Multiscale Wettability Characterization of Anhydrite-Rich Carbonate Rocks: Insights into Zeta Potential, Flotation, and Contact Angle Measurements
Authors: Isah Abubakar , Mahmoud Mohamed , Kamal Muhammad Shahzad , Arif Muhammad , Jawad Murtada Al 
Published: February 2023

Summary
Anhydrite (CaSO4) is a chemically reactive rock/mineral found predominantly as a constituent of carbonates. The main constituents of anhydrite are calcium and sulfate ions. The presence of anhydrite, its distribution, and the associated anhydrite-fluid interactions are important to precisely evaluate the effectiveness of oil recovery techniques. While anhydrite dissolution is the key interaction mechanism in anhydrite-rich rocks, its presence may also lead to complex rock wetting behavior. The underpinning logic is that pure anhydrite is strongly water-wet, while pure calcite and dolomite are somewhat intermediate to weakly oil-wet, thus the question remains unclear as to what the wettability would be of anhydrite and calcite, and anhydrite and dolomite combinations. Moreover, because anhydrite is negatively charged while dolomite and calcite in formation water (FW) are positively charged, depending on the mixture composition, pH, and brine type, it is not clear what the charge would be of a combination of anhydrite-calcite or anhydrite-dolomite, and, consequently, what the wetting behavior of calcite and dolomite would be due to anhydrite presence. Therefore, this research explores the effect of anhydrite mineral on carbonate wetting characteristics. The effect of mineralogical heterogeneity, specifically the presence of anhydrite minerals in calcite and dolomite wettability, is investigated across a range of scales.
The results show that anhydrite dissolution occurs in deionized (DI) water, seawater (SW), and FW as evident from the general increase in sulfate ions concentration with increased anhydrite content in the anhydrite-carbonate system. We also found that zeta potential demonstrates an unstable colloidal system, which is indicated by near-zero and low zeta potential values (less than ±10) of the anhydrite-carbonate-brine systems. It also shows a nonmonotonic wetting behavior with brine salinity and pH variations. Accordingly, the zeta potential is not a general and valid candidate to justify the wettability behavior of heterogeneous carbonates. However, based on flotation and contact angle techniques of wettability estimation, anhydrite presence has the tendency to alter the wetting state of anhydrite-carbonate-brine-oil systems to more water-wet. Thus, findings from this research will provide answers to the question of how the mineralogy affects the wetting characteristics of carbonates. What will be the changes in carbonate wetting behavior with mineralogical heterogeneity? Specifically, what would be the wettability of calcite-anhydrite and dolomite-anhydrite combinations? This research therefore provides a systematic investigation of rock/fluid interactions and their implications on wettability and ultimate recovery of oil at different range scales. The findings from this study will significantly enhance our knowledge of fluid-rock interactions, in particular, anhydrite-rich carbonate wetting behavior, thereby reducing the uncertainties associated with laboratory-scale predictions and oil recovery planning.