Kerogen Pore Size Distribution of Barnett Shale using DFT Analysis and Monte Carlo Simulations

Abstract

Abstract Porosity and permeability are fundamental properties of hydrocarbon reservoirs that are important to estimation of the original hydrocarbon in-place and the production rates. These petrophysical properties are often characterized by the pore size distribution. Scanning electron microscopy studies performed on organic-rich gas shale samples have provided two- and three-dimensional images of organic pores with maximum resolution of 4-5 nm. However these images also suggest that the some of the pores and the paths connecting the pores are even smaller. In this study a different set of experimental and computational procedures based on low-temperature gas adsorption measurements are used to provide information about the porous structure of kerogen. The experimental approach involved nitrogen adsorption on crushed Barnett shale samples with sizes less than 500 microns at 77°K. Following the measurements, BET, t-Plot and DFT analyses are carried out and statistical analyses were performed to determine the distribution and the effective pore size of each sample. Monte Carlo simulations are performed in the Gibbs ensemble to predict thermodynamic states of methane in model pores with the resulting effective sizes and to estimate the amounts of adsorbed and free gases under Barnett shale reservoir pressure and temperature. Analysis show that the average organic pore widths of the samples range between 5-7 nm. The amount of adsorbed gas in a pore of this size under reservoir pressure and temperature conditions constitutes about 25% of the total gas in-place. The work is important for reservoir characterization and engineering analysis of gas shales.