Effect of Pore Size Distribution and Connectivity on Phase Behavior and Gas Condensate Production from Unconventional Resources

Abstract

Transport properties and mechanisms as well as phase behavior under nanoscale confinement exhibit significant 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, production from shale gas reservoirs is strongly influenced by pore sizes and their effects on phase behavior and transport properties. In this study, we focus on analyzing and determining the effect of phase behavior and transport properties change due to pore proximity on production from a shale gas condensate reservoir. Additionally, the effect of different connectivities between pore sizes on production is studied. The effect of pore size on phase behavior is considered by using modified critical properties for different pore sizes in the phase behavior calculations. A shale gas condensate reservoir with a ternary mixture of methane (80 mol%), n-butane (10 mol%), and n-octane (10 mol%) as the reservoir fluid is modeled. The reservoir pressure and temperature are 5000 psia and 180 °F, respectively. The dew point pressure is 3600 psia. Pore sizes change between 5-150nm. Based on Scanning Electron Microscopy (SEM) studies on shale reservoir rocks, the pore volume of the reservoir was divided into five regions: bulk (stimulated area and pore sizes more than 50nm (17% PV)), 20-50nm (4% of PV), 15-20nm (6% of PV), 10-15nm (45% of PV), and less than 10nm (28% of PV). Three different types of connectivities between pores were considered: 1-completely random distribution 2-pore sizes from smallest to largest connected to the SRV in series, and 3-pore sizes from largest to smallest connected to the SRV in series. Our study has shown that by decreasing the pore size, dew point pressures decrease between 5 to 17%. Also by decreasing pore size, two-phase region shrinks therefore condensate drop-out and near wellbore permeability impairment are reduced. After 10 years of production, condensate saturation around SRV is 6-10% less under confinement effects. Gas and condensate viscosities under confinement decrease 3-16% and 10-45% respectively. Considering effect of confinement did not affect gas production significantly but the liquid production increased significantly and doubled. The effect of different pore size connectivities caused a 20% change in liquid productions. The results of this study can have a significant impact on our understanding of gas condensation and transport in shale formations thereby enabling improved field planning, well placement, completions design and facilities management.

Flow Units in Shale Condensate Reservoirs

“…Slot porosity has been reported in tight (Rahmanian et al, 2010) as well as shale (Sonnenberg, 2011) reservoirs. Singh et al's (2009) results have been used by Devegowda et al (2012), Jin et al (2013) and Sanaei et al (2014a) to quantify the effect of pore diameter on gas condensate properties. This requires the transformation from a slit to a circle.…”

“…This observation has been used by various authors (Devegowda et al, 2012;Jin et al, 2013;Sanaei et al, 2014a;Sanaei et al, 2014b) to develop empirical equations for determining the shift in critical temperature and pressure as function of pore size with a view to investigate the effect of two-phase envelope shrinkage on gas and liquid recoveries. In this work a method that uses a variable shape distribution (VSD) equation (Aguilera, 2006) is used taking into account all critical temperature shift data available in the literature at this time.…”

“…The data was published originally by Vishnyakov et al (2001) and Singh et al (2009Singh et al ( , 2011, and has been used by Jin et al (2013), Sanaei et al (2014a), and in their studies of shale reservoirs. The raw data and correlations by Jin et al (2013) and are shown in Figure 8.…”

“…The critical pressure shift is calculated with the use of an equation adapted from Sanaei et al (2014a) and Sanaei et al (2014b): (9) where ⌬P c is the critical pressure shift, P cb is the critical pressure of bulk state (psia), P cp is the pore-size dependent critical pressure (psia), and is the effective molecular diameter in nm. Figure 10 shows the flow-units graph as a function of , k and r p35 including two-phase envelopes for different values of r p35 .…”

Numerical Study of Production Mechanisms in Unconventional Liquid-Rich Shale Volatile Oil Reservoirs

“…It is the major cause of unusual fluid flow and production mechanisms in LRS oil wells. Many studies have been conducted in recent years to understand geology, rock and fluid characterization and production mechanisms of these resources (Orangi et al, 2011;Honarpour et al, 2012;Whitson, 2012;Sanaei et al, 2014;. PVT property suppression, severe reduction of permeability due to compaction, and enhancement of critical gas saturation are the most important impacts of nano pores on thermodynamic and fluid phase behavior, formation rock characterization and coupled rock and fluid properties of LRS oil reservoirs (Honarpour et al 2012;Orangi et al 2014;.…”