弱胶结地层采动覆岩活动规律及渗透性演化特征研究

Study on movement laws and permeability evolution features of mining-induced overburden in weakly cemented strata

伊犁四矿含煤地层为中生代侏罗系地层,形成时间短,赋存条件具有典型的浅埋深薄基岩厚煤层特征。现阶段矿区主采煤层开采主要受顶板古近系弱胶结砂砾岩含水层影响,该含水层以静储量为主、连通性较差、松散弱胶结、富水性弱至中等。矿井主采煤层顶板基岩主要为泥岩、粉砂岩、细砂岩等软质岩类,为保证工作面安全生产,矿井已通过现场实践开展了工业性试验,同时从覆岩多场耦合机理、保水开采机理等方面进行了基础研究。然而,由于该区弱胶结地层特殊的成岩环境和沉积过程,其物理力学性质特别是水理特性与我国东部地区具有明显的差异性,受开采扰动弱胶结岩石隔水性能演变、地下水流场响应特征等,是弱胶结煤系地层安全高效生产和保水开采中亟待解决的关键基础科学问题。在浅埋煤层矿区实现保水开采,关键是煤层推采后导水裂隙带是否贯通隔水层到达含水层。以新疆伊犁四矿地质条件为背景,通过相似模拟试验,研究了地层采动后覆岩破断、裂隙发育规律,进一步采用固液耦合数值模拟技术,定量分析了覆岩的渗透性演化规律。研究结果表明:(1)随着煤层的推采,导水裂隙带最终发育至70 m,未贯通古近系上部隔水层,采空区中部裂隙基本被压实,几乎全部闭合。隔水层完整性良好,能够实现保水开采;(2)采空区正上方基岩底部渗流速度最大,由于中部基岩被压实,孔隙度减小,所以两端头围岩的渗流速度大于中部围岩的渗流速度,采空区上方岩层渗透性普遍提高,下方范围内渗透性略有降低。

The coal-bearing strata of Yili No.4 Coal Mine belongs to the Mesozoic Jurassic strata,which was formed in a short period of time and have the typical characteristics of shallow buried deep thin bedrock and thick coal seams.At present,the mining of the main coal seam in the mining area is mainly affected by the Paleogene weakly cemented glutenite aquifer in the roof,which is characterized by static reserves,poor connectivity,loose weak cementation,weak to medium water abundance and so on.The bedrock of the roof of the main coal seam of the mine is mainly soft rocks such as mudstone,siltstone and fine sandstone.As a way to ensure the safe production of the workface,the mine has conducted industrial tests through on-site practice,as well as basic research on the overburden multi-field coupling mechanism and water retention mining mechanism.Nevertheless,the special rock-forming environment and deposition process of the weakly cemented stratum in this area make its physical and mechanical properties,especially its hydraulic properties,differ significantly from those in the eastern part of the country,and it is also affected by the evolution of weakly cemented rocks’water-isolation properties and the response characteristics of groundwater flow field due to mining disturbance.These effects are the critical basic scientific problems that need to be solved in the safe and efficient production and water conservation of weakly cemented coal formations.The key to realize water-preserved mining in shallow coal seam mining area is whether the water-conducting fracture zone passes through the aquifuge and reaches the aquifer after coal mining.Based on the geological conditions of Yili No.4 Coal Mine in Xinjiang,the development law of overburden failure and fracture after mining is studied through similar simulation experiment.Furthermore,the permeability evolution law of overlying rocks is quantitatively analyzed by using solid-liquid coupling numerical simulation.The results show that:(1)with the retreat mining of the coal seam,the water conducted fracture zone finally developed to 70 m,and the upper aquifuge of the Paleogene system was not connected.The fractures in the middle of goaf are basically compacted and almost completely closed.The integrity of the aquifuge is good,which can realize water-preserved mining.(2)The seepage velocity at the bottom of bedrock directly above the goaf is the highest.Because the central bedrock is compacted and the porosity decreases,the seepage velocity of the surrounding rock at the two ends is greater than that of the central rock.The permeability of rock layer above goaf is generally improved,while the permeability of rock layer below is slightly decreased.