table ?. Thermal- <br />mechanical rock properties, pre-mining track temperature distribution, cavity height, and radial <br />growth rate are identical to those presented in the AAI (2002) study, including tumperature- <br />dependent rock properties. Cavity pressures, temperatures, and diameters are varied in the <br />analysis over respective ranges of 100 to 900 psi (in t00-psi increments), 350°F' to 420°F, and <br />100 to 300 fr (in approximately 25-ft increments). The study also models pressure cycling for a <br />200-ft-diameter cavern by varying the pressure from 900 to 100 psi, and back to 900 psi in <br />'Woodcock, C. R. and 1. S. Mason (1957), Bulk Solids Handling, .4n Gw~nduc~ainn an alre Ponnicr and Trrhnolugr. <br />Chapman and Hall, New York, NY, pp. 49-52. <br />Agapito Associates, Inc. <br />