214 Figure 5 - Schematic diagram of upward reverse drilling rig enlarging the borehole from top to bottom. Figure 6 - Schematic diagram of upward reverse drilling rig enlarging the borehole from bottom to top. 2.3 Blasting and drilling method and reverse well enlargement for well formation In the early stage of the development of special drilling equipment, due to limitations in equipment capabilities and the lifespan of rock-breaking tools, the drilling efficiency and tool lifespan were low, resulting in the mechanical rock-breaking cost being much higher than that of the drilling-blasting method. Therefore, the practice was to adopt a special drilling process only for drilling small-diameter pilot wells, and then, use the drilling-blasting method to expand upwards and downwards (also known as "widening") to increase the diameter. Drawing on the drilling method for well construction, the process of drilling a well in reverse using an anti-well drilling machine was proposed. The disadvantage of a reverse-boring rig for a single expansion drilling and well formation is that it is limited by the geological conditions. It is difficult to provide timely support for unstable strata during the drilling process, and there is a certain risk of the well side structure collapsing. In complex strata, it needs to be carried out under conditions of freezing or ground pre-annexation grouting curtain. After the expansion excavation is completed, temporary support in the form of self-top-down anchornet-spewing should be carried out, and then permanent support for the well wall should be carried out according to the functional requirements of the well shaft. 2.6 Shaft Expansion Drilling for Well Completion using Tunnel Boring Machines With the limitations on the use of explosives, particularly regarding construction safety and efficiency, domestic vertical shaft tunneling machine technology has recently begun to develop. Among these, the most effective method is the pilot shaft type slag removal vertical shaft tunneling machine, which utilizes the weight of rock cuttings for well sinking. This has led to the development of a pilot shaft type gravity slag removal vertical shaft tunneling machine sinking process, as shown in Figure 7.
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