Thermal breakthrough calculations to optimize design of a multiple-stage Enhanced Geothermal System
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This study presents an optimization procedure and sensitivity analysis to examine designs for Enhanced Geothermal Systems (EGS) that involve horizontal wells and multiple fracturing stages. The sensitivity analysis included calculations of thermal breakthrough and the maximum flow rate that could be achieved through the system. Conventionally, EGS wells have been nearly vertical and stimulated with open-hole completion in a single stage. This paper investigated a design with two parallel horizontal wells. The first well would be drilled and completed with casing, and then stimulated sequentially in stages with cased-hole packers rated to high temperature. The second well would be drilled through the stimulated region created around the first and completed open-hole. For different combinations of well spacing, lateral length, formation permeability, and number of stages, the optimal flow rate was determined to maximize present value of revenue. The calculations showed that stimulating with multiple stages would radically improve economic performance, delaying thermal breakthrough and allowing a higher overall flow rate to be circulated through the system. At low well spacing and low number of stages, it is optimal to circulate fluid more slowly than the maximum possible rate, in order to delay thermal breakthrough. With greater well spacing and with more stages, thermal breakthrough will be relatively delayed, and it is optimal to circulate at the highest possible flow rate. Overall, it is optimal to use the lowest well spacing where present value is maximized by circulating at the maximum possible rate. When it is optimal to circulate at the maximum possible rate, present value is sensitive to reservoir transmissivity. When it is optimal to circulate at less than the maximum possible rate, present value is unaffected by reservoir transmissivity. Designs with medium to high well spacing and a 1000 m lateral had only modestly lower present value than designs with a 2000 m lateral. Longer lateral length is more beneficial for designs with lower well spacing.