不同煤级储层氮气闷压增渗机理研究

To reveal the mechanism of permeability enhancement in different coal rank induced by nitrogen injection and seal (long time interaction between high pressure nitrogen and coal reservoirs), a series of studies are to be carried out. Physical tests will be conducted to investigate permeability changes, strain characteristics and successive depressurization desorption on different methane-saturated coals before and after high pressure nitrogen injection and seal. Numerical modeling is employed to examine the relation of permeability enhancement to the coal matrix shrinkage during coal desorption process under pressure after injecting high pressure nitrogen.The research results can assist in understanding changing characteristics of pore-fissure structures, permeability, diffusion coefficients and permeability enhancemen with the coal matrix shrinkage of different methane-saturated coals during the desorption process under pressure after high pressure nitrogen injection and seal. Results from this study are of great theoretical and engineering significance for CBM recovery in low-permeability coal reservoirs and production enhancement of low-yielding wells.

为了揭示不同煤级储层氮气注入闷压（高压氮气与煤长时间相互作用）增渗机制，拟通过实验室物理模拟还原不同煤级储层氮气注入闷压后的逐次降压解吸过程，测试分析不同煤级煤岩体氮气注入闷压后的渗透率变化以及含不同饱和度甲烷煤岩体氮气注入闷压后的应变规律；利用数值计算分析氮气注入闷压后不同煤级储层带压解吸过程中的渗透率增量和煤基质收缩量的关系；对比研究不同煤级储层氮气注入闷压前后孔裂隙结构、渗透率与扩散系数、含不同饱和度甲烷煤岩体带压解吸过程中煤储层渗透率随煤基质收缩的变化规律。研究成果对于我国低渗煤储层煤层气开发和低产井增产改造具有理论和工程意义。

我国煤储层渗透率普遍偏低，注气压裂改善煤储层渗透率是提高煤层气产量和采收率的有效途径之一。不同饱和度、不同含气量煤储层N2–ECBM开发过程中的解吸率、煤基质收缩变形、渗透率变化规律尚不明晰。因此，揭示不同煤级储层氮气闷压增渗机理是目前亟待解决的关键科学问题。本次利用核磁共振仪、煤吸附/解吸应变测试平台、高压渗透率测试仪对17件不同煤级煤柱样，开展了氮气驱替不同饱和度甲烷实验、N2+CH4不同组合浓度下煤吸附/解吸应变实验、氮气闷压过程中煤中孔裂隙、基质收缩特征和渗透率演化规律等模拟研究。研究结果表明不同煤级、不同温度、不同CH4+N2组合浓度、不同孔径、不同相态（吸附态、游离态）甲烷的解吸不同步，吸附–解吸体积变形也存在差异，煤级越低，温度、含气饱和度、含气量越高，解吸效率越高；N2注入闷压促进了CH4解吸，CH4累计解吸率随煤级先增大后减少（拐点在Ro,av,ran=1.5%附近）；N2–ECBM过程中残余体积变形量随N2浓度的增加而减少，注氮闷压压力高于临界解吸压力N2–ECBM效果好；煤储层渗透率随逐级降压解吸煤基质收缩变形量的增大呈指数形式增大；氮气闷压一方面对大孔与裂隙起重张和“疏通”作用，另一方面通过降低甲烷分压促使甲烷解吸，煤基质收缩，渗透率增大。