页岩纳米级孔隙中甲烷解吸的微观机理研究

As an important unconventional natural gas resource, the occurrence states of the shale gas in the formation include the free gas, the adsorbed gas and the dissolved gas. The present exploitation experiences indicated that the main reason why the late production of the shale gas wells reduced slowly is the desorption effect of the adsorbed gas in the shale gas reservoirs. Currently, aimed at the researches about the adsorbed gas desorption on the shales, the academia mainly investigated the desorption behaviors of the methane on the shals from the macroscopic angle. However, it can’t reflect the microcosmic mechanism of the methane desorption in the shale nanopores. The functional mechanisms of the influences of the different fluids on the methane desorption in the shale nanopores is not investigated clearly, neither. Therefore, the methods of macroscopic experimental and microcosmic simulation would be adopted in the project. On the basis of the process of the methane desorption on the shales, the methane desorption model and the kinetic model during the process of desorption would be developed. The molecular simulation methods are used to simulate the process of the methane desorption in the shale nanopores, which would reveal the microcosmic mechanism of the methane desorption in shale pores. The microcosmic mechanism of the effects of different fluids and different temperatures on the methane desorption in the shale nanopores through simulating the process of the methane desorption in the shale nanopores under the different fluids (water, carbon dioxide and nitrogen) and the different temperatures. The achievements of the project would provide the theoretical basis to deeply investigate the mechanism of the methane desorption on the shales, and is of great significance to the development of the shale gas reservoirs in China.

页岩气作为一种重要的非常规天然气资源，其赋存形式包括游离态、吸附态和溶解态等。现有页岩气藏开采经验表明页岩气井后期产量递减缓慢，主要原因在于页岩中吸附气的解吸作用。目前，针对页岩中吸附气解吸的研究，学术界主要从宏观角度研究了页岩中甲烷的解吸规律，但不能反映出甲烷在页岩纳米级孔隙中解吸的微观机理，且不同流体对页岩纳米级孔隙中甲烷解吸影响的作用机理认识也不够清晰。为此，本项目拟采用宏观实验和微观模拟手段，通过分析页岩中甲烷的解吸过程，建立页岩中甲烷的解吸模型与解吸过程的动力学模型；通过模拟甲烷在页岩纳米级孔隙中解吸过程，揭示甲烷在页岩孔隙中解吸的微观机理；通过模拟不同温度条件下和不同流体作用下页岩孔隙中甲烷的解吸过程，揭示温度、流体对页岩纳米级孔隙中甲烷解吸影响的微观机理。本项目研究成果为深入认识页岩中甲烷的解吸机理提供理论依据，对我国页岩气藏的开发具有重要意义。

现有页岩气藏开采经验表明页岩气井后期产量递减缓慢，主要原因在于页岩中吸附气的解吸作用。目前，针对页岩气解吸的研究，主要从宏观角度研究了页岩中甲烷解吸规律，但不能反映出甲烷在页岩孔隙中解吸微观机理，且不同流体对页岩孔隙中甲烷解吸影响的作用机理认识也不够清晰。本项目以四川盆地下志留统龙马溪组页岩为研究对象，以宏观实验和微观模拟等为手段开展相关研究工作。研究结果表明：页岩等温吸附/解吸曲线存在较明显存在着较为明显的滞后环，即存在较明显解吸滞后现象；D-A方程和吸附特征曲线结合得到改进D-A模型，其能用于描述甲烷等温吸附或解吸过程；随着压力降低或温度增加，页岩中甲烷解吸效率呈增大的趋势，但甲烷解吸效率对压力的敏感性要大于温度；干酪根和不同矿物的甲烷解吸效率顺序为石英>绿泥石>伊利石>高岭石>蒙脱石>干酪根；甲烷在干酪根中的吸附解吸迟滞系数最大，而甲烷在石英上的吸附解吸迟滞系数最小；甲烷扩散量随着时间增加而呈上升趋势，而甲烷扩散速度随着时间增加而呈下降趋势，且甲烷扩散系数随着压力增大而增大，随着温度增大而增大；根据结构信息表征的四川盆地龙马溪组干酪根的平均分子结构的化学式为C206H158O19N4S4，该分子结构模型与实测数据较吻合；甲烷分子在页岩孔隙中以分散形式存在，其在孔壁面附近区域聚集从而形成吸附层，其为吸附相，而远离孔壁区域，甲烷分子分散于孔中，其为游离相，且吸附相与游离相处于动态平衡状态；随着压力降低，处于吸附相的甲烷分子向游离相转变的解吸量增大；相同压力下，随着孔径增大或温度增大，处于吸附相的甲烷分子向游离相转变的解吸量增大；不同流体注入将使流体分子占据甲烷分子吸附空间，且使甲烷分子吸附位发生变化，造成处于吸附相的甲烷分子向游离相转变的解吸量增大，且随着流体注入量增大，这种现象更明显。该研究成果对页岩气渗流规律及提高采收率技术研究具有重要的理论意义和应用价值。