Study of the stress relief and gas drainage limitation of a drilling and the solving mechanism

The stress relief mechanism and the limitation of a borehole drilling are pointed out by theoretical and numerical analysis. The stress is relieved around the borehole wall, and the gas can be drained out easily, but the periphery of the stress relief area generates annularity stress concentration to embay around the boreholes, and forms the “Bottle Neck Effect” (BNE) which restricts the gas drainage ability. Jetting high pressure water in the boreholes to form several slots can break the BNE, which makes the stress above and below the slots relieve sufficiently. The stress relief area is nearly 200 times larger than the stress concentration area under the same condition. The slots around a borehole improve the borehole’s stress relief ability, drain out more gas, and enlarge the drainage range, which can reduce the number of the boreholes to achieve the same aim.