Phase Behavior in Shale Organic and Inorganic Nanopores From Molecular Simulation

Production from shale reservoir is getting more attention from the oil industry. However, the shale is not understood as well as conventional reservoirs. One complexity is the unclear fluid phase behavior in shale nanopores. In previous work, the shale rock is usually built as a simple model without consideration of the material effect on phase behavior. To have a better understanding of phase diagrams in different shale environment, three materials (two inorganic minerals and one kerogen) are used in this work to generate two types of nanopores (slit and cylinder), which are adopted in the phase behavior simulations of pure fluids (methane and propane) and one ternary fluid (C₁/C₃/nC₅). From the pure fluid tests in pores of three diameters (4, 7, 10 nm), confined liquid densities are decreased while vapor densities are increased in slit and cylinder pores. Critical points are shifted to lower densities and higher temperatures. Group tests are conducted for the ternary fluid in slit and cylinder pores at one temperature (160 °F). Under confinement, there is a big shift in the nC₅ composition of the vapor phase in the ternary diagrams while only small changes have been observed in the liquid composition. 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 pore types. Phase separation is computed in slit pore tests while only one phase is formed in tests of cylinder pores. Since the cylinder pore has more adsorption surface area, compared with the slit pore, a stronger adsorption effect is introduced in all tests of cylinder pores. Based on the comparison of all results (pure fluids and ternary case) from three materials, the calcite pores provide stronger confinement effect on fluids and the other two materials have the similar confinement effect on phase diagrams.