关闭矿井采空区破碎岩体再断裂机制及空隙结构演化特性

Research on re-fracturing mechanism and cavity structure evolution characteristics of broken rock mass in goaf of closed mine

受“双碳”政策影响,关闭矿井地热开采技术逐渐受到关注。关闭矿井采空区内储热流体提取效率与其渗流特性密切相关,而破碎岩体空隙结构是决定采空区渗流特性的关键因素。因此,需要研究地热开采复杂环境下储热空间破碎岩体空隙结构变形演化特性。笔者基于颗粒离散元数值方法,建立浸水及侧向约束压缩条件下不同级配的破碎岩体数值模型,分析破碎岩体变形行为及二次断裂演化特性,追踪颗粒从岩体骨架结构剥离及空隙内运移规律。得到如下结论:破碎岩体压实过程应力随应变增长可分为3个阶段:初始阶段(0<ε≤0.175),缓慢增长阶段(0.175<ε≤0.275)以及快速增长阶段(ε>0.275)。快速增长阶段应力-应变曲线出现明显波动,破碎岩体二次断裂及应力重分布现象在该阶段最明显。破碎岩体热储环境下空隙率的变化值与初始空隙率成正比,最大达到0.2。当破碎岩体内岩块粒径悬殊大时,接触键应变能最大,断裂能增长缓慢。岩块与颗粒密集区接触部分断裂分布多,与空隙接触部分断裂分布极少。颗粒未剥离岩块时,随岩块运动,整体运动路径复杂速度较小;颗粒剥离瞬间速度突然增大,与岩块碰撞导致速度改变。当颗粒速度减小到与周围岩块相近时,将造成空隙空间的堵塞。研究结果可为关闭矿井储热空间换热效率评估提供理论依据。

Due to the“Dual Carbon”policy impact,geothermal extraction technology in closed mines has garnered increasing attention.The efficiency of extracting thermal fluid within the closed mine goaf is related to the permeability characteristics,with the broken rock mass porosity structure playing a key role in determining the permeability characteristics of the goaf.Therefore,it is of great significance to investigate the deformation and evolution characteristics of the porosity structure of broken rock mass in the complex environment of geothermal extraction.Numerical models of broken rock mass with different size grading indexes were established using the particle discrete element numerical method under conditions of immersion and lateral-constrained compression.The deformation behavior and evolution characteristics of the broken rock mass were analyzed,and the movement rules of particles within the rock voids were tracked.The following conclusions were obtained:the stress-strain curve during the compaction process of the broken rock mass can be divided into three stages,namely the initial stage(0<ε≤0.175),the slow growth stage(0.175<ε≤0.275),and the rapid growth stage(ε>0.275).In the rapid growth stage,the stress-strain curve shows significant fluctuations,and the phenomena of secondary fracturing and stress redistribution in the broken rock mass are most pronounced.The variation value of porosity in the broken rock mass under the thermal storage environment is directly proportional to the initial porosity,with a maximum value of 0.2.When there is a large size difference between rock blocks in the broken rock mass,the contact bond strain energy is the largest,and the growth of bond breakage energy is slow.Fractures are more common in the contact part between rock blocks and particles,while fractures in the contact part between rock blocks and voids are very rare.When the particles are not separated from the rock fragments,they move with the rock fragments,resulting in a complex overall trajectory.In the instant when the particles are detached,their velocity suddenly increases and the collision with the rock fragments causes a change in velocity.When the particle's velocity decreases to a level similar to the surrounding rock fragments,it will lead to the blocking of the void space.The research results can provide a theoretical basis for evaluating the thermal storage efficiency in the goaf of closed mines.