含瓦斯煤渗透率演化模型和实验分析

与气体压力有关的煤层渗透率变化规律是煤矿开采和煤层气开发过程中的重要问题,不同应力条件下,不同类型煤样的渗透率演化特征不同。为了研究瓦斯压力变化过程中煤样渗透性的变化规律,以开滦赵各庄煤矿9号煤层的煤样为研究对象,利用含瓦斯煤热-流-固耦合三轴伺服渗流装置,在恒定温度、轴压和围压,降低瓦斯压力的实验条件下测定了煤样应变和瓦斯渗透率。实验结果表明:随着瓦斯压力的降低,煤样收缩应变加剧,渗透率表现为两种变化趋势:逐渐增大和先减小后增大(渗透率回升对应的瓦斯压力小于1. 0 MPa)。瓦斯压力降低至0. 3 MPa时,渗透率为初始条件下(瓦斯压力2. 0 MPa)渗透率的1. 9～2. 9倍。考虑到煤样在径向和轴向的收缩应变数值接近,针对三维变形煤样建立了渗透率模型,模型同时体现了气体压力和气体解吸对渗透率的影响。理论分析表明,降压过程中煤的渗透率将在某一气体压力(反弹气体压力pr)时由降低转为升高。推导的反弹气体压力pr计算公式显示pr的取值由煤样的体积模量K、与吸附效应有关的Langmuir系数εp和pL共同决定;体积模量K与吸附变形系数εp越大,pr越大。值得注意的是,pr的取值与煤样的外部应力以及内部的气体压力无关。结合本文和前人的实验数据,由本文的渗透率模型计算得到了不同应力和瓦斯压力条件下的煤样渗透率变化曲线以及相应的反弹气体压力pr。模型计算结果与实验数据接近,最大相对误差低于8. 9%。研究表明,实验测得煤样的渗透率表现为何种变化趋势,取决于反弹气体压力pr和实验气体压力的关系。当pr≥pmax(实验测点中最大的气体压力值)时,渗透率随着气体压力增大而降低;当pr≤pmin(实验测点中最小的气体压力值)时,渗透率随着气体压力增大而增大;当pminprpmax时,随瓦斯压力的增大,煤样渗透率呈"V"形变化,即先减小后增大。

Modeling and experimental analysis on the permeability evolution of coal containing methane

The change of coal permeability related to gas pressure is important to coal mining and CBM exploitation.The permeability evolution of various kinds of coal is different and changes with stress conditions.To investigate the permeability evolution of coal samples changed in gas pressure,the strain and permeability of coal samples collected from the No.9 coal seam in Zhaogezhuang mine,Kailuan,China were measured during gas pressure reduction under constant temperature,axial and confining stress by using Triaxial servo-controlled seepage rig.The experimental results showed that coal shrinkage strain increased with the decrease of gas pressure.And the permeability of coal cores may increase gradually or decrease firstly and then increase (The gas pressure cor-responding to permeability rebound is less than 1.0 MPa).When the gas pressure reduced to 0.3 MPa,the perme-ability was 1.9-2.9 times of the initial condition (gas pressure was 2.0 MPa).Considering that coal axial and radial strains are approximately equal,a theoretical model was established to describe the permeability evolution of the three-dimensional deformation coal.The model also reflected the effects of gas pressure and gas desorption on coal permeability.The analysis showed that in the process of reducing gas pressure,the permeability of coal will change from decreasing to increasing at a certain gas pressure (rebound gas pressure pr).The equation of rebound gas pressure pr was derived.It indicated that pr was determined by coal volume modulus,K,and Langmuir coefficients related to the adsorption,εp and pL.The greater volume modulus K and the adsorption deformation coefficient εp are,the higher pr is.And pr was independent of the external stress and the internal gas pressure.According to the derived permeability model,the permeability curves and the rebound gas pressure pr of coal samples under various stresses and gas pressures were calculated by referring to the experimental data of this paper and other published articles.The numerical results were close to the experimental data with a maximum relative error less than 8.9%.The trend of coal permeability changing with gas pressure in the experiment depends on the rebound gas pressure pr and the experimental gas pressure.When pr≥pmax (the maximum gas pressure in the experiment),coal permeability decreases with gas pressure increasing.When pr≤ pmin (the minimum gas pressure value in the experiment),coal permeability increases with gas pressure.When pmin