多尺度煤粒与瓦斯多尺度动扩散系数模型特征参数关系研究

摘要

To study the influence of coal matrix scale on the diffusion and migration capacity of gas,the gas diffusion experi-ments of coal particles with 5 different scales under different initial adsorption equilibrium pressures were carried out,inclu-ding 0.18 mm-0.25 mm,0.25 mm-1mm,1mm-3mm,3mm-6mm and 6mm-10mm.Based on the new model of multi-scale dynamic diffusion coefficient,the relationship between the scale of coal particles and the characteristic parameters of the multi-scale dynamic diffusion coefficient model was studied.The results showed that the description accuracy of the new model for the whole diffusion process was better than that of the classical model,and the maximum relative error of the classical model was 18.4%,while that of the new model was only 3.9%.The initial diffusion coefficient D0 increased with the increase of particle size under the same conditions,and increased by 45.3times to the maximum.The diffusion attenua- tion coefficient β decreased with the increase of particle size,and the maximum attenuation was 89.8% .The experiments re-vealed the multi-scale diffusion characteristics of gas in the multi-scale coal particles.The large scale coal particles contained larger external pores,which caused the larger the scale of coal particles,the larger the initial diffusion coefficient D0,but the large pores occupied a relatively large proportion,which caused a smaller variation of pore scale level difference,so there was a smaller attenuation coefficient β.The small scale coal particles had smaller external pores,and the D0 values were smaller.However,the small pores occupied a relatively large proportion,and the small pore scale level difference varied greatly,so there was a larger attenuation coefficient β.The variation characteristics of diffusion characteristic parameters D0 and β under different scales of coal particles reflected the phenomenon of large initial volume but rapid attenuation in the process of gas drainage in the low permeability coal seam at present,and it also provides the direction to the reservoir transformation for in-creasing production and stable production of gas (CBM).%为研究煤基质尺度对瓦斯扩散运移能力的影响,在实验室内开展了0.18~0.25 mm,0.25~1mm,1~3mm,3~6mm,6~10mm等5种不同尺度煤粒在不同初始吸附平衡压力下的瓦斯扩散实验.基于多尺度动扩散系数新模型,研究了煤粒尺度与多尺度动扩散系数模型特征参数的关系.研究结果表明:新模型对全扩散过程的描述精度要好于经典模型,经典模型最大相对误差达18.4%,而新模型仅为3.9%;相同条件下,初始扩散系数D0值随着粒径的增大呈递增趋势,最大增大了45.3倍;扩散衰减系数β值随着粒径的增大呈递减趋势,最大衰减了89.8%.实验揭示了多尺度煤粒内的瓦斯多尺度扩散特征,大尺度煤粒包含了更大的外在孔隙,导致煤粒尺度越大,初始扩散系数D0值越大,但因大尺度孔隙占比较大,导致孔隙尺度级差变化较小,因而有较小的衰减系数β值;小尺度煤粒因具有较小的外在孔隙,其D0值较小,但因微小孔隙占比较大,且微小孔隙尺度级差变化较大,因而有较大的衰减系数β值.不同煤粒尺度的扩散特征参数D0和β值的这种变化特征反映了当前低渗煤层瓦斯抽采过程中初期抽采量大但衰减迅速的现象,也为瓦斯(煤层气)增产及稳产提供了储层改造方向.