Interactions of Organic Molecules with Calcite Surfaces: A Simulation and Experimental Study

Shale gas, as a clean unconventional hydrocarbon resource, faces significant challenges in safe and efficient development due to wellbore instability. Traditional drilling fluid strategies primarily rely on density control, inhibition, and plugging but are insufficient to prevent shale structural degradation caused by fluid invasion. Calcite, an important brittle mineral in shale, is highly susceptible to dissolution and structural damage. Its role in wellbore stability thus warrants an in-depth investigation. In this study, calcite in shale was selected as the research focus, and a combination of molecular simulation and experimental approaches was employed to systematically screen the interfacial interactions of representative organic functional groups with calcite to elucidate the underlying mechanisms. The results show significant differences in interfacial interaction strength among functional groups, with adsorption strength following the order: COO⁻ > HCONHOH > HCOCH₂OH > CH₃NH₂ > C₃H₆O ≈ HCHO > C₂H₆O, indicating that carboxyl groups have the strongest binding ability on the calcite surface. Oleate sodium (OAS), containing carboxyl groups, was then selected as a model compound for the experimental validation. Fourier-transform infrared spectroscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy analyses confirmed the successful incorporation of the OAS into shale and its chemical bonding with the calcite surface. Mechanical tests demonstrated that the OAS treatment significantly enhanced shale strength, increased the surface contact angle from 21.7 to 94.0°, and markedly increased surface hydrophobicity, effectively mitigating fluid-induced weakening. This study systematically reveals the interfacial chemical interaction mechanism between carboxyl functional groups and calcareous minerals in shale and proposes a chemical consolidation design strategy based on functional group-mineral interactions, providing new theoretical guidance for wellbore stabilization in shale formations.