Abstract:

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.

Key words: shale oil, digital core, pore network model, multi-scale pore structure, permeability