Molecular dynamic simulation study on admissibility of green additives in hydraulic fracturing fluid

CONTEXT: Low interfacial tension (IFT) between fracturing fluids and reservoir fluids is essential for enhancing gas recovery and mitigating formation damage in shale gas systems. However, conventional surfactants used in hydraulic fracturing pose environmental and sustainability concerns, necessitating the development of greener alternatives. This study evaluates the potential of the bio-derived surfactant D-limonene in reducing methane-water interfacial tension under both ambient and high-pressure high-temperature (HPHT) conditions representative of shale reservoirs. A systematic relationship between IFT and solvation free energy (ΔGₛₒₗᵥ) was observed across all investigated conditions. Increasing temperature significantly reduced IFT, with reductions of up to ~ 80% under ambient conditions (18-90 °C), whereas the reduction was less pronounced under HPHT conditions (~ 7 mN/m). Pressure exhibited a comparatively minor influence, with IFT increasing by approximately 8% between 70 and 120 MPa. Increasing D-limonene concentration consistently lowered IFT, although higher concentrations were required under HPHT conditions to achieve comparable reductions. METHODS: All calculations were performed using classical molecular dynamics simulations. Molecular interactions among methane, water, and D-limonene were described using the Optimized Potentials for Liquid Simulations (OPLS) force field. Molecular topologies were generated using the Automated Topology Builder, and simulations were conducted using GROMACS®. Systems were equilibrated under constant number of particles, volume, and temperature (NVT) and constant number of particles, pressure, and temperature (NPT) ensembles. The solvation free energy (ΔGₛₒₗᵥ) was computed using thermodynamic integration (TI), where the ensemble-averaged energy derivatives ⟨∂V/∂λ⟩ were evaluated across 21 λ-coupling states and numerically integrated using the trapezoidal rule. Interfacial tension was subsequently determined via its thermodynamic correlation with ΔGₛₒₗᵥ.