Optimum Layout of Multiple Tree-type Boreholes in Low-Permeability Coal Seams to Improve Methane Drainage Performance

Abstract

Extracting coal mine methane (CMM) is important for underground mining safety. The tree-type borehole drainage (TTBD) technique can effectively remove methane from coal seams. Determining a suitable drilling pattern for multiple tree-type boreholes will promote the efficient application of this technique in coal mines. Aimed at solving the problem that the optimum methane extraction layout for multiple tree-type boreholes is unclear, this study first constructed a full-coupled thermo-hydro-mechanical model to simulate methane flow in coal. This model and data from a coal mine were used to investigate the effect of multiple tree-type borehole layouts, tree-type borehole spacing, different Langmuir volume and different Langmuir pressure constants, and initial coal permeabilities on CMM drainage. The results show that the different tree-type borehole layouts result in significant differences in drainage and that the use of a rhombic sub-borehole layout can reduce the methane pre-drainage time by up to 44.4%. As the tree-type borehole spacing increases, the total time required for pre-drainage increases as a power function. As the Langmuir pressure constant, the fracture permeability, or the matrix permeability increases, the effective drainage zone expands. The effective drainage zone also expands when the Langmuir volume constant decreases but all these changes are accompanied by a shortening of the drainage completion time. These results can provide a reliable basis for optimizing tree-type borehole drilling layouts.

Fracture Mechanism of Coal Seams Induced by a Multibranched Borehole Drilling (MBBD) Technique to Enhance Coalbed Methane (CBM) Extraction

“…In this section, six cases with different BBL values are simulated, and the influence of the BBL on the induced fracture amount and the fracture distribution pattern are investigated. In the six numerical models carried out in this section, which draw upon extensive references to previous field experiments and research findings on the evaluation of rock-breaking capability of drill bits, the values of BBL are set to 3–8 m, respectively. ,, All other parameters are the same in each of the six cases: specifically, the in situ stresses are set to σ h = 6 MPa and σ H = 20 MPa and the order of drilling is set to A-B-C-D. That is to say, branch borehole A was drilled first, and after reaching 7 m, the drilling direction was rotated 60° counterclockwise in order to drill hole B (see Figure ). This process was repeated until the drilling of all of the BBs was completed.…”