1470 3. MEHANICAL ROCK BREAKING GRAVITY SLAG REMOVAL DRILLING TECHNOLOGY AND EQUIPMENT 3.1 Pilot Hole Drilling Technology In the gravity slag removal process system, after the pilot hole directly and precisely connects the two horizontal levels, the pilot hole is enlarged and the drill pipe is lowered to the lower level. After the drill pipe is connected to the reaming bit, the drill pipe transmits the power output from the upper drilling rig to the reaming bit. The reaming bit rotates and breaks the rock, and the rock slag falls due to its own weight, thus forming a pilot well. Similar to ordinary drilling methods, conventional raise boring machines use a topdown pilot hole drilling method. Upward reverse drilling rigs use a bottom-up pilot hole drilling method, as shown in Figure 3. The drill bit breaks up rock cuttings during pilot hole drilling, which are then discharged by gravity. Compared to forward circulation cuttings discharge, the rig's drive torque and the power of the circulating cuttings pump are significantly reduced. Compared to conventional reverse drilling rigs drilling pilot holes, energy consumption can generally be reduced by about 70%. However, compared to the forward drilling pilot hole method, the upward drilling pilot hole method is more difficult to control due to the drill bit being under pressure. Figure 3 - Schematic diagram of a reverse drilling rig drilling pilot hole from bottom to top. Figure 4 - Schematic diagram of a dedicated directional drilling rig drilling pilot hole. For deep wells, deviated wells, and wellbore projects with high deviation requirements, including single-drilling well completion projects using raise boring machines (RBMs), precise control of deviation is necessary to ensure the effective cross-section and functionality of the wellbore. Using a dedicated directional drilling rig and related instruments (Figure 4), technologies such as MWD (Measuring While Drilling), screw drill string deviation correction, and mud pulse signal transmission are implemented to ensure the pilot hole follows a predetermined trajectory. However, the power for deviation correction using the screw downhole motor is generated by the mud pump, requiring a high-power mud pump and resulting in high energy consumption. 3.2 Pilot Well Drilling Technology A conventional raise boring machine is used. The drill pipe extends into the lower horizontal roadway, the pilot hole bit is removed, and a reaming bit is connected. Drilling proceeds from bottom to top. Upon reaching the upper horizontal level or surface, the raise boring machine is removed, the reaming bit is retrieved, and the pilot well construction is complete. For raise boring machines, two reaming methods can be used: The first is the top-guide-bottom-reaming method, where the pilot hole bit is removed in the upper horizontal roadway, and after connecting the reaming bit, drilling proceeds from top to bottom (Figure 5). In this method, the annular space between the pilot hole and the drill pipe is relatively small, increasing drilling
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