The Effects of Surface Roughness On Contact: Angle With Special Reference to Petroleum Recovery
Authors: Morrow Norman R. 
Published: March 2010

The effect of surface roughness on contact-angle hysteresis was investigated though a study of capillary rise in PTFE (polytetrafluoroethylene) tubes. Systems were chosen to give intrinsic contact angles (contract angles measured at a smooth PTFE surface) covering the complete range from 0 to 180 degrees. Advancing and receding contact angles were determined before and after deliberately roughening the insides of the tubes by abrasion with particulate solids. These angles showed systematic dependence on intrinsic angle and were independent of tube diameter. With sufficient roughening, there was a marked change in the general nature of the observed contact-angle hysteresis. Results or the well-roughened tubes were reasonably consistent and showed no dependence on the particle size of the powder used to roughen the tubes or on further roughening. Relationships between intrinsic contact angle and advancing and receding contact angles were used in a test of the Wenzel theory of surface roughness. Applications of results and of contact-angle studies in general to the complex problems of reservoir wetting behavior are discussed. When considering contact angles which are operative during oil recovery, distinction should be drawn between imbibitions and drainage, which respectively correspond to advancing and receding conditions. For systems of intermediate wet ability, it appears that oil recovery by low-interfacial-tension flooding will be adversely affected, because contact-angle hysteresis will tend to increase the capillary forces which resist displacement.

Introduction & Background

WETTABILITY determines the distribution of commingled fluids within reservoir rocks and directly affects the microscopic mechanism of oil recovery. Contact angle, for example as exhibited by the sessile drops of water in oil shown in Figure 1 (a) and (b), provides a basic measure of the wetting properties of reservoir fluids with respect to selected mineral surfaces. Preparation of a smooth solid surface is recognized to be an important factor in obtaining reproducible results. However, as may be seen from the electron micrographs shown in Figure 1 (c) through (f), the surfaces of pores which determine wetting behavior within reservoir rocks are generally rough and extremely complex in character because of cementation and other diagenetic effects. Mixed mineral composition., surface roughness, pore geometry and adsorption effects can all be expected to influence reservoir wetting and hence the capillary forces which control recovery. The development of a sound understanding of the significance of observed contact angles with respect to recovery mechanisms requires that each of these factors be studied systematic1ly, The present investigation concerns the effect of surface roughnesson contact angle.

The circumstances under which contact angle provides a single valued and unambiguous measure of wet ability aloe somewhat restrictive and do not generally apply to reservoir systems, The requirements are that fluids (say a liquid, 1, and gas, g) are free of polar impurities and that the surface of the solid (s) is smooth, no deformable and homogeneous with respect to surface energy, Contact angle is then a fundamental property of the system and is referred to as the intrinsic angle 0E(-).