基于数字岩心的页岩油储层孔隙结构表征与流动能力研究

Pore structure characterization and flow ability of shale oil reservoir based on digital cores

页岩油藏孔隙结构非均质性较强且具有多尺度特征,既有纳米级有机质粒内孔隙、纳米—微米级非有机粒间孔隙,还发育微裂缝。页岩低孔低渗特征导致岩心流动物理实验开展较为困难,无法测量不同尺度孔隙介质内的页岩油渗流参数,难以准确认识不同尺度孔隙介质内的页岩油流动能力。为解决该问题,提出基于数字岩心的页岩油储层孔隙结构表征与流动能力计算方法。首先建立考虑纳微尺度运移机制、赋存状态的页岩油纳米孔隙流动数学模型,分析了孔隙表面的物理化学性质、孔隙尺寸对页岩油流动规律的影响。进一步拓展至三维多孔介质,建立孔隙网络页岩油流动数学模型,结合不同尺度下的页岩油储层岩心扫描成像结果,构建了不同介质内的数字岩心,提取孔隙网络模型,研究了页岩油储层多尺度孔隙结构特征与油相流动能力。研究结果表明,孔隙半径在5 nm以下时,页岩油渗透率主要取决于吸附相渗透率;晶间型孔隙介质主导页岩油流动能力,微尺度效应对页岩油储层油相渗透率影响较小,可忽略不计;有机质孔隙介质内页岩油流动微尺度效应较强,滑移现象较为明显,对页岩油储层流动能力贡献取决于有机质内部孔隙联通性。

Shale oil reservoir bears heterogeneous pore structure with multi-scale pore sizes.Nano-scale organic intra-granular pore,nano-micro scale inorganic inter-granular pore and micro-fracture coexist in shale oil reservoir.The ultra-low porosity and ultra-low permeability characteristics make the laboratory core flow experiment unavailable for shale oil core sample.As a consequence,shale oil flow parameters in different scale of porous medium can not be measured and it is difficult to accurately evaluate the shale oil flow ability in different scale of porous medium.To solve this problem,a calculation method for pore structure characterization and flow ability of shale oil reservoir is proposed based on digital cores.The nanopore shale oil flow model is first established considering nano-micro scale transport mechanisms and occurrence state,and the influences of shale pore surface physicochemical property and pore size on shale oil flow are analyzed.Then,the nanopore shale oil flow model is further extended to 3D porous media by establishing pore network shale oil flow model.The digital cores in different medium and its pore network are constructed based on the multi-scale shale core imaging data in shale oil reservoir.The multi scale pore structure characteristic and shale oil flow ability are studied in detail on this basis.The analysis results indicate that when the pore radii are less than 5 nm,the shale oil permeability is dependent on adsorbed phase permeability.Inter-granular pore dominates shale oil flow ability.The micro-scale effect on shale oil permeability is very small which can be neglected.The micro scale effect and oil slippage are more obvious in organic pores.However,the contribution of organic pore permeability on total shale oil permeability relies on the connectivity of organic pore structure.