Over the last 50 years, PDC [polycrystalline diamond compact] bits have gradually become the standard drill bit for drilling oil and gas wells, replacing the previous roller-cone bit technology. As PDC bits have improved, they’ve made it possible to drill faster, and to drill for longer without having to change out the bit, both of which make it cheaper to drill a well. By making drilling faster and cheaper, PDC bits have helped enable the shale revolution, and are one of the drivers of progress in enhanced geothermal energy.
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Figuring out how to make a bit drill faster and last longer in large part comes from experience: every failed or damaged drill bit is an opportunity to understand the causes behind that failure, and to modify the drill bit and drilling process to try and avoid it. 50 years of constant modifications based on both real-world and laboratory drill bit failures have steadily pushed PDC drilling performance higher and higher.
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A major advance in PDC drilling in the late 1980s was the discovery of the effects of “bit whirl," a type of vibration in the drill string. Bit whirl is when a drill bit doesn’t just spin, but rotates eccentrically around a center point. A whirling bit changes the path a cutter takes, resulting in loading on it from undesirable directions and impacts that can damage the cutters. Based on field studies of PDC bits, researchers at Amoco discovered that bit whirl was a major cause of damaged cutters and bit failure: cutters were being bashed against the side of the hole, causing damage and rapid wear. Bit failures that were thought to be due primarily to thermal effects (ie: the bit heating up) were discovered in many cases to be primarily due to bit whirl, with damaged cutters rapidly failing under high heat.
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One major limiting factor was the quality of the synthetic diamond cutters. In the early 2000s one of the major cutter manufacturers, US Synthetic, began to investigate whether the diamond itself could be made higher-quality. Though the diamonds were made from synthetic diamond chemically identical to natural diamond, the process of turning diamond grit into solid diamond tables wasn’t perfect, and synthetic diamond cutters weren’t as hard as natural diamond. US Synthetic began to run extensive experiments, tweaking manufacturing process parameters and testing the resulting cutters to determine the impact on performance.
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Thanks to continued improvements in cutters and drilling methods, by 2015 PDC bits were responsible for roughly 90% of oil and gas drilling. Ironically, however, the purpose which originally inspired their development — geothermal energy — still remained the domain of roller-cone bits. Historically, PDC bits’ poor thermal performance and inability to drill the hardest rock prevented them from being used in geothermal applications. However, we’re now seeing the gradual adoption of PDC bits in the enhanced geothermal industry, where the same learning-driven improvement mechanisms at work in the oil and gas industry can be observed.
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Enhanced geothermal startup Fervo is also using limiter-based redesign to steadily improve their drilling practices and achieve substantial reductions in drilling time and cost. By studying wear patterns and modifying the bits in response — changing things like number of cutter rows, shapes of cutters and where they’re positioned, and the size of gauge pads — it has achieved very high drilling learning rates, and increased the lifetime of PDC bits by roughly a factor of five from its first well drilled.
From the article:
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