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Most of the water flood reservoirs in China, such as Shenli Oil Field and Daqing Oil Field have entered into an ultrahigh water cut period. The study of remaining oil percolation characteristics under microscopic pore scale plays an important role in enhanced oil recovery for ultrahigh water cut period water flood reservoirs. In this paper, glass etching microscopic model experiments and computer image processing & recognition techniques are combined to reclassify the microscopic remaining oil flowing patterns which considering the flowing shape and the linkage among the oil, water and rock. Then we design different etching models, study the effect of pore-throat ratio, pore throat radius, coordination number, inject speed and oil viscosity on the flowing patterns and variation of microscopic remaining oil. Research results indicate all the remaining oil in ultra-high water cut period can be classified into five categories: clustered stream, multi-porous stream, columnar stream, membranous stream and droplet stream. Among of the five categories, clustered stream possesses the largest proportion and is also the main factor of relative permeability curves bended. Meanwhile, as water saturation increases, clustered stream gradually transformed into other patterns, like multi-porous stream, columnar stream, membranous stream and droplet stream. Sensitivity analysis based on pore structure shows that: (1) the larger pore radius, the greater the maximum water saturation, the stronger capability of the continuous phase flow through porous media, and the smaller the clustered stream percentage; (2) the larger pore throat ratio, the easier broken the remaining oil and occurring as non-continuous phase in the pores, and the smaller the maximum water saturation; (3) the larger the coordination number, the better the pore connectivity, the smaller the maximum water saturation; (4) the smaller shape factor, the more complex the porosity, the smaller the maximum water saturation, the easier the remaining oil is transformed into a non-continuous phase". Meanwhile, the greater the oil viscosity, the smaller the maximum water saturation, while the greater the injection rate, the greater the maximum water saturation. Also we found that the occurring occasion of clustered stream inflection point is positively correlated to the maximum water saturation. In this study, we not only provide the computation code which can recognize and analyze flowing remaining oil, but also explain the oil-water relative permeability curves non-linearity during ultra-high water cut period from the microscopic point of view. The achievements of this study are also the foundation of water flood reservoir ultra-high water cut period improved remaining oil sweeping efficiency and enhanced oil recovery.

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Extensive pore-structure studies reveal that the fabric of shale systems are comprised primarily of micro-nano meter matrix pores and complex natural fractures. Measured permeability of shale samples containing natural fractures varies significantly with different cores in the same shale gas reservoir. The significant variation is mainly caused by the stochastic distribution of natural factures. These natural fractures’ effects on permeability under reservoir conditions significantly influence oil and gas production. This work proposes a predictive permeability model that considers the gas transport mechanisms both in the natural factures and matrix pores. In this work, permeability variation is accomplished by considering that the shale core consists of matrix pores and natural fractures. A simple conceptual flow model with shale gas apparent permeability is developed that couples the cubic flow effects of natural fractures and micro- and nano- scale seepage effects of matrix micro- and Nano- pores based on the principle of equivalent flow resistance and the Tandem -Parallel rules of circuit theory. The matrix apparent permeability is derived based on the dusty gas model (DGM) by considering the contribution of Knudsen diffusion and gas slippage. The fracture apparent permeability is established based on a parallel-plate model by considering the influence of the fracture density. Then the comprehensive apparent permeability of shale samples is achieved by coupling the matrix apparent permeability and the natural fractures apparent permeability. The matching results with experiment permeability of the Cambrian Niutitang formation from Sichuan Basin in South China show that the naturally fractured shale permeability is strongly affected by the natural fractures’ characteristic parameters. This research proves that shale measured permeability is the result of seepage interaction between the natural fractures and matrix space. The new permeability model, which overcomes the defect of conventional models that only considered the contribution of the fracture system or the matrix system, can accurately estimate the total contributions of matrix pores and natural fractures. In turn, this will enable engineers to complete the shale gas simulation studies more readily. In the future, this new methodology can be used to develop the next generation reservoir simulators for the shale reservoirs.