Pore-scale investigation of microscopic remaining oil variation characteristics in water-wet sandstone using CT scanning

With the aid of CT scanning and image processing techniques, water displacing oil experiments were performed to analyze six natural water-wet sandstone cores of different permeability and porosity visually and quantitatively. Microscopic remaining oil was categorized on the basis of quantitative characterization parameters, such as shape factor, contact ratio, Euler number etc. The remaining oil characteristics with different flow types were presented in different water displacement stages. Experimental results show that the remaining oil can be divided into five categories. They are, respectively membranous flow, droplet flow, columnar flow, multi-porous flow and clustered flow from hard-to- produce to easy-to-produce. The relative permeability of oil phase is represented by the macroaverage relative permeability of all the above five flow regimes. Among them, clustered flow possesses strong producing capacity and high relative permeability, and the other four flow regimes have weaker producing capacity and lower relative permeability. Variation of number and average volume of five types remaining oil and several measurements of the ganglion size distribution have been performed in the literature. The nonlinear knee point on the relative permeability ratio curve is intrinsically caused by the decreasing of volume and quantity fraction of clustered flow when water saturation increases. This paper has studied the flowing law of microscopic remaining oil, explained the intrinsic mechanism for the appearance of an inflection point on the relative permeability ratio curve and microscopic sweep phenomena, and also presented a new effective way of upscaling to a certain extent. (C) 2017 Elsevier B.V. All rights reserved.