Research advances and prospects in trajectory control technologies for small-diameter directional drilling in underground coal mines

Abstract:

To clarify the key issues and development directions of borehole trajectory control for small-diameter directional drilling in underground coal mines， this paper systematically reviews research progress in borehole deviation mechanisms， trajectory-control tools， measurement while drilling， near-bit sensing， trajectory prediction， and intelligent control， with emphasis on the requirements of long-distance near-horizontal boreholes for trajectory accuracy， borehole stability， and drilling efficiency. The analysis shows that the alternating distribution of coal seams， partings， broken soft coal， hard roof and floor rocks， and structural fracture zones causes continuous variations in bit loading， borehole stability， weight-on-bit transfer， and cuttings transport conditions. These variations may result in deviation along bedding planes， cross-layer deviation at soft-hard interfaces， reduced efficiency of borehole deviation correction in hard-rock intervals， and cumulative borehole-trajectory errors in long-distance drilling. Small-diameter conditions further constrain the coordinated integration of positive displacement motors， stabilizers， measurement subs， power-supply and communication modules， and cuttings-removal passages. At present， single-bend positive displacement motors remain the basic tools for deflecting and borehole deviation correction. Compound directional drilling， rotary steerable drilling， PDC bits， multi-power drilling assemblies， and multi-parameter measurement-while-drilling systems provide complementary functions in angle holding， rock breaking， flushing and cuttings removal， and information feedback. However， their adaptability to small-diameter boreholes， operational reliability in complex coal-measure strata， and field validation of closed-loop control remain insufficient. Future research should strengthen the coupling between borehole deviation mechanisms and engineering data， develop miniaturized near-bit sensing and reliable data-transmission technologies， promote the integration of mechanism-based and data-driven models， and convert prediction results into executable parameters， including tool face angle， weight on bit， rotary speed， pump discharge， and switching between sliding drilling and rotary drilling， thereby establishing a closed-loop control system integrating sensing， prediction， decision-making， execution， and verification.