Using Energy Balance in Porous Media to Determine Wettability

Abstract Recent derivation of a thermodynamically consistent contact angle estimates from high-resolution 3D x-ray imaging has proven successful to determine rock wettability based on energy balance assumptions. This method involves computing the differences in interfacial areas and curvatures from images acquired during a waterflood displacement, which are highly sensitive to the image resolution and segmentation quality. In this study, three different wettability rock samples were analysed, namely (i) a water-wet Bentheimer sandstone, (ii) a mixed-wet Ketton limestone, and (iii) an oil-wet reservoir carbonate. The average thermodynamic contact angles for the water-wet, mixed-wet, and oil-wet rock samples were 48.5° ± 5°, 121° ± 15°, and 111° ± 10° respectively. This work extends its novelty approach to analyse the spatial wettability variations by discretising each rock sample into smaller sections along the flow direction. A wider range of contact angle was calculated in the water-wet sample; however, because of the tiny sample size, a supplementary study is needed to further evaluate the local rock heterogeneity. Although the same wettability alteration protocol was implemented for mixed-wet and oil-wet rock samples, the displacement exhibited different behaviours. Interestingly, waterflooding in the mixed-wet rock governed by wettability control, in which water preferentially invades the more water-wet and then the more oil-wet regions of the pore spaces, resulting in an increase trend of contact angle over time. On the other hand, the oil-wet case has a more consistent contact angle because the pore filling sequence was controlled by the pore size; therefore, water was filling the larger pores preferentially. Haines jump was observed in the oil-wet rock during the initial water displacement, resulting in the abrupt change of mean curvatures and capillary pressure. Positive curvatures and capillary pressure in water-wet rock measurements validated that oil bulges into the water, in contrast to the mixed-wet and oil-wet samples. For oil-wet case, capillary pressure has larger negative values, as compared to the mixed-wet, implying that water needs higher pressure to bulge into the oil in the pore spaces.