Reservoir rock wettability is a critical parameter in any reservoir characterization. To assess this parameter, conventional techniques such as imbibition tests (Amott-Harvey and USBM), contact angles and NMR are used in oilfield. These techniques provide a relative assessment of rock wettability, but do not provide an accurate wettability measurement and neither one considers the rock surface morphology and rock/fluids interactions at sub-micron scale.
Atomic force microscopy (AFM) is currently used extensively for imaging surfaces and getting force-distance measurements between a tip and a surface. AFM images have been used to confirm the deposition of oil components on mineral surfaces as described in “The role of interfacial rheology in reservoir mixed wettability,” by M. Freer, T. Svitova, C. J. Radke, Journal of Petroleum Science and Engineering, Vol. 39, No. 1, pp. 137-158, 2003, and H. Toulhoat, C. Prayer, G. Rouquet, Colloid Surf. A: Phys. Eng. Asp., Vol. 91, pg. 267, 1994. AFM imaging has also been used to study the effect of adding resins to asphaltene solution on aggregation at a mineral surface in M.-H. Ese, J. Sjoblom, J. Djuve, R. Pugh, Colloid. Polym. Sci., Vol. 278, pg. 532, 2000. They found that the addition of resins increased the size of asphaltene aggregates. In H. Toulhoat, C. Prayer, G. Rouquet, Colloid Surf. A: Phys. Eng. Asp., Vol. 91, pg. 267, 1994, AFM was used to study the adsorption of asphaltene extracted from different crude oils and observed a change in contact angles due to asphaltene adsorption. In “Atomic Force Microscopy Study of Wettability Alteration” by K. Kumar, E. K. Dao, K. K. Mohanty, International Symposium on Oil Field Chemistry, Houston, Tex., (2005), SPE 93009, it was demonstrated that wettability of mica (Muscovite) and calcite with SARA fractions (Saturates, Asphaltenes, Resins and Aromatics) is controlled by the adsorption of asphaltene components and the force of adhesion measured for minerals aged with just the asphaltene fraction is similar to that of the whole oil. D. L. Lord, J. S. Buckley, Colloid Surf. A: Phys. Eng. Asp., Vol. 206, No. 53, pgs. 1-546, 2002 observed micron- and nanometer-scale changes in topography on mica surfaces before and after exposure to crude oil. Density of coverage and film structure is found to be related to exposure time and crude oil chemistry. A. Abudu, L. Goual, Energy & Fuels, Vol. 23, pgs. 1237-1248, 2009 concluded that adsorption of crude oil results in rigid films with an average thickness of about 3.5 nm. O. Karoussi, A. A. Hamouda, J. Colloid Interface Sci Vol. 317, pgs. 26-34, 2008 studied the effect of Mg2+ and SO42− on wettability alteration of calcite surface at nano-scale by AFM and contact angle measurements and they found that magnesium ions increases the water wetness of calcite and showed good correlation between contact angle and AFM measurements. The same agreement was found by J. Liu, B. Yu, B. Ma, X. Song, X. Cao, Z. Li, W. Yang, F. Zhou, Colloid Surf. A: Phys. Eng. Asp., Vol. 380, pgs. 175-181, 2011. T. Hassenkam, L. L. Skovbjerg, S. L. S. Stipp, PNAS, Vol. 16, No. 15, pgs. 6071-6076, 2009 using a hydrophobic AFM probe to investigate wettability at pore scale level and concluded that wetting, as a macroscopic parameter, averages the nanoscopic behavior along fluid pathways, and mixed-wet samples have patches with vastly different properties.