The invention relates to a method for extracting
coal bed methane through down-hole
bedding long borehole consecutive
cave building and a hydraulic mechanical
drill with the
diameter changed and enlarged while drilling. A
coal bed bedding long borehole is combined with a
cave building technology, and the permeability of low-permeable
coal reservoirs and the coal
bed borehole extraction capacity are improved. A hole is opened at 1 / 3 coal thickness or 1.5m upwards away from the coal heading floor on the extracting
drill site coal wall of an underground coal mine along with the coal bed, the hydraulic mechanical
drill with the
diameter changed and enlarged while drilling performs
cave building on the borehole from inside to outside successively after the borehole reaches the preset depth, and the cave diameters and distances can be calculated through formulas.
Coal bed stresses around the cave are released and coal bed prior cracks are widened and lengthened after the caves are completed, tensional and shear failures occur to the coal bed, novel cracks are produced, and accordingly, the coal bed crack
connectivity is increased, the permeability is improved, the coal bed
methane can be depressurized and desorbed rapidly, the coal bed
methane extracting efficiency is improved dramatically, and the coal bed methane extracting time is shortened.