Coupling Effect of Temperature, Gas, and Viscoelastic Surfactant Fracturing Fluid on the Chemical Structure of Deep Coal: An Experimental Study

The chemical structure of coal has an important influence on coalbed methane mining. Through Fourier transform infrared (FTIR) and Raman analysis of coal samples after coupling treatment of temperature, gas, and viscoelastic surfactant fracturing fluid (VES), we studied the evolution law of coal chemical structure. It was determined that the functional groups of coal decreased by approximately 30% with increasing temperature, showing an obvious temperature sensitivity. The VES fracturing fluids can destroy the functional groups of coal, and high temperature and high gas pressure conditions can seriously weaken the degree of destruction of the functional groups of coal by VES fracturing fluids but significantly enhance the destruction of the functional groups of coal by deionized water. Temperature, deionized water, and the use of VES fracturing fluid under a low temperature barely affect the macromolecular structure of coal, but when the temperature increases above 323.15 K, the coupling effect of temperature and VES fracturing fluid causes the position of the Raman peaks D1 and G for coal to move toward higher frequencies, the peak position difference to move toward lower frequencies, and the range of motion to be 10–18 cm–1. That is, the coupling effect of high temperature and VES fracturing fluid can make the macromolecular structure of coal become more ordered.