Conference Paper

Drilling Practices and Workflows for Geothermal Operations

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Over a three-well program the FORGE drill teams reduced well times by more than half, with instantaneous ROP increased over 400% in the thick granite pay zone. At the same time, record footage per bit was increased over 200%. The physics-based, limiter-redesign workflow utilized is described, along with specific changes in design and operational practices. Both are expected to yield similar results in any hard rock, geothermal or similar operations. In 2020, the U.S. Department of Energy (DOE) funded a group at Texas A&M University to develop physics-based practices for the geothermal industry, similar to those that have enabled large gains for many operators in the petroleum industry. Wells in the Frontier Observatory for Research in Geothermal Energy (FORGE) were used to develop and test the workflow and practices. The effort began with sixteen hours of training for all team members, including drilling management. Changes were made in the daily workflow, such as periodic parameter testing, real time recognition and response to the common drilling dysfunctions, limiter identification, and daily discussion of the physics of each limiter and the immediate response required that included remote support personnel. The continual daily emphasis on identification of limiters, combined with training in how each limiter physically worked, created an enabling environment for change Some of the key performance limiters addressed in these FORGE wells included previously held beliefs about limitations on WOB with PDCs, modifications to reduce BHA whirl, use of high WOB to suppress bit whirl, identification and avoidance of resonant RPM, BHA design and drilling practices to reduce the amplitude of borehole patterns to improve weight transfer, and the use of high spurt loss fluid (water) to achieve brittle rock failure. It was eventually possible to increase WOB to the structural limit of the bits (i.e., 68k lbs on 10-5/8" PDC). The bit vendor was engaged continually in daily analysis of digital data and dulls, and bits were redesigned to redistribute cutter wear, increase aggressiveness, and improve life through the increased use of shaped cutters. A significant finding was that contemporary PDC cutters remained relatively unworn for long distances in the FORGE granite regardless of WOB used, if the team is trained to manage dysfunction The mechanism through which the cutters eventually fail is discussed, along with operational and design practices to further extend the run lengths. This paper is intended to serve as a reference, with the basic concepts, science, and real-time practices an operator may consider in developing its physics-based, limiter-redesign workflows.

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Polycrystalline diamond compact (PDC) cutter is the key component of PDC bits, whose rock-breaking characteristics are particularly crucial for the bit performance. Based on the understanding on rock-breaking characteristics of various shaped cutters, the multiridge-shaped cutter has been developed to improve the cutting efficiency and durability of PDC cutters. The design of multiple ridges combines the advantages of both the shaped cutter and the reasonable narrow cutter spacing of the bit cutting structure. More specifically, a triple-ridge-shaped cutter (TSC) was compared with the widely used axe-shaped cutter (ASC) in this work to recognize the superiority of the novel multiridge cutter shape. Numerical simulation, laboratory experiments, and field testing were performed to investigate the cutter-rock interactions of TSC and ASC. Numerical simulations were carried out using nonlinear finite element software LS-DYNA and verified by the single cutter testing. Both numerical and experimental results proved that the stress concentration caused by the triple-ridge cutter shape of TSC is significantly greater than that of ASC during the cutter-rock interaction. And the tangential force of TSC is much less than that of ASC under the same cutting conditions, indicating that the PDC bit equipped with TSCs requires less torque while drilling. According to the results of the fractal analysis model of cuttings and the mechanical specific energy (MSE), TSC consumed less drilling energy than ASC to break the same volume of rock, which demonstrated that the TSC has greater cutting efficiency than ASC. Beneficially, the greater cutting aggressivity of TSC in turn improves the wear resistance. The laboratory wear and impact tests presented that TSC outperforms ASC in both wear resistance and impact resistance, indicating better durability under downhole. Although only one controlled experiment was conducted in the field, the field test result showed over 100% improvement in average rate of penetration. In short, the numerical, laboratory, and field testing have confirmed a great potential of TSC in improving the cutting efficiency and durability of PDC bits. The results of this work will provide valuable guidance for the development of shaped PDC cutters to overcome engineering limitations.
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