1469 Figure 1 - The positive circulation slag removal principle in small-diameter well drilling Figure 2 - The principle of reverse circulation slag removal in large-diameter drilling For large-diameter wells, such as those drilled using vertical shaft drilling rigs, rock breaking is typically achieved through roller cutting or scraper shearing, producing rock fragments with a diameter of approximately 50mm. Large rock particles can exceed 200mm in diameter, and with drilling diameters reaching up to 10.8m, the annular space between the drill pipe and the wellbore is enormous. If a forward circulation cutting technology is used, the mud flow velocity within this annular space is very low. Large rock fragments, due to their greater gravity, sink faster in the slow-flowing mud and cannot be removed to the surface. Therefore, vertical shaft drilling rigs employ a compressed air reverse circulation cutting technology. The principle of compressed air reverse circulation cutting in large-diameter drilling is shown in Figure 2. 2.2 The Proposal and Development of Gravity-Based Slag Removal Technology Shaft engineering using mechanical rock-breaking drilling still requires rock breaking, slag removal, and support operations, among which the slag removal process has a particularly significant impact on drilling efficiency, engineering costs, and construction safety. In actual engineering projects, whether in the mining field of underground mining or other underground engineering fields, some shaft projects already have a production system at the bottom, which can meet basic functions such as ventilation, transportation, and drainage. Therefore, considering the engineering conditions where the lower part of the proposed shaft has a roadway, this paper proposes a slag removal method based on the weight of the rock slag, making full use of the existing production system and leveraging the advantages of continuous mechanical rock-breaking operations. Based on the limitations of geological and engineering conditions in wellbore crossings, and relying on the core idea of removing slag by its own weight, the existing large-diameter wellbore drilling technologies mainly include small-diameter reverse drilling to form pilot wells, reverse drilling to directly drill large-diameter wellbores, direct upward reverse drilling, and forward enlargement drilling.
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