Impact of texture and mineralogy on the wettability of rock surfaces

Wettability, the tendency of a solid medium to stay in contact with one fluid in the presence of another, is a key factor influencing multiphase flow in porous rocks. This occurs for example during production in oil-reservoirs, where water or gas phases are typically also present. In addition to the average wettability, which is often described through adhesion forces, surface energies or contact angle measurements, the spatial distribution of wettability in the porous rock is significant. Variation in wettability may be related to the surface chemistry of constituent minerals, and also to both surface texture and roughness.

In this work, we assessed the relationship between wettability, mineralogy and surface structure for two representative porous rocks: Ketton carbonate , which is effectively chemically homogeneous comprising 99.9% calcite; and Bandera Brown sandstone which contains different minerals such as quartz, kaolinite, plagioclase, calcite and illite. These were studied using atomic force microscopy (AFM), environmental scanning electron microscopy (ESEM) and inverse gas chromatography surface energy analyser (iGC-SEA).

For both rocks, the surface structure was found to have a large impact on the wettability of the rock. Roughness and texture variation along the rock surface constitute the respective capillary pressure required to drain prevailing thin water films. The water films prohibit crude-oil-rock contacts and subsequent wettability alteration to occur. The water films were measured in-situ and in 3D using AFM on a 10 x 10 μm surface with nm-resolution. ESEM was used to generate larger scale high-resolution representative images of the rock surfaces and to correlate these to the features observed in the AFM. Finally, iGC-SEA was utilized to compare the results with the reduction of surface energy due to the presence of water within cores containing a surface area larger than 1 m²/g. The results obtained for sandstone, indicate that, in addition to the surface structure, differences in surface chemistry are also significant.

The presented work demonstrates that rock surfaces need to be investigated in their native state in order to determine the wettability characteristics of a rock-fluid system. The introduced different techniques can be used in combination for this purpose.