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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