Research progress of drilling and borehole completion technologies in broken soft coal seam in underground coal mine

In China, broken soft coal seams are in many strata and widely distributed. It is generally characterized by complex tectonic stress, high gas pressure, low mechanical strength and poor permeability, therefor, the hole is easy to collapse, outburst, which will probably cause many problems such as drilling difficulties, frequent accidents in the hole, inadequate hole depth, borehole blockage, drainage blind area and so on. With the continuous increase of coal mining depth, it is more difficult for drilling and completion in broken soft coal seam. Therefore, the high efficiency, deep hole and precise drilling technology, as well as the completion technology, production increasing and borehole protection technologies have always been the major requirements and research hotspots of gas governance in broken soft coal seam. From the aspects of key technical problems, such as borehole protection, cutting remove, trajectory control, borehole completion, the research status, application of inseam drilling and completion technology in broken soft coal seam are summarized. The problems existing in drilling technologies of broken soft coal seam are analyzed, and the suggestions for improvement are proposed. New technology progress analysis on internal controlled and guided rotary directional drilling technology, screen pipe with porous medium filling and folded expansion tube completion technology are conducted, which provides ideas for new research direction and improvement of existing drilling and completion technologies of broken soft coal seam. © 2021 Meitiandizhi Yu Kantan/Coal Geology and Exploration. All rights reserved