Raman Spectroscopy Study on Ternary Model Coal Mine Methane Hydrates

[Image: see text] The microfeatures of coal mine methane (CMM) hydrates, synthesized with three gas samples (CH(4)/C(2)H(6)/N(2), G1 = 43 : 47 : 10, G2 = 60 : 30 : 10, and G3 = 74 : 16 : 10) in a self-made transparent high-pressure cell at 275.15 K and 5 MPa were investigated using Raman spectroscopy. As a discriminator, the vibrational band frequencies in the C–C regions of the recorded hydrate Raman spectra for C(2)H(6) show that G1∼G3 hydrates are structure I. The three principal parameters used to study the microfeatures of the model CMM hydrates, including cavity occupancies, hydrate guest compositions, and hydration numbers, were calculated. The large cavity occupancies for C(2)H(6) constantly decrease from 85.12 to 79.32%, while the small cavity occupancies for CH(4) have a continuous increase from 73.75 to 96.42%. However, CH(4) competes with C(2)H(6) on entering the large cavities for their large cavity occupancies of 12.79–17.31%. The cavity occupancies of N(2) are less than 1.2%. The hydrate composition calculations show that the molar fractions of C(2)H(6) are the maximum. The hydration numbers range from 6.221 to 6.00. Based on the hydrate guest compositions and hydration numbers, the molecular formulas of the three CMM hydrates are presented.