Determining the Zeta Potential of Intact Shales via Electrophoresis

Abstract

The wettability of rocks is of fundamental importance to the understanding of fluid transport within hydrocarbon reservoirs. Wettability is intimately tied to the zeta potential, which is determined by the electrostatic forces among the rock's mineral constituents. Extensive prior research has been conducted into the relationship between wettability and surface forces of shale samples that have been ground up and/or otherwise been altered. However, very little attention has been given to measuring the zeta potential of the surface of unbroken, intact shale samples. A new technique is explored in this paper that investigates the zeta potential of the actual surface of shale samples in one intact piece. The surface zeta potential of shales was found to be strongly determined by the mineral composition and the cation exchange capacity (CEC) of the shale. Shales containing higher amounts of silicates (e.g. quartz, feldspar, and clays such as Illite and Smectite) tended to be water-wet and exhibit higher negative zeta potential values. Shales that contained higher amounts of carbonates (dolomite, aragonite, calcite, etc.) were more oil-wet with zeta potential values that were less negative or slightly positive. Measurements were also performed on Berea and Boise sandstone in order to compare organic vs. inorganic samples. Results for common shales showed Mancos and Marcellus to be intermediate-wet, Oxford and Eagleford to be moderately oil-wet, and Arne to be highly water-wet. Additionally, the zeta potential was also studied in various ionic and nano-particle solutions.

A Surface Charge Approach to Investigating the Influence of Oil Contacting Clay Minerals on Wettability Alteration

“…This means that across all pH, due to the low negative ζ-potential values, the surface of kaolinite presents an oil-wet surface even though the surface has mostly negative surface charge. 42 Figure 4C,D presents the effect of 0.1 M salt solutions of NaHCO 3 and Na 2 SO 4 , respectively, on kaolinite clay minerals, and as can be observed, they both have similar trend patterns, which could mean that the same mechanism acts on the suspended particles. The wave-like pattern, which repeats cross the pH, can be attributed to competing mechanisms of adsorption of the HCO 3 − and SO 4 2− on the surface with the presence of H + and OH − as well as the double-layer collapse around the particle.…”

“…This results in near-zero ζ-potential and the development of an adsorption-prone surface, which would promote wettability alteration . Contrary to the observation of oil wetness of montmorillonite, Hoxha et al asserted that montmorillonite possesses water wet characteristics that are congruent with our base case without salt interactions; however, in the presence of divalent cations, this argument of water wetness may not hold as depicted in Figure B. The effect of NaHCO 3 and Na 2 SO 4 is depicted in Figure C,D, respectively.…”

“…The author highlighted that in the case of steric stabilization, ζ-potential does not correlate with colloidal stability measurement. This means that across all pH, due to the low negative ζ-potential values, the surface of kaolinite presents an oil-wet surface even though the surface has mostly negative surface charge …”

“…Thus, generally, the surface charge of illite particles is dependent on the adsorption/desorption of H + and OH − as well as the presence of cations in the solution resulting in different PZCs with different ions. 25 Thus, the notion that silicate groups (quartz, feldspar, illite, and smectite) are all water-wet as claimed by Hoxha et al 42 is misleading especially with interactions with divalent and trivalent cations.…”

Characteristics of Shale Wettability by Contact Angle and Its Influencing Factors: A Case Study in Songliao

“…Nowadays, shale wettability is characterized by five main methods viz. contact angle method (Siddiqui et al, 2018) (CA), spontaneous imbibition (Li et al, 2019) (SI), zeta potential method (Hoxha et al, 2016), nuclear magnetic resonance (NMR) (Chen et al, 2006), atomic force microscopy (AFM) (Kumar et al, 2005), and molecular dynamics simulation (MD) (Xue et al, 2021). The CA method is convenient, with a test range from strong water-wet to strong oil-wet, and has clear mechanical and thermodynamic significance, satisfying qualitative and quantitative studies and suitable for mechanistic studies.…”