Modeling the density profiles and adsorption of pure and mixture hydrocarbons in shales

Abstract

The production from shale resources in the US has shifted from the gas window to the condensate and oil windows recently due to the low natural gas price. Liquid-rich shales, such as Barnett, Woodford and Eagle Ford Shales, etc., are brought more attentions than ever before. Therefore, it is critical to understand the fluid phase behavior and their impacts on production in the condensate systems.

Fluid phase behavior in porous media is governed by not only fluid molecule–fluid molecule interactions but also fluid molecule-porous media wall interactions. In the shale formations, a large amount of hydrocarbons are stored within the organic matters where the pore sizes are in the order of nanometers. Inside these nanopores, the interactions between the fluid molecules and porous walls play such an important role that can change the fluid properties of the stored fluids. Our work focused on the predictions of fluid density distributions of both dry gas and liquid rich systems inside nanoporous media. Simplified Local-Density theory coupled with Modified Peng–Robinson Equation of State was used to predict the density profiles of pure and mixture hydrocarbons in confined pores. Adsorption isotherms were generated based on the density profiles calculated. The adsorption isotherms of pure methane and the methane/ethane binary mixture were calculated and compared to experimental data and molecular simulation results in the literature with excellent accuracy.

Our results showed that due to the fluid-wall interactions, the fluid density is not uniformly distributed across the pore width. In general, the fluid density is higher near the porous media wall than that in the center of the pore. It also showed the fluid density profiles are temperature, pressure, pore size and fluid composition dependent. In general, the adsorbed amount increased by increasing pressure and decreased by increasing temperature. The pore size range of interest is from 2 nm to 20 nm. In order to present the condensate system, a binary mixture of 80% methane and 20% n-butane was used. It was found that fluid composition for the fluid mixture was not uniformly distributed across the pore. Heavier component (n-butane) tended to accumulate near the wall while lighter component (methane) would like to stay in the center region of the pore. For the methane/ethane binary mixture, the composition of methane in confined space was found much smaller than the bulk methane composition. For example, the methane composition in confined silicalite is 7% when the bulk methane composition is 50% at 355 kPa and 300 K.