Automated Drilling of High Aspect Ratio, Small Diameter Holes in Remote, Confined Spaces

We describe the development and prototype performance characterization of a mechatronic system for automatically drilling small-diameter holes of arbitrary depth. The characterization of the integrity of oil and gas wells requires measurements to be taken in confined spaces at distances up to several miles from human operators. A new approach to wellbore integrity requires small holes to be precisely drilled into, and potentially through, the steel well casing in order to enable direct measurements of the cement that surrounds the casing and the interfaces between casing, cement, and rock formation, e.g. by placing sensors in these areas. These holes must be deep enough to reach the desired depths (up to 8-12”) but their diameters must be minimized to avoid lasting damage to the wellbore. The precise drilling of very small diameter, high aspect ratio holes, particularly in dimensionally constrained spaces such as a wellbore, presents several challenges including bit buckling, limited torsional stiffness, chip clearing, and limited space for the bit and mechanism.  These issues can be significantly mitigated by controlling the length of the unsupported drill bit throughout the drilling process.  We describe the design, prototyping and testing of a novel mechanism for small diameter, high aspect ratio, precise drilling. The mechanism includes various motors to spin the drill bit, release and grasp the bit, feed the drill bit, and advance the mechanism towards the workpiece as well as various other components to facilitate precise drilling. The mechanism begins drilling with a minimal unsupported drill bit length. Once the full length of the unsupported drill bit has been reached, the mechanism releases the bit from the chuck, and then simultaneously feeds the drill bit into the newly drilled hole and reverses until the desired free supported length is reached. The linear actuators are also used to inject energy for chip clearing. This process is repeated an arbitrary number of times, and is only limited by the length of the drill bit. When used with flexible drill bits, holes of arbitrary depth and aspect ratio may be drilled orthogonal to the wellbore. This entire operation may be conducted remotely, without operator involvement. Comparative testing of the mechanism and a conventional drilling system performing deep, small diameter hole drilling operations is described. The experimental results are compared, and the performance of both systems is evaluated based on maximum achievable hole depth. A qualitative comparison of the hole quality is completed based on geometric qualities of the hole. I.e. roundness, straightness, and uniformity. We show that the mechanism is able to achieve holes with substantially greater aspect ratios and more consistent profiles than using conventional methods with very long drill bits.

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