An NMR Experimental Study and Model Validation on Capillary Condensation of Hydrocarbons Confined in Shale Gas Condensate Reservoirs

Abstract

Unlike conventional gas reservoirs, shale gas reservoirs contain organic mesopores that have pore sizes ranging from 2 to 50 nm. These organic pores may cause capillary condensation of confined hydrocarbons due to the non-negligible capillary pressure. A novel phase equilibrium model has been developed to quantify effects of pore size distribution on the phase behavior of confined hydrocarbons, including the occurrence of capillary condensation. However, it remains a challenge to assess the phase behavior of confined hydrocarbons by laboratory experiments. This is because the conventional pressure-volume-temperature (PVT) method measures the phase behavior of a bulk fluid. Here, we employ low- and high-field nuclear magnetic resonance (NMR) techniques to experimentally probe the capillary effect on phase behavior using retrograde condensates in synthetic porous media and shale rock samples. In low-field NMR experiments, water-wet porous glass and oil-wet polymer-based spherical activated carbon (PBSAC) beads are used as porous media. NMR relaxation times are used to observe the occurrence of capillary condensation for pure and mixed hydrocarbons at room temperature under controlled pressure. High-field NMR is employed to gain further sensitivity and resolution for the phase behavior of a confined methane-butane mixture. NMR spectroscopic signatures of the dew point were identified, enabling the comparison of dew-point pressures of the bulk hydrocarbons and hydrocarbons confined in grinded shale rock. NMR-measured dew point of confined hydrocarbons is ~115 psi higher than that of bulk phase. This pressure shift agrees well with simulation results. In summary, we present NMR experimental studies and model validation on the capillary condensation effect, showing a shift of dew-point pressures of confined hydrocarbons mixtures in porous media. The agreement between NMR and simulation results validates the novel phase equilibrium model implemented in the newly developed PVT simulation software. The lab measurements and model validation results show that a) oil-wet is one key condition for the occurrence of capillary condensation of confined hydrocarbons; b) the shift of an upper dew-point pressure of hydrocarbons confined in shale rock can be tens of psi to slightly over 100 psi for the retrograde condensate system being used; c) the phase equilibrium model is valid for modeling phase behavior of multi-component hydrocarbons confined in mesopores.