Role of Organic Acid Partitioning and Bulk Brine in Calcite Surface Modification and Wettability Alteration

The rock wettability is an important property governing the multiphase flow in subsurface porous media. Wettability alteration of native water-wet rocks is often linked to the destabilization of the native brine film present on the rock surface, followed by the adsorption of polar components of an organic phase. Thus, pores having intact brine films are assumed to be water-wet. In this study, we show that the calcite surface covered with intact brine film (and bulk brine) could also become oil-wet. To understand this phenomenon, calcite chips and crushed calcite powder were aged by using two different approaches. In the first approach, calcite chips and powder were first aged in brine and then separately aged in oil. In the second approach, calcite chips and powder were aged simultaneously in brine and oil. The amount of brine in the second approach was such that the calcite surface and powder were completely submerged in the brine. The aged samples were analyzed using various analytical techniques, including X-ray photoelectron spectroscopy (XPS). The effect of the brine composition on the partitioning of stearic acid and rock wettability was studied. Calcite chips become oil-wet in both the approaches, with contact angles varying from 69 ± 5° to 151 ± 6°, depending on the brine composition and the aging technique. XPS analyses confirmed the surface modification of the calcite surface with the organic acid in the second approach. Organic acid partitioning was found to be similar in all brines. However, not all calcite surfaces became oil-wet, indicating that interactions at the rock-brine interface also play important role in organic acid adsorption and calcite wettability alteration. Furthermore, oil-wet calcite surfaces prepared using both the approaches showed wettability alteration in low-salinity brines such as seawater (SW) and sulfate-rich seawater (4S-SW).