Wettability alteration of oil-wet calcite: A mechanistic study

Enhancing oil recovery in naturally fractured reservoirs by injecting chemistry-optimized water has been widely investigated recently and has demonstrated its efficiency at both laboratory and field trials. There is an extensive ongoing efforts in the industry to characterize and understand the responsible mechanisms at scales ranging from nano-scale to field scale. The ionic formulation of the injected brine affects dramatically the crude oil/brine/rock interfaces, altering rock wettability and improving oil recovery efficiency. In this experimental work, a mechanistic study is performed utilizing analytical methods to study the effect of the ionic composition and ionic strength on the rock sample wettability. The combination of Thermogravimetric Analysis (TGA) and Fourier Transform Infrared (FTIR) spectroscopy is a time saving experimental approach, suitable for wettability alteration quantification of rock samples. The results indicate that stearic acid stretching vibrational bands decrease with the decrease of brine ionic strength indicating a partial release of adsorbed organic material from calcite surface. Single ion brines impacted the calcite wettability and sulfate ions were found to be the most effective in stearic acid release followed by sodium, calcium and magnesium. Thermogravimetric analysis confirmed the observed trend and the calcite weight loss due to stearic acid decomposition decreased with decreasing brine ionic strength and confirmed the fact that sulfates ions are the most effective in partial release of adsorbed stearic acid from calcite surface.