Abstract
Understanding surface properties such as wettability and CH4/shale interfacial tension (IFT) is essential to decipher CH4 productivity and hydrocarbon-in-place estimates. However, these factors are poorly understood due to lack of data despite their importance. We thus measured the equilibrium contact angle (θe), reflecting the degree of wetting for shale/CH4/water systems, and determined the governing interfacial tensions. In particular, water/CH4, water/shale, and CH4/shale IFTs are determined at pressures 0.1, 5, 10, 15, and 20 MPa and temperatures 303.15, 323.15, 343.15, and 373.15 K. Shale samples from Eagle Ford and Mancos were used to examine the effect of treating the shale samples with cetyltrimethylammonium bromide (CTAB, a cationic surfactant) on contact angle and IFTs. Finally, excess CH4 adsorption on the shale surfaces was determined before and after adding CTAB surfactant. The results showed that shale/CH4/water equilibrium contact angles (θe) increased with pressure but decreased with temperature. The Eagle Ford sample exhibited a contact angle of 138°. In comparison, the Mancos shale sample exhibited 97°, at both 20 MPa and 373.15 K. These contact angles suggest that Eagle Ford shale is more CH4-wet than Mancos shale due to the higher TOC of Eagle Ford shale (4.95 wt %, while Mancos shale had a TOC = 1.02 wt %). Water/CH4 IFT decreased from 71.47 to 56.02 mN/m as pressure and temperature increased from 0.1 MPa and 303.15 K to 20 MPa and 373.15 K. Shale/CH4 IFT also decreased with increasing pressure and temperature. Finally, CTAB surfactant treatment increased contact angles but reduced water/CH4, water/shale, and CH4/shale IFT. CTAB also reduced the amount of adsorbed CH4 and increased the amount of free CH4, which implies improved CH4 recovery from shale reservoirs. These results provide new insights into the first-principles that govern CH4 recovery from shale reservoirs.