Wettability Alteration of Carbonate Rocks Using Surfactants - a Comprehensive Analysis Using Centrifuge, NMR and Micro Ct Techniques

Altering the wettability of a reservoir rock to achieve better sweep efficiency is the main focus of most modern EOR projects. The primary objective of this study is to assess the effect of various classes of commercially available surfactants on the wettability of carbonate rocks using different assessment techniques. Centrifuge, Nuclear Magnetic Resonance (NMR) and Micro Computed Technology techniques are used for wettability assessment and comparison of results is discussed.

In brief, effect of three different surfactants from different classes on wettability of similar carbonate samples was studied in this project. Primary drainage, imbibition and secondary drainage tests were conducted on a centrifuge where the core samples immersed in a mobile injection fluid were spun at predefined speeds for a certain period. The produced fluid was translated into saturation thereby producing capillary pressure curve. Wettability of a core is ascertained from the area under the imbibition and secondary drainage curves.

Results obtained using different surfactants reveal that wettability alteration strongly depends on the surfactant type. The fluorinated zwitterionic surfactant was most effective to alter the wettability of the Indiana limestone from oil-wet to neutral-wet and water-wet, followed by hydrocarbon zwitterionic surfactant. The three equipment centrifuge, NMR and micro-CT deploy different techniques of USBM method, saturation profile and high resolution imagery respectively to study initial and altered state of wettablibty of these carbonates. The merits and de-merits of these techniques are also discussed in this study but all three techniques when correlated with each other largely tend to agree in terms of final results.

This work is first of its kind where wettability alteration was studied by a combined approach using a centrifuge, micro CT and NMR analysis. Developing newer and better wettability assessment techniques leads to a more reliable understanding of rock-fluid interactions thereby enabling us to manage a reservoir better.