Transport properties and mechanisms as well as phase behavior under nanoscale confinement exhibit deviations from their bulk behavior. This is due to the significant effect of molecule-wall interactions as well as molecule-molecule interactions in shale formations which are mainly characterized by nanopores. Consequently, the critical temperatures and pressures and transport properties of hydrocarbon mixtures under nanopores confinement are influenced strongly by fluid molecule-pore wall interactions.
The effect of phase behavior on production from a shale gas condensate reservoir is studied where nanopores present either as the dominate storage region and when dispersed with pores with bulk behavior. The effect of pore size on phase behavior is considered by using modified critical properties for different pore sizes in the phase behavior calculations.
Permeability, porosity, and mercury injection capillary pressure (MICP) tests are run on several Eagle ford core plugs. Using experimental results, an equation for estimation of mean pore size as a function of permeability and porosity is presented. A shale gas condensate reservoir with an Eagle Ford gas condensate as the reservoir fluid is modeled. The reservoir pressure, dew point pressure and temperature are 5000 psia, 3800 psia and 180 °F, respectively. Pore size distribution of one of the samples is used in the reservoir modeling. Based on MICP experiments and pore-throat size distribution, the pore volume of the reservoir was divided into five regions: bulk (pore sizes more than 50nm (10% PV)), 20-50nm (12% of PV), 12-20nm (29% of PV), 7-12nm (39% of PV), and less than 7nm (10% of PV). For each region, a specific permeability was assigned using the new developed correlation. Three different types of connectives between pores were considered: 1-pore sizes from smallest to largest connected to the fracture in series, 2-pore sizes from largest to smallest connected to the fracture in series, and 3-completely random distribution.
Results showed that by decreasing the pore size, dew point pressures decrease between 5 to 24%, fluid tends to behave as a dry gas and the two-phase region shrinks therefore condensate drop-out and near wellbore permeability impairment is reduced. After 15 years of production, condensate saturation around fracture is up to 7% less under confinement effects. Gas and condensate viscosities under confinement decrease 3-16% and 10-50% respectively. Confinement did not affect gas production significantly but condensate production increased more than 30%. Production analysis showed that Permeability is overestimated by 30% if bulk PVT is used instead of modified PVT with pore size. Phase behavior effect has a positive contribution to production while considering permeability variation with pore size has a negative impact on production. Connectivity type between different pore sizes has a pronounced effect and determines which of these factors has more impact on production.