Mineralogy and Salinity Control Nano-Assisted Wettability Alteration in Sandstone and Carbonate Reservoirs: Contact Angle Screening for Middle East EOR Optimization

Abstract

Abstract Low-salinity waterflooding (LSWF) combined with nanoparticle (NP)-enhanced formulations shows promise for Middle East reservoirs, yet the relative contributions of salinity, rock mineralogy, and interfacial tension (IFT) to wettability alteration (WA) remain poorly quantified. This study systematically evaluates how brine salinity (30,000 vs. 15,000 ppm) and mineral composition (quartz, calcareous, calcite) control contact angle (CA) reduction in nano-assisted systems, establishing quantitative screening criteria for field-scale Enhanced Oil Recovery (EOR) design. Six cores—four sandstones (including one calcareous) and two carbonates—were experimentally characterized for porosity (13.0–33.6%), permeability (2.4–2927 mD), and mineralogy. Cores were oil-aged to establish oil-wet conditions (CA 64–90°). Nano-enhanced brines containing 70-nm MgO NPs at two concentrations (0.5 wt% high-salinity; 0.1 wt% low-salinity) with 0.5 wt% sodium dodecyl sulfate (SDS) surfactant were tested at ambient temperature. Static contact angles quantified wettability alteration; IFT and viscosity were monitored. Multivariate regression analysis (R2 = 0.908) correlated CA with salinity, NP concentration, IFT, rock type, mineralogy, porosity, and permeability to identify dominant controlling parameters. All treatments achieved 61–92% CA reductions, shifting systems toward water-wet conditions. High-salinity (HighSal) nano-brine (0.5 wt% MgO NPs, 30,000 ppm) produced significantly lower CA (14.2° ± 8.7°) than low-salinity (LowSal) formulations (0.1 wt% MgO NPs, 15,000 ppm; 23.0° ± 8.4°), despite the LowSal system achieving greater IFT reduction (96.1% vs. 88.3%), demonstrating that NP–mineral surface interactions dominate over bulk IFT effects. Sandstones consistently outperformed carbonates (mean final CA: 15.5° vs. 24.8°), with calcareous sandstone exhibiting optimal response (6.6–8.7°, representing 90–92% reductions from initial oil-wet state). Pure calcite carbonates showed weakest alteration (29–33° final angles). Multivariate analytics revealed mineralogy and rock type as primary controls (standardized regression coefficients: porosity ±30.8, permeability ±29.1, rock type ±12.4) while treatment variables showed uniform secondary effects (±1.3 each). High-porosity, high-permeability sandstones achieved absolute lowest CA (5.5° for 33.6% porosity, 2927 mD sample), indicating enhanced NP accessibility amplifies effectiveness. This work demonstrates with 18 CA measurements across mixed-mineralogy cores that (1) mineralogy and salinity govern nano-LSWF effectiveness more than IFT reduction; (2) calcareous and quartz-rich sandstones are priority targets for nano-EOR (nEOR) pilots; (3) simple CA screening combined with rock-property analytics provides field-deployable reservoir prequalification before costly pilot implementation. These insights de-risk EOR investments in heterogeneous Middle East reservoirs where mixed mineralogy is common.