The thermodynamic response behavior and mechanism of oil-bearing coal under ultrasonic excitation

In order to study the thermodynamic response behavior and its influence mechanism of oil-bearing coal under ultrasonic excitation, a self-designed experimental platform is constructed to monitor the thermodynamic response and temperature rise of ultrasonically excited coal, and oil-bearing coal is studied under an ultrasonic power of 900 W and a frequency of 28 kHz to determine the law of temperature rise of ultrasonically excited oil-bearing coal. At the same time, in combination with nuclear magnetic resonance (NMR), Fourier infrared spectroscopy (FTIR) and other experiments, the relationship between the thermodynamic response behaviour of ultrasound and the porosity structure of oil-bearing, the crude oil distribution and the functional groups of oil-bearing coal is explored, and the underlying mechanism of action by which ultrasonic excitation affects the thermodynamic response behaviour of oil-bearing coal is elucidated. Under ultrasonic excitation, the heating rate exhibits a three-phase evolution (rapid, slow, and decelerated heating), with crude oil content and distribution governing the spatial temperature profile on coal surfaces, where higher oil content expands the relatively high-temperature zone, peaking at a 118.3 % increase for 10 % oil-bearing coal. The rule for the temperature rise of the various liquid media is: crude oil > oil–water mixture > water. During ultrasonic excitation, the thermal damage caused by the thermodynamic response of coal is mainly reflected in the change of the pore structure of coal and the change of the content of functional groups in coal and crude oil. After ultrasonic excitation, the cumulative porosity of coal with different oil content first increases and then decreases. The degree of thermal damage of the cumulative porosity of 5 % oil-bearing coal is the highest with an increase of 24.53 % due to the indirect thermal effect of the collapse of cavitation bubbles. However, as the temperature increases, the viscosity of the crude oil decreases and the bound oil turns into mobile oil. Although the oil content of the coal continues to increase, the expansion effect of a single pore decreases. At the same time, the thermodynamic response of oil-bearing coal to ultrasonic excitation leads to a decrease in the content of oxygen-containing functional groups, hydroxyl groups and aliphatic hydrocarbons. In addition, the effect of the thermodynamic response by ultrasound on oxygen-containing functional groups is less pronounced compared to hydroxyl groups. These results improve the theoretical and technological basis for ultrasonic-assisted fracturing, permeability enhancement and efficient gas recovery in coexisting coal-oil–gas reservoirs.