Tree-Type Boreholes in Coal Mines for Enhancing Permeability and Methane Drainage: Theory and an Industrial-Scale Field Trial

Zhou

et al. 2021

Energy Fuels

The commercial production of coalbed methane (CBM) in China has made significant achievements. To further improve the production of CBM and realize its development, it is necessary to establish a unique, high-efficiency CBM production method that considers the characteristics of coal reservoirs in China. This paper presents the current status of China's CBM exploitation over the past 15 years from three aspects: reservoir characteristics, production mode, and exploitation technology. The achievements, in terms of exploration and development of CBM resources in China, were summarized. The CBM reservoirs in China are characterized by high coal rank, low permeability, high gas content, and low reservoir pressure gradient. Strategic measures for coal and CBM co-mining were proposed creatively as "Huainan", "Jincheng", and "Songzao" modes. CBM production enhancement measures have achieved significant innovations and breakthroughs in drilling and completion technologies and reservoir reconstruction, effectively reducing development costs and increasing single well production. Finally, based on existing basic research, this study proposes efficient CBM exploitation technologies suitable for the characteristics of CBM reservoirs in China. In particular, the uniform permeability enhancement measures of hydraulic grid slotting and fracturing have the advantages of safety, high efficiency, economy, feasibility, and broad application prospects under complex mining conditions. It is of great strategic significance to improve the CBM recovery rate and optimize China's energy structure.

Section: Challenges and Perspectivesmentioning

confidence: 99%

Current Status and Effective Suggestions for Efficient Exploitation of Coalbed Methane in China: A Review

Zhou

et al. 2021

Energy Fuels

“…To better connect conventional boreholes to coal cleats and to create a large stress relief zone around a conventional borehole, a new method, tree-type borehole drainage (TTBD), was introduced by Lu et al (2015) and Lu et al (2019) (Figure 1). This method has been developed and tested in low-permeability coal seams (Xiao et al, 2018;Zhang et al, 2020). The TTBD method uses high-pressure waterjets generated by a self-propelled radially build multi-layer deep and long sub-boreholes in an existing cross-measure borehole.…”

Section: Introductionmentioning

confidence: 99%

“…After employing TTBD, the conductivity pathways for CMM drainage are greatly increased (Ge et al, 2021;Xiao et al, 2021). Previous studies (Zhang et al, 2020;Zhang et al, 2021) focused mainly on analyzing the permeability enhancement and methane drainage capability of a single tree-type borehole. These studies showed that a single tree-type broehloe can deliver the desired levels of methane drainage from low-permeability coal seams.…”

Section: Introductionmentioning

confidence: 99%

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

Deng

et al. 2021

Front. Energy Res.

Self Cite

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.

“…The content of adsorbed methane (CH 4 ) in coal reservoirs generally increases with depth [2–5]. As deep‐burial coal resources are increasingly mined, great methane content and high stress levels are encountered [6–9]. Methane‐related mining accidents excited by dynamic disturbances such as vibration and loading stress impact from mine seismic events or natural earthquakes readily happen due to the combustibility and explosibility of methane [10–14].…”

Section: Introductionmentioning

confidence: 99%

Modelling and simulation of earthquake‐induced changes in methane emission from the working face in an underground coal mine

The Journal of Engineering

Zuo

et al. 2021

Self Cite

Evidence demonstrates that unusual methane emissions in coal mines may happen after earthquakes or mine tremors. Here, a seismicity‐induced permeability change model is constructed and validated using field data from the Laohutai coalmine. The validated model is employed to explore the extent to which different peak ground velocities (PGVs), seismic acceleration coefficients, remnant coal seam methane pressures, and initial coal seam permeabilities affect earthquake‐induced methane emissions. Detailed simulation results suggest that the permeability of a coal seam subjected to earthquakes is both controlled by the dual competing mechanism of earthquake‐imposed strain and excess pore pressure. A higher PGV leads to a rapid increase in the coal seam permeability and the maximum of methane concentration in the airflow, while higher seismic acceleration coefficients could somewhat reduce them. Meanwhile, increasing the remnant methane pressure and initial permeability in a coal seam subjected to an earthquake increases the maximum of methane concentration in the airflow at working faces. These results have provided reliable guidance for how to adopt emergent measures to prevent earthquake‐triggered methane accidents in underground coal mines.