Design analysis of a drilled geothermal well

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Virginia Polytechnic Institute and State University

An insulated coaxial pipe for a 50,000 ft drilled geothermal well was designed, in order to remove heat from the earth's crust by the circulation of water through a single well. The design took into consideration means of insulating and supporting the inner pipe, thermal expansion, the large hydrostatic pressures involved, and a feasible means of installing the ten mile pipe.

A thermodynamic analysis of the aboveground power generation system, including a once-through steam generator, condenser, circulating pumps, and a feed-water heater system, was made in order to obtain the maximum thermal efficiency. The drilled geothermal power generation equipment is different from that of conventional fossil or nuclear-fired equipment in that the water temperature arriving at the wellhead is continuously varying due to the removal of heat from the earth's crust.

Due to the fact that the earth is a poor conductor of heat, continuous operation of the well results in the water at the wellhead falling below an acceptable power generation temperature after a relatively short time period. Therefore, intermittent operation of several wells in staggered fashion using the same aboveground power generation equipment is required. A determination of the well operating characteristics including optimum mass flow and well cut-on and cut-off temperatures was made. From this a justifiable well cost was determined in order to provide geothermal electrical power and process heat at a reasonable cost.