基于动态扩散的煤层多场耦合模型建立及应用

Solid-gas coupling model for coalseams based on dynamic diffusion and its application

为研究瓦斯抽采过程中煤层瓦斯流场演化规律,本文基于双重孔隙介质的假设,建立了应力场、渗流场和扩散场多场耦合模型,引入了动态扩散系数,使模拟结果更接近实际情况.结果表明:随着测点与钻孔距离的减小,煤层渗透率先减小后增大;瓦斯扩散系数的衰减系数λ在抽采初期对渗透率空间分布影响不显著,后期随着扩散系数的快速衰减,渗透率空间分布差异变大.当λ≤1×10^(-7)时,渗透率随时间先降低后升高;当λ≥2×10^(-7)时,随时间的增加,渗透率由先降低,再升高,然后再降低的变化趋势转为一直降低.衰减系数越大,裂隙内瓦斯压力衰减越快,基质孔隙压力衰减越慢.采用裂隙压力测得的有效抽采半径随时间增长越来越快,而采用基质孔隙压力测得的有效半径随时间增长逐渐趋于平缓,说明基质孔隙压力是确定钻孔有效抽采半径的合理指标.

Based on the assumption of dual-porous media,a solid-gas coupling model was developed with the consideration of effective stress,matrix shrinkage effect and Klinkenberg effect.A dynamic diffusion coefficient was introduced into the model,which makes the simulation results closer to the actual situation.The results show that with the decrease of the distance to the borehole,the coal permeability decreases first and then increases.The attenuation coefficient has little effect on the permeability distribution at the initial stage,but at the later stage,the difference between the permeability distributions is significant.Whenλ≤1×10^（-7）,the permeability deceases first and then increases with the drainage time and whileλ≥2×10^（-7）,the variation trend is complicated.In addition,the attenuation coefficient has a marked influence on the gas pressure distribution.A higher value of attenuation coefficient will lead to a rapid attenuation of the fracture gas pressure and a slow decay of the matrix pore pressure.The effective drainage radius determined by the fracture pressure increases faster and faster with thetime,while that determined by the pore pressure increases fast with time at first,but later gradually trends gently.The pore pressure is the reasonable index for the determination of the effective gas drainage radius of the borehole.