Analysis and jet drilling test of coiled tubing drilling string for underground coal mine

Due to continuous tripping, uninterrupted circulation and other technical advantages, coiled tubing drilling has become one of the fastest developing oil-drilling-technologies in recent years. Developing the coiled tubing drilling technology for underground coal mine is an essential technical means to realize “less humanized” or even “unhumanized” coal drilling. Aiming at solving the key problems existing in the application of this technology in underground coal mine, such as pipe string optimization and drilling method, the coiled tubing jet directional drilling method was proposed. According to the mud pump capacity and borehole depth design for underground coal mine drilling, the fluid friction resistance of coiled tubing with different diameters and bending ratios (r₀/R_b) was analyzed. Numerical simulation was used to analyze the buckling morphology of horizontal drilling string and the contact stress with borehole wall at different ratios of coiled tubing diameter to borehole diameter (r_c). Besides, the optimum jet bit parameters were given through the study on rotary jet hydraulic parameters and rock breaking effect test. Specifically, the deviation rule of jet directional drilling, as well as the relationship of different flow rate, drilling speed and borehole diameter, were studied based on the coiled tubing jet directional drilling test. The results show that the bending radius ø19 mm‒ø31.75 mm of coiled tubing can meet the space requirements of coal mine tunnel.Tubing diameter and flow rate are the main factors affecting the fluid friction resistance of coiled tubing. Under the conditions of a flow rate of 200 L/min, a pressure of 31.5 MPa and a drilling depth of 200 m, ø31.75 mm coiled tubing is preferred for the optimal drilling string scheme. When the tube hole ratio is r_c=0.454, the contact stress between the coiled tubing and borehole wall is stable and the compressive stress changes linearly. Drilling with ø34.5 mm/5×1.0 mm jet bit + ø40 mm hydraulic commutator + ø31.75 mm coiled tubing assembly, the average inclination increase capacity was 0.67 (°)/m, the average inclination decrease capacity was 0.61 (°)/m, and the average azimuth increase/decrease capacity was 0.45 (°)/m. By controlling the jet flow rate and drilling speed, the borehole diameter can be controlled around ø70 mm. Generally, the research results could provide a theoretical foundation for the development of coiled tubing drilling technology and equipment in underground coal mine.