Discussion of shale rock wettability and the methods to determine it

Because of ultra-low permeability of shale rocks, capillary pressure is very high. Wettability not only determines the value of capillary pressure, but, more importantly, it also determines whether the capillary pressure is a driving force or a resistance to fluid flow. This paper is to address the wettability characteristics of shale rocks. First, the common methods to determine wettability are reviewed and discussed for their feasibility for shale rocks. It is found that the real reservoir wettability must be determined in a system with oil and water being present; it cannot be determined by comparing the water and oil-wetting angles measured on dry rock surfaces. It is also interesting to note that more shale rocks are water wet, although the oil-wetting angles are smaller than water-wetting angles on dry rock surfaces. The factors that affect shale wettability are also reviewed.