Influences of Different Types and Concentrations of Additives in Fracturing Fluids on the Wettability of Coal: Simulations and Mechanisms

Influences of different types and concentrations of additives in fracturing fluids on the wettability of coal were investigated through molecular simulations and laboratory experiments. Taking coal samples collected from Pingdingshan Shoushan No. 1 mine as research objects, two types of composite models for the wettability (coal + different types of additives (viscosifier polyacrylamide (PAM), surfactant sodium dodecyl benzenesulfonate (SDBS), clay stabilizer potassium chloride (KCl), and a combination of gel breakers ammonium persulfate (APS) and hydrogen peroxide (H2O2)) and coal + different concentrations of additives) were established. The mechanisms underpinning the influence of the types and concentrations of additives on the wettability of coal were revealed from the microscopic perspective. Results show that these additives are listed (in descending order of their contact angle with coal) as follows: KCl, H2O, PAM, APS + H2O2, and SDBS. K+ and Cl- ions dissociated by KCl in water enhance the interaction with water molecules, causing the liquid surface to contract and the cohesion to increase, thus increasing the contact angle. Acylamino (-CONH2) in PAM is adsorbed on coal surfaces, which strengthens the interaction with coal molecules and reduces the hydrophobicity of coal surfaces, thus decreasing the contact angle. APS + H2O2 oxidize the nonpolar C-H bonds on coal surfaces into polar groups including -OH, which enhance the hydrophilicity; meanwhile, the unoxidized hydrophobic hydrocarbon structure is exposed, leading to reduced contact angle. The sulfonic acid group (-SO3Na) in SDBS is adsorbed on coal surfaces, which reduces the hydrophobicity and the contact angle of coal surfaces. As the number of water molecules increases, the additive concentration decreases, the interaction energies of KCl, APS + H2O2, and SDBS solutions with coal molecules decrease, and the contact angle increases; the viscosity of PAM solution decreases, the interaction energy with coal molecules increases, and the contact angle decreases.