The Effect of Wettability Heterogeneity on the Capillary Pressure and Relative Permeability

摘要

The effect of wettability on fluid flow properties in porous media has been extensively studied in the literature, and it is still a subject that attracts researchers. Most of the work is focused on cores of homogeneous wettability but little attention has been paid to wettability heterogeneity effects at the core or pore scale. In a previous paper we reported on a series of centrifuge experiments performed to study the effect of wettability heterogeneity on capillary pressure. An experimental technique, named cyclic aging, was developed to create regions of different wetting in the same core sample. In this paper the work is extended to study the effect of wettability heterogeneity on both capillary pressure and relative permeability curves using centrifuge, continuous injection and steady state techniques. The experimental procedure consists of three steps: 1) the core plug is fully saturated with brine and subsequently a drainage experiment is performed targeting initial oil saturation S_(oi), 2) after aging, oil is displaced by water to residual oil saturation S_(or), and 3) oil is injected targeting higher initial oil saturation. In this secondary drainage experiment, oil will first displace water from the pores exposed to crude oil in the primary drainage and will then enter fresh pores which were not exposed to crude oil before. In the previous study it was found that wettability heterogeneity caused a step change in the capillary pressure which correlated very well with the saturation where wettability contrast was expected. However, the height of the step could not be explained by wettability contrast and/or water trapping alone. An experimental artifact caused by the centrifuge technique made the step higher than expected. The experimental artifact was the result of the non-uniform saturation profile developed across the core at the end of the centrifuge experiment. In this study new techniques were used which result in a uniform saturation profile along the core sample during the primary drainage experiment. It was found that in this case the step in the capillary pressure is determined by wettability contrast and water trapping. It was also found that the relative permeability curve changes its characteristics when oil accesses the pores not previously exposed to crude oil. The results of this study shows that 1) only the part of the pore space exposed to crude oil undergoes wettability changes on both core scale and pore scale, and 2) ignoring wettability heterogeneity can lead to large errors in the estimated two phase flow functions with important consequences on fluid flow in porous medium.