The temperature of conduit center may be as high as 600 degree C. Even by simple heat conduction model, however, such high temperatures are limited to the immediate proximity of the conduit. Seiji Saito and Nobuo Hatakeyama of our project simulated the hole bottom temperature during operation of conduit drilling (Saito and Hatakeyama, 2001). They clearly showed that the temperature is able to be controlled at low values during the operation, as a function of bottom diameter, pumping rate of mud circulation, and water inlet temperature (below). The bottom temperature increases rapidly when round trip starts, but it decreases soon after pumping begins again. The simulated temperature does not exceed the limitations of the Positive Displacement Motor (PDM) and Measurement-While-Drilling (MWD) tools (currently about 150 degree C).
Hole bottom temperature during operation of conduit drilling, simulated using insulated drill pipe of 3.5 W/(mK), mud flow rate of 1000 l/min., and inlet mud temperature of 20 degree C. A 8-1/2" casing was assumed from 1500m depth to the bottom. The bit lifetime, rate of penetration, speed of pull-out of the hole (POOH) and running in the hole (RIH), and operation time between POOH and RIH were 50 hours, 1 m/hour, 800 m/hour, and 4 hours, respectively. (S. Saito and N. Hatakeyama, 2001, Drilling Plan into the Recently Erupted Unzen Volcano and Borehole Temperature Simulations. Geothermal Resources Council Transactions, 25, 149-153.)
Casing program for conduit drilling. Numbers in parenthesis are drilled depths of optional casing that was designed to reduce risk during operation.
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