For the characteristics of low pressure and low permeability of coal reservoir in China, conventional drilling may damage reservoirs and reduce the production of CBM. In order to find efficient drilling technology used for CBM development, three kinds of under-balanced drilling technologies—gas drilling, aerated liquid drilling and circulated micro-bubble drilling have been used for CBM development. Considering the different characteristics of three kinds of under-balanced drilling technologies, firstly, by analyzing the use of gas drilling, characteristic of coal seam and technical measures for gas drilling, besides that, considering the characteristics of CBM and the state of drilling technology in China, the feasibility of gas drilling technology used in China's CBM development. Secondly, the research result and field application of aerated liquid and circulated micro-bubble under-balanced drilling technique are introduced. In the end, the importance of under-balanced drilling for reservoir protection and layer-loss prevention is stated.

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