Summary
Production from shale reservoirs is receiving more attention from the petroleum industry. However, there are many complexities associated with optimal production of shale reservoirs. One complexity is the unclear fluid-phase behavior in shale nanopores. In the shale nanopores, the interaction between the pore wall and fluid molecules can significantly change the fluid-phase behavior from the bulk (unconfined) condition. Therefore, the type of pore material can influence the fluid-phase behavior. There have been several recent efforts to model the confined fluid-phase behavior in shale reservoirs. However, the effect of pore material on fluid-phase behavior in shale reservoirs has not been addressed in the previous studies. In this paper, to have a better understanding of phase diagrams in different shale environments, we perform molecular simulations with three materials (two inorganic minerals and one kerogen) and two types of nanopores (slit and cylinder) to model the confined-phase behavior of pure fluids (methane and propane) and one ternary fluid (C1/C3/nC5). From the pure-fluid simulations in the pores of three diameters (4, 7, and 10 nm), confined liquid densities are decreased, whereas vapor densities are increased in the slit and cylinder pores. When pore diameters are increased, critical points are shifted to low densities and high temperatures. Critical temperature and density under confinement can deviate as high as 15 and 60%, respectively, from the original values in bulk. Molecular simulations are conducted for the ternary fluid in the slit and cylinder pores at 160°F. Under confinement, there is a large shift in the nC5 composition of the vapor phase in the ternary diagrams, whereas only small changes have been observed in the ternary diagrams of liquid phase. The density difference between two phases is reduced. Additional tests are performed at one typical shale temperature (290°F) for this ternary fluid in both nanopore types. Phase separation is observed in the slit-pore tests, although only one phase is formed in the tests of the cylinder pores. Because the cylinder pore has more adsorption surface area compared with the slit pore, a stronger adsorption effect is introduced in all tests of the cylinder pores. Based on the comparison of all results (pure fluids and ternary fluid) from the three materials, the calcite pores provide a stronger confinement effect on fluids, and the other two materials have a similar confinement effect on the phase diagrams.
