1. Relationship Between SMAS Inhibition Mechanism and Classical DLVO Theory
The DLVO theory (Derjaguin-Landau-Verwey-Overbeek) explains colloidal stability through the balance of van der Waals attraction (vdW) and electrostatic double-layer repulsion (EDL). For SMAS (Sodium Methallyl Sulfonate) in nano-porous shale, its inhibition mechanism partially follows DLVO theory, but modifications are required due to the unique shale environment:
- Enhanced Electrostatic Repulsion:
The sulfonate group (–SO₃⁻) of SMAS forms a strongly negatively charged layer on shale surfaces, increasing the double-layer thickness (κ⁻¹) and inhibiting clay particle aggregation. - Modulation of van der Waals Forces:
In nano-pores, vdW forces are amplified due to confinement effects, but SMAS adsorption can partially shield clay-clay vdW attraction.
Limitations:
DLVO theory assumes an equilibrium state, whereas shale-fluid systems often operate under non-equilibrium conditions (e.g., dynamic fluid shear, temperature/pressure gradients), requiring additional force mechanisms.
2. Non-Equilibrium Effects to Consider
In shale nano-pores, the following non-equilibrium effects significantly influence SMAS inhibition:
(1) Hydrophobic Forces (Non-DLVO Forces)
- Shale organic matter (e.g., kerogen) surfaces are hydrophobic. SMAS sulfonate groups may alter water molecule arrangement via polar-nonpolar interface reorganization, generating additional hydrophobic attraction or repulsion.
- Experimental Evidence: Atomic force microscopy (AFM) reveals spontaneous cavitation at hydrophobic surfaces under nanoscale confinement, affecting SMAS adsorption kinetics.
(2) Ion Bridging and Specific Adsorption
- Multivalent ions (Ca²⁺, Mg²⁺) in shale pore water may form ion bridges between SMAS and clays, causing localized aggregation (unpredictable by DLVO).
- Dynamic Adsorption-Desorption: Non-equilibrium adsorption under flow conditions may lead to transient charge reversal (e.g., Ca²⁺ competitive adsorption).
(3) Nano-Confinement Effects
- In nano-pores (<50 nm):
- Double-Layer Overlap: When pore size approaches the Debye length (κ⁻¹), EDL repulsion nonlinearly intensifies.
- Steric Hindrance: Conformational entropy loss of SMAS molecular chains in confined spaces reduces inhibition efficiency.
(4) Dynamic Shear and Mass Transfer Effects
- Fracturing Fluid Flow: Shear forces may strip SMAS adsorption layers, necessitating analysis via Reynolds lubrication theory for film stability.
- Non-Steady-State Diffusion: SMAS transport in shale microfractures follows fractional diffusion models (non-Fickian).
(5) Phase Transitions and Interfacial Energy Perturbations
- Oil-water-gas three-phase coexistence in shale pores may cause SMAS enrichment at interfaces, altering local wettability (contact angle hysteresis).
- Temperature/Pressure Fluctuations: At high temperatures (>80°C), SMAS sulfonate groups may protonate (–SO₃H), weakening electrostatic repulsion.
3. Integrated Model and Future Directions
To accurately describe SMAS behavior in shale, an extended DLVO + non-equilibrium model should incorporate:
- Modified EDL equations (accounting for nano-pore geometry).
- Hydrophobic interaction terms (based on augmented Young-Laplace equations).
- Dynamic adsorption-shear coupling models (validated by surface force apparatus (SFA) data).
Experimental Validation Recommendations:
- In Situ AFM/QCM-D: Real-time monitoring of SMAS adsorption kinetics on shale surfaces.
- Molecular Dynamics (MD) Simulations: Quantify SMAS conformational changes under nano-confinement.
Conclusion
SMAS inhibition in nano-porous shale partially aligns with DLVO theory but requires integration of hydrophobic forces, ion bridging, nano-confinement, and dynamic shear effects. Future research should focus on multi-physics coupling models to optimize SMAS applications in unconventional oil/gas development.
