Correlation of Oil-Water Flow Behaviour in Reservoir Rocks With Dynamic Contact Angles
Authors: Rao D.N. 
Published: March 2010

Laboratory corefloods have constituted a major step in understanding many aspects of fluids flow behaviour in reservoirs. Oil-water relative permeabilities are perhaps the most widely derived parameters from such corefloods, and they are the only recourse to account for all of the rock-fluids interactions in the mathematical models developed to describe the reservoir flow phenomena. One of the factors that has a strong influence on reservoir fluid mechanics is rock wettability. Therefore, attempts to derive wettability from corefloods are abundant in the literature spanning several decades. On the other hand, contact angles observed in crude oil-water-solid systems have been traditionally regarded as a true, or universal, measure of wettability. Hence, the obvious question becomes?is there a correlation between wettability derived from corefloods and contact angles?

A definitive answer does not seem to emerge from the literature for two main reasons:Craig's broad rules-of-thumb, while enabling approximate comparisons, especially between extreme cases of wettability; do not allow direct deduction of rock-fluids interactions to the extent that is required to characterize wettability, andthe conventional contact angle measurements have had their own share of problems in terms of reproducibility. The dual-drop-dual-crystal (DDDC) contact angle technique, reported elsewhere in the literature(1), appears to resolve this long-standing reproducibility problem with contact angle measurements. This paper aims to compare wettability derived from reproducible DDDC tests in widely differing rockfluids systems with their corresponding oil-water relative permeabilities derived from waterflood experiments using reservoir and Berea cores. In all, a total of 10 different case studies are compared, in which eight rock-fluids systems appear to yield similar wettabilities from corefloods and contact angles, while the other two differ markedly. Explanations are sought for these agreements and differences in an effort to shed more light on this important aspect of correlating core analysis with the distribution and flow mechanics of reservoir fluids.

Introduction

In spite of its well-recognized importance, wettability has remained an elusive reservoir parameter to characterize with certainty. The waterflood data on reservoir cores obtained in the laboratory are used to calculate oil-water relative permeabilities, which are then analysed through the application of Craig's broad rules-of-thumb(2) to discern wettability. These rules are based on the values of the connate water saturation, the water saturation at the crossover-point of relative permeability curves, and the end endpoint water relative permeability at residual oil saturation.

Due to the practice of using the end-point oil permeability as the basis for calculating relative permeabilities, the effect of wettability on the end-point oil relative permeability at connate water saturation has been missing from these rules-of-thumb. However, it is well recognized(3) that the end-point oil relative permeability at connate water saturation decreases as the system becomes more oil-wet. In the analysis presented in this paper, the relative magnitude of the end-point oil relative permeability (at connate water saturation) is used as the fourth rule, in addition to Craig's three rules, to infer wettability states from oil-water relative permeability data.