Gas Migration Patterns with Different Borehole Sizes in Underground Coal Seams: Numerical Simulations and Field Observations

Haibo Liu,Zhihang Shu,Yinbin Shi,Xucheng Xiao,Xuebing Wang,Jia Lin

doi:10.3390/min11111254

Haibo Liu, Zhihang Shu + Show 4 more

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https://doi.org/10.3390/min11111254

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Abstract

Gas flow in a coal seam is a complex process due to the complicated coal structure and the sorption characteristics of coal to adsorbable gas (such as carbon dioxide and methane). It is essential to understand the gas migration patterns for different fields of engineering, such as CBM exploitation, underground coal mine gas drainage, and CO2 geo-sequestration. Many factors influence gas migration patterns. From the surface production wells, the in-seam patterns of gas content cannot be quantified, and it is difficult to predict the total gas production time. In order to understand the gas flow patterns during gas recovery and the gas content variations with respect to production time, a solid-fluid coupled gas migration model is proposed to illustrate the gas flow in a coal seam. Field data was collected and simulation parameters were obtained. Based on this model, different scenarios with different borehole sizes were simulated for both directional boreholes and normal parallel boreholes in coal seams. Specifically, the borehole sizes for the directional boreholes were 10 m, 15 m, and 20 m. The borehole sizes for the normal parallel boreholes were 2 m, 4 m, and 6 m. Under different gas drainage leading times, the total gas recovery and residual gas contents were quantified. In Longwall Panel 909 of the Wuhushan coal mine, one gas drainage borehole and five 4 m monitoring boreholes were drilled. After six months of monitoring, the residual gas content was obtained and compared with the simulation results. Of the total gas, 61.36% was drained out from the first 4 m borehole. In this field study, the effective drainage diameter of the drainage borehole was less than 8 m after six months of drainage. The gas drainage performance was tightly affected by the borehole size and the gas drainage time. It was determined that the field observations were in line with the simulation results. The findings of this study can provide field data for similar conditions.

Highlights

Coal seam gas (CSG), often referred to as coal bed methane (CBM), exists in underground coal seams [1]
In terms of natural gas production, it is a form of clean energy [2], but in terms of coal mining, coal seam gas brings a potential hazard for underground activities, such as coal and gas outbursts [3,4] or gas explosions
Both the directional borehole scenarios and the normal parallel borehole scenarios were simulated in this study

Summary

Introduction

Coal seam gas (CSG), often referred to as coal bed methane (CBM), exists in underground coal seams [1]. Small pores exist in the coal matrix and most of the coal seam gas is stored in these small pores in an adsorption phase state [10]. For the gas recovery process (the coal seam gas discharge process or gas drainage process), the adsorbed gas desorbs from the internal surface of the micropores and diffuses into the coal fractures [11,12]. These gas molecules flow through the fractures into boreholes or gas production wells [13]. The gas flows towards the wells or boreholes due to the low gas pressure in the wells or boreholes [16,17]

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