Equilibrium dihedral angles in the system quartz-CO 2sbnd H 2O sbnd NaCl at 800°C and 1-15 kbar: the effects of pressure and fluid composition on the permeability of quartzites

An experimental study of fluid-solid-solid dihedral angles in the system quartz-H ₂O sbnd CO ₂sbnd NaCl at 800°C and a range of pressures from 1 to 15 kbar confirms the presence of a marked pressure control on dihedral angles in this system. At 1.5 kbar, the dihedral angle increases from about 77° in a pure water system to 98° in CO ₂-rich fluids ( X_{CO 2} of about 0.96). For pure water the dihedral angle increases from 77° at 1 kbar to 84° at 6 kbar, but decreases above 6 kbar at about 7° /kbar until angles below 60° are reached at pressures between 9 and 10 kbar. Similar behaviour is observed for brines but the rates of change with pressure are more marked. For fluids close to pure CO ₂ ( X_{CO 2} > 0.96) the dihedral angle remains constant at 98° in the pressure range investigated. The dihedral angle of intermediate fluids ( X_{CO 2} = 0.5) remains constant within error at about 92° until 6 kbar. At this point the dihedral angle decreases with increasing pressure at about 7° /kbar. The quartz-argon dihedral angle was measured at 4 kbar and found to be 96 ± 2°. It is suggested that this angle represents textural equilibrium between the quartz grain boundary and a "clean" quartz surface. Comparison of the argon angle with that obtained for CO ₂, coupled with the Gibbs adsorption equation, shows that CO ₂ adsorption on quartz is negligible, and that both H ₂O and NaCl adsorb positively. From a consideration of the rate of change of dihedral angle with pressure, using the approach of Passerone and Sangiorgi, it is shown that both the interfacial energy and the grain boundary energy in aqueous fluid-bearing systems decrease with pressure until 6 kbar and increase again with further pressure increase. Use of the Gibbs adsorption equation leads to the suggestion that such changes may be due primarily to the concentration and spacing of the adsorbed H ₂O molecules on the quartz surface compared to the bulk fluid. The results also suggest that the grain boundaries of the quartz contain adsorbed H ₂O or NaCl.