The variations and mechanisms of coal wettability affected by fracturing fluids with different concentrations of Ca2+ during fracturing

Abstract

In low-porosity, low-permeability, and high-adsorption coalbed methane (CBM) reservoirs, the flow of fracturing fluid and methane gas is strongly affected by the wettability of coal. Ca2+ in mine water can affect the wettability of coal by changing the properties of the fracturing fluid, thus affecting the recovery efficiency of CBM. To investigate the mechanism of the wetting properties of bituminous coal by fracturing fluid with different concentrations of Ca2+, contact angle and T2 distribution curves were tested using an interfacial rheometer and low-field nuclear magnetic resonance (LF-NMR), and changes in water absorption and functional groups of coal soaked by fracturing fluid with different concentrations of Ca2+ were described using a precision electronic balance, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) to evaluate the wetting properties of coal. Molecular dynamics simulation was used to characterize the microscopic wetting properties of water molecules on the coal surface. Results show that when the concentration of Ca2+ is lower than 50 mg/L, the surface tension of the fracturing fluid gradually decreases, and the lowest value is 27.66 mN/m at 50 mg/L. The overall change in the contact angle and T2g value was lower than that of the control group (no Ca2+). The content of oxygen-containing functional groups on the surface of coal increased by 22.30%, and the content of C-O functional groups inside coal increased by 3.55%. The relative concentration and density of water molecules on the coal surface increase with increasing molecular diffusion ability. These results indicate that when the Ca2+ concentration is lower than 50 mg/L, the wettability of coal is enhanced, and thus, the fracturing fluid and methane gas flow capacity is improved. However, with increasing Ca2+ concentration, this phenomenon is reversed, and the water absorption of coal decreases, indicating that the wetting ability of coal decreases. The results of this study provide theoretical guidance that can be used to improve the recovery of CBM by fracturing and permeability enhancement measures, and preparing fracturing fluid in mine water to strengthen the protection of freshwater resources.