David Shakhovskoy, Geodynamics; Aaron Dick, Gary Carter and Martin Jacobs, Baker Hughes
When drilling into oil, gas and geothermal reservoirs located in deep and/or geologically active areas, high temperatures place great demands on the thermal performance of drilling systems. Additionally, the lithologies drilled to gain access to these reservoirs are often hard rock, requiring the use of roller-cone drill bits. Conventional rubber seal components used in roller- cone drill bits are typically rated for temperatures up to 150°C (302°F), and become thermally degraded—leading to bearing failure—when subjected to higher temperatures than these.
To make a roller-cone drill bit capable of operating at higher downhole temperatures, new rubber compounds were developed and tested in laboratory experiments to characterize their high-temperature behavior. Then, drill bits containing these rubber compounds were used to drill the 8½-in.-diameter vertical well section of a geothermal well in the Cooper Basin of South Australia. Maximum bottomhole circulating temperatures were estimated to be 170°C
(338°F) while drilling the granite basement below 3,500 m (11,500 ft); however, temperature indicators showed that the bits were exposed to temperatures greater than 260°C (500°F).
Seventeen drill bits were used: 14 containing mechanical face seals and high-temperature-rated rubber components and three containing mechanical face seals and conventional rubber components. Filtered run data demonstrated that the bits with high-temperature rubber ran for a 62% greater duration and drilled 27% longer intervals than the bits containing conventional rubber components, although changes in formation temperature and borehole deviation may have skewed the results. Post-drill examination of the high-temperature rubber components found them pliable and resilient, suggesting that they were still functional.
The improved performance of the high-temperature rubber components extends the advantages of mechanical seals in high-temperature roller-cone drilling applications. Longer runs and the resultant greater footage drilled positively impact the well installation costs that consume between 42% and 95% of capital expenditures for a typical geothermal power facility.
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