吸附条件下含裂隙煤体渗透率演化规律数值模拟分析

    Numerical simulation of permeability evolution of fractured coal under adsorption condition

    • 摘要: 基质−裂隙相互作用是影响煤体渗透率非均匀演化的关键因素。为探究吸附条件下裂隙渗透行为的真实演化过程,基于含裂隙煤体的CT二维扫描图像重构模型,对比了吸附作用下渗透演化的长期实验数据,验证了数值模型的可靠性,并实现了裂隙结构引导的基质−裂隙相互作用过程可视化观测。结果表明:气体注入产生的吸附作用导致裂隙附近的基质发生局部膨胀;吸附膨胀行为随气体渗透作用由近裂隙区域扩展至远裂隙区域;达到吸附平衡时,局部膨胀效应减小,形成新的基质−裂隙平衡状态,当基质膨胀并导致裂隙压实时,裂隙体积减小;非均匀膨胀行为导致渗透率存在增强、下降、反弹、稳定4个阶段的演化过程;注入压力的增加会抑制渗透率的下降幅度,提升最终渗透率;Langmuir应变常数与膨胀期间的裂隙渗透率下降过程呈正相关关系;基质初始渗透率越低,由局部膨胀引起的裂隙渗透率下降幅度越大,渗透率反弹期持续时间越长。

       

      Abstract: Matrix-fracture interaction is a key factor affecting the non-uniform evolution of coal permeability. In order to explore the real evolution process of fracture permeability behavior under adsorption condition, a reconstruction model based on two-dimensional CT scan images of fractured coal was used to compare the long-term experimental data of permeability evolution under adsorption, and the reliability of the numerical model was verified. The visual observation of matrix-fracture interaction guided by fracture structure is realized. The results show that the adsorption caused by gas injection leads to the local expansion of the matrix near the crack. The adsorption expansion behavior extends from near fracture region to far fracture region with gas permeation. When adsorption equilibrium is reached, the local expansion effect decreases, and a new matrix-fracture equilibrium state is formed. When the matrix expands and causes the cracks to compact, the volume of the cracks decreases. The heterogeneous expansion behavior leads to a four-stage evolution process of permeability enhancement, decline, rebound and stability; the increase of injection pressure can inhibit the decrease of permeability and increase the final permeability; Langmuir strain constant is positively correlated with the fracture permeability decreasing process during expansion; the lower the initial permeability of matrix, the greater the decrease of fracture permeability caused by local expansion, and the longer the permeability rebound period.

       

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