Revisiting Mercury Wetting Properties and their Implications for Pore Throat and Pore Body Analysis Using Mercury Injection Capillary Pressure in Porous Media Characterization

Abstract The wetting characteristics of mercury, specifically contact angle and surface tension (or interfacial tension, IFT), are required parameters, for the conversion of MICP (mercury injection capillary pressure, Pc) data into pore-throat sizes, Pc curves of reservoir systems, and pore body-to-throat ratio. The current study investigates the effect of lithology and surface preparation on mercury contact angle and IFT measurements in air with the objective of enhancing MICP based porous media pore structure characterization, including pore typing and pore body-to-throat ratio assessment. In this work, mercury contact angle and IFT (between mercury and air) were measured using an optical pendant drop tensiometer on a variety of lithological surfaces including limestones, dolostones, coarse-grained and fine-grained sandstones, and clay-rich sands. The contact angle was measured on three different areas of a prepared rock surface, in both preserved rough surface and diamond-polished smooth surface, to assess effect of surface roughness on contact angle measurement. The tests were done under static conditions to minimize the influence of surface pores and the possibility of mercury drops falling into pore cavities. As such, the mercury drop was observed for a short period of time; only five seconds. The optical tensiometer captured image was then analyzed with an automated drop-profile fitting method to determine contact angle, and in the process, IFT was also obtained by analyzing the shape of the pendant drop. The measured contact angles at the three different locations on a rock surface were close to each other for the limestone and dolostone samples, indicating homogeneity of their surfaces. However, discrepancies between measurements on the three different locations were observed for the sandstone and clay-rich sand samples. XRD (X-ray diffraction) tests showed that the limestone and dolostone were over 98% calcite and 99% dolomite, respectively. However, the sandstone and clay-rich sand showed a richer variety of mineralogy content, which suggests that heterogeneity in surface mineralogy may result in heterogeneity in the contact angle measurement. These discrepancies can also be explained by the presence of pore cavities on the rock surfaces, which could make the mercury drop unstable and the measurement less accurate. From this study, the average mercury contact angle measured was 138°, only slightly less than the commonly used value of 140°, while average IFT was 417 dynes/cm, 14% less than the often defaulted 485 dynes/cm. The combined effect resulted in an average 17% reduction in pore throat size derived from MICP intrusion and pore body size from MICP extrusion measurement, with little effect on the derived pore body-to-throat ratio.