Gas pre-drainage through underground-to-inseam (UIS) horizontal boreholes is one of the common practices for the management of environmental and safety concerns in underground coal mining. In the design of UIS boreholes, local experience is invaluable; however, an experienced-based design is not necessarily optimal from both economic and technical perspective. In this study, we present an informed, data-driven design and optimization methodology that employs a multiphysics coal seam gas flow model for simulating gas pre-drainage on different scenarios of UIS borehole pattern. The implementation of the methodology is presented through a case study from an Australian underground coal mine. The methodology uses an in-house three-dimensional finite element (FEM) model (referred to as NETCoal) that has been developed based on the non-equilibrium thermodynamics and continuum mechanics. NETCoal considers the interacting processes of matrix gas diffusion, fracture gas flow and rock stress evolution in a coupled manner. This allows the permeability tensor of each grid element to be updated in response to evolving stresses on fractures induced by matrix shrinkage due to gas desorption (sorptive-mechanical effect) and gas pressure depletion (hydro-mechanical effect). The proposed methodology for the UIS borehole design requires a three-dimensional geological/structural grid model to be constructed for the area under consideration using available field data. Material and flow properties of coal (e.g., Isotherm, flow, mechanical properties etc.) are then calculated particularly using downhole geophysical logs (DGL) and physical equations/empirical correlations at borehole locations and distributed onto other grid cells using the kriging method to build a property model. NETCoal imports these geological and property models in a corner point gridding format and generates its own finite element mesh that suits the UIS borehole pattern under consideration. A Warren-Root type shape factor is used for the calibration of the numerical results of gas content with actual borehole flow data. Once calibrated, the model simulates gas drainage for different scenarios to determine an optimal borehole pattern which is the one satisfying the threshold limit value (TLV) and lead time constraints while requiring least drilling length. In addition to introducing an innovative practical methodology for the design of UIS pre-drainage boreholes, this study provides insights into the effect of coal seam and geometry parameters on gas pre-drainage through a sensitivity analysis.

通过井下至内层 (UIS) 水平钻孔进行瓦斯预抽是管理地下煤矿环境和安全问题的常见做法之一。在 UIS 钻孔的设计中，当地的经验是非常宝贵的；但是，从经济和技术角度来看，基于经验的设计不一定是最佳的。在本研究中，我们提出了一种知情的、数据驱动的设计和优化方法，该方法采用多物理场煤层气流模型来模拟 UIS 钻孔模式不同场景下的瓦斯预抽。通过澳大利亚地下煤矿的案例研究介绍了该方法的实施。该方法使用基于非平衡热力学和连续介质力学开发的内部三维有限元 (FEM) 模型（称为 NETCoal）。NETCoal 以耦合的方式考虑了基质气体扩散、裂缝气体流动和岩石应力演化的相互作用过程。这使得每个网格单元的渗透率张量可以更新，以响应由气体解吸（吸附-机械效应）和气体压力消耗（水力-机械效应）引起的基质收缩引起的裂缝上不断变化的应力。UIS 钻孔设计的建议方法需要使用可用的现场数据为所考虑的区域构建三维地质/结构网格模型。煤的材料和流动特性（例如，等温线、流动、然后特别使用井下地球物理测井 (DGL) 和钻孔位置处的物理方程/经验相关性计算机械特性等），并使用克里金法将其分布到其他网格单元上以建立特性模型。NETCoal 以角点网格格式导入这些地质和属性模型，并生成适合正在考虑的 UIS 钻孔模式的自己的有限元网格。Warren-Root 型形状因子用于用实际钻孔流量数据校准含气量数值结果。校准后，该模型会模拟不同情况下的瓦斯抽放，以确定最佳钻孔模式，即满足阈值限值 (TLV) 和前置时间限制，同时需要最短钻孔长度的井眼模式。