Field tracer tests indicate that the effective matrix diffusion coefficient of fractured media increases with the increase of observation scale. To understand the mechanisms of this phenomenon, a 3D fracture-matrix model considering a horizontal fracture as well as the damage zone around the fracture is developed. The effect of the damage zone properties, including width, porosity and permeability, on the effective matrix diffusion coefficient is analyzed through a series of solute transport simulations. The results show that when the damage zone has a larger thickness and porosity, more solute enters the rock matrix, which leads to an increase in the measured effective matrix diffusion coefficient. For natural fractures, the permeability and porosity of damage zone increase with the decrease of the distance from the fracture surface. With the increase of fracture scale, the thickness, permeability and porosity of the damage zone increase. The simulation results of different observation scales show that a positive relationship exists between the scale factor of the effective matrix diffusion coefficient and observation scale. This study interprets part of the mechanism of the scale dependence of the coefficient and contributes to the understanding of the solute transport process in complex fracture-matrix systems.

根据现场示踪剂试验的统计结果, 裂隙介质中的有效基质扩散系数会随着观测尺度的增加而增加。为了研究其背后的机制, 建立考虑裂缝周围损伤区的三维裂缝–基质模型, 利用溶质运移过程的数值模拟结果, 分析损伤区厚度、渗透率以及孔隙度对有效基质扩散系数的影响。结果表明, 当损伤区具有较大的厚度和孔隙度时, 会使得更多溶质进入岩石基质, 导致测得的有效基质扩散系数增大。对于天然裂缝来说, 损伤区内部岩石的渗透率及孔隙度会随着与裂缝面距离的减小而增大。当裂缝尺度增大时, 损伤区的厚度增加, 岩石的渗透率和孔隙度也整体上增大。不同观测尺度下的模拟结果表明, 有效基质扩散系数的尺度因子与观测尺度间存在正向的关系。该研究结果可以解释有效基质扩散系数尺度效应的部分机制, 有助于了解复杂裂隙–基质系统中的溶质运移过程。