煤层气直井排采过程中渗透率变化规律研究

Research on permeability variation law of coal reservoir in drainage process of CBM vertical wells

基于煤层气储层地质参数的差异对排采过程中煤储层内部应力状态、力学结构性质、能量系统变化的影响，采用孔隙结构、弹塑性、损伤力学，能量平衡转化理论等方法，构建了排采过程渗透率变化的数理模型．提出了“回升系数”的概念，量化了不同条件下渗透率变化规律的差异．结果表明：排采过程中渗透率总体呈现“先降低后增大”的趋势，储层参数不同，渗透率降低幅度和回升能力存在较大差异，埋深越浅，渗透率回升至初值所需的弹性模量越小．当埋深在400m以浅，煤体弹性模量只需4～5GPa渗透率即可回升至初值，当埋深增至600m时，弹性模量需达到7～8GPa渗透率才能回升至初值，当埋深超过800m时，煤体弹性模量在8GPa内渗透率基本不能回升至初值．排采过程渗透率变化的临界地质参数值的确定，为现场的工程实施或渗透率理论研究提供了理论依据．

Based on the effect of geological parameters differences in coal bed gas reservoirs on the internal stress state, the mechanical structure properties and the energy system changes of coal reservoir in drainage process, the permeability variation mathematical model in drainage process has been established according to the pore structure, elastic plasticity, damage mechan- ics and the balance of energy conversion theory. The differences of permeability variation law under different conditions were quantitated by proposing the concept of ＂rebound coefficient＂. The results show that, the permeability variation law presents the general trend of ＂decrease first and then increase in drainage process＂. But there would be large differences in permeabili- ty decrease level and rebound level when the reservoir parameters are different. The shallower the depth is, the smaller the elastic modulus for permeability recovering to initial value is. When the depth is above 400 m the permeability can recover to initial value with 4--5 GPa elas- tic modulus, while the 7--8 GPa elastic modulus is needed when the depth is about 600 m, and the permeability cannot recover to initial value within 8 GPa elastic modulus when the depth is up to 800 m. The quantitative geological parameters in the critical state of permeability varia- tion during the drainage process were very important theoretical basis for engineering and theo- ry research.