Investigating the Effect of Pore Proximity on Phase Behavior and Fluid Properties in Shale Formations
Authors: Jin Luchao , Ma Yixin , Jamili Ahmad 
Published: June 2014

Phase behavior and fluid properties are governed by molecule-molecule and molecule-pore wall interactions. The effect of molecule-pore wall interactions is negligible in conventional reservoirs because pore sizes are much larger than molecular mean free paths. However, this effect is very important in shale formations because the pore sizes in shale formations are in the order of nano-scale. Reservoir fluids properties and phase behavior under nanoscale confinement exhibit significant deviations from their bulk properties.

This work used two methods to investigate the effect of pore proximity on the phase behavior and fluid properties: Developing a new flash calculations algorithm: the influence of difference between oil and gas pressures (i.e. capillary pressure) is neglected in the flash calculations of vapor-liquid equilibrium for conventional reservoirs. However the capillary pressure is very high and cannot be ignored in phase behavior calculations of shale formations. A new mathematical expressions for chemical potentials of pure components and their mixture as a function of capillary pressure is proposed, andModifying the critical properties of pure components under the effect of confinement: new correlations based on molecular simulation studies are developed for taking into account the effect of pore size (i.e. molecule-pore wall interactions) on critical properties of each component. The modified critical properties are used in phase behavior calculations.

Both methods were tested against experimental data of Sigmund et al. (1973) for C1- nC5 mixtures. The relative errors between the prediction of bubble point pressures and gas compositions from the models and the reported experimental data were less than 5 %. Then phase behavior and fluid properties of a mixture of Methane, n-Butane and n-Octane with different compositions were studied under confinement for pore size range from infinity to 2 nm by using both methods. In general, the two-phase envelope shrinked slightly in method 1 and significantly in method 2 with decreasing the pore size. The effect of pore size on two-phase envelope becomes significant when pore radius is smaller than 10 nm. For method 1, critical point does not change and the closer of the temperature to the critical point, the smaller the change in saturation pressures. For method 2, the critical point decreases with the pore radius. Interfacial tension for bulk fluid and confined fluid remain about the same for pore sizes more than 50nm. For pore sizes less than 50nm, interfacial tension from method 1 did not change significantly, but it decreased dramatically especially for pore sizes less than 10nm when method 2 was used. K-values from both methods were almost the same for pore sizes more than 10nm. From method 1, k-value decreases with decreasing the pore radius for all components. But from method 2, it decreases for light component and increases for intermediate and heavy components.

The results of this study can have a significant impact on our understanding of the gas condensation and transport in shale formations thereby enabling improved field planning, well placement, completions design and facilities management.