Capillary Pressure Correlations For Uniformly Wetted Porous Media
Authors: Morrow Norman R. 
Published: March 2010

The dependence of capillary pressure on wettability as defined by contact angle has been investigated. Drainage and imbibition capillary pressure measurements are presented for six different types of polytetratfluoroethylene (PTFE) porous media. Changes in capillary displacement curvature are shown to be reasonably consistent from one medium to another. The form of these results suggests that operative contact angles which cannot be measured directly within porous media are in reasonable correspondence with contact angles measured at toughened surfaces. To a fair approximation, changes in drainage and imbibition displacement curvatures with contact angle are respectively proportional to the cosines of receding and advancing contact angles observed at rough surfaces.

General correlations of the results have been developed.

Conditions for observation of spontaneous imbibition behaviour, as predicted by these correlations, are used as a basis for classifying systems as wetted, intermediately-wet or non-wetted. Discussion is given of some aspects of the role of wettability in obtaining improved oil recovery.

Introduction

Incentives for the study of wettability in regard to economic methods of obtaining improved recovery of oil from reservoir rocks have been discussed previously(1). In the only extensive investigation of reservoir wetting behaviour yet reported, advancing contact- angle measurements showed that a wide range of wetting conditions may be encountered from one reservoir to another(2). Contact angles were measured for reservoir fluids at formation temperatures on minerals which, from examination of thin sections of core samples, were judged to be predominant at the pore walls. It was found that wettability, as determined by contact-angle measurements at smooth surfaces, was qualitatively consistent with relative permeability behaviour. The distribution of wettabilities for the 55 reservoirs for which advancing contact angles were reported is presented as a histogram in Figure 1. Although these results may not be typical of reservoirs in general, they present a distinct contrast to the view that most reservoirs are preferentially water-wet.

In practice, when wettability is considered to be important, laboratory data are obtained for preserved cores and fluids at conditions considered to satisfactorily simulate those in the given reservoir. Such data cannot be readily reproduced from one laboratory to another and are difficult to interrelate or quantify. Systematic study under controlled wettability conditions is needed in order to develop an understanding of the role of wettability in oil recovery and other displacement phenomena. Investigations were therefore undertaken of the effects of wettability on spontaneous imbibition(3) capillary pressure and relative permeabilities(4); a companion study was made to determine the effects of surface roughness on contact angle(5). A survey of this work has been prepared(6).

The present paper concerns the effects of wettability on relationships between capillary pressure and saturation.

Background

Intrinsic Contact Angle

The most reliable method of obtaining precise wettability control in displacement studies is through the use of low-energy solids and pure liquids. Problems which arise in attempting to obtain satisfactory wettability control at high-energy surfaces have been discussed previously(7).