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

Liang Zhang,Yingjie Liu,Qingjie Qi,Kai Deng,Shaojie Zuo

doi:10.3389/fenrg.2021.732827

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.

Highlights

Over the past few centuries, coal has been one of the world’s primary fossil fuel energy sources (Gyamfi et al, 2021; Li et al, 2021; Tsaa et al, 2021)
During methane pre-drainage, the most important parameter for evaluating drainage effect produced by multiple boreholes in a coal seam is the effective drainage zone where the methane pressure is less than 0.74 MPa (Gao et al, 2016; Zhang et al, 2019)
The effective drainage zone produced by multiple tree-type boreholes and the corresponding drainage completion times are first analyzed with different subborehole configurations

Summary

INTRODUCTION

Over the past few centuries, coal has been one of the world’s primary fossil fuel energy sources (Gyamfi et al, 2021; Li et al, 2021; Tsaa et al, 2021). Because the TTBD method can effectively stimulate methane drainage from conventional boreholes and multiple boreholes are commonly needed to extract methane from the coal seams in underground coal mines, it is important to determine the optimum layouts for tree-type boreholes to guide future CMM drainage programs. As a result, existing research cannot guide the largescale application of TTBD program and the optimum tree-type borehole layout needed to conduct TTBD in underground coal mines is still unclear To solve this problem, a full-coupled THM model correlating methane flow within coal fractures and matrix has been developed and is described in this paper. The numerical models’ input parameters are given in Table 1 (Zhu et al, 2011; Xia et al, 2014; Li et al, 2016; Zheng et al, 2016; Zhang et al, 2021)

NUMERICAL RESULTS AND DISCUSSION

CONCLUSION

DATA AVAILABILITY STATEMENT