Fully coupled thermo-hydro-mechanical model for extraction of coal seam gas with slotted boreholes

Abstract

China has abundant coal seams with high gas content and low matrix permeability, which is unfavorable for efficient gas extraction. Popular extraction technology that uses high-pressure water jet slotting can overcome this difficulty, increase the contact area of coal and air, and expand the influence radius of boreholes. Thus, this technology improves gas extraction rates and ensures mine safety. Although the high-pressure water jet slotting technology has been comprehensively investigated and tested, fewer studies have been conducted on coal seam gas extraction with slotted boreholes using a fully coupled thermo-hydro-mechanical model. Neglecting this coupled thermo-hydro-mechanical interaction may result in misleading estimates of the gas extraction effect, which could be detrimental to the production prediction, drainage optimization design and gas control.In this study, we first create a fully coupled thermo-hydro-mechanical (THM) model using a fully coupled finite element approach and a history data matching to ensure the reliability and validity of this model. Second, we perform simulation experiments to investigate the sensitivities of gas extraction with single slotted borehole to changes in the intrinsic and extrinsic factors (for example, permeability, gas pressure, temperature and radius of the slots). Finally, we examine the effect of the interaction of multi-boreholes on the optimization design of multi-boreholes in practical engineering applications. The results demonstrate that: (1) higher permeability, initial gas pressure, temperature and slot radius has an effect on the radius of a single slotted borehole. (2) For both conventional and slotted arrangements, the estimated completion time increases exponentially with an increase in the hole spacing. For the same hole spacing, the drainage time of the conventional arrangement is longer than that of the slotted arrangement. Thus, our fully coupled THM model can improve the current understanding of the gas drainage process with slotted boreholes and provide scientific guidelines to evaluate gas extraction and optimize the design of gas extraction systems.