Changes in Pore Structure of Coal Associated with Sc-CO2 Extraction during CO2-ECBM

Run Chen,Pengfei Zhang,Linlin Wang,Youyang Wang,Yong Qin,Chongtao Wei

doi:10.3390/app7090931

Run Chen, Pengfei Zhang + Show 4 more

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https://doi.org/10.3390/app7090931

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Journal: Applied sciences	Publication Date: Sep 10, 2017
Citations: 27	License type: CC BY 4.0

Affiliation: China University of Mining and Technology

Abstract

Supercritical CO2 (Sc-CO2), a supercritical solvent, can extract small organic molecules (fluid) from coal, changing pore structures to affect gases storage and migration in the coal matrix. Five undeformed coals before and after the second coalification jump were collected to simulate Sc-CO2 extraction performed with supercritical extraction equipment. Pore structures of the samples before and after Sc-CO2 extraction were characterized using mercury porosimetry. The results show that there are significant changes in pore size distribution of samples. ΔVMa and ΔVMe of coal samples are positive, ΔVTr and ΔVMi are positive for most coals, and ΔVMi of higher coals are negative; the ΔSMa and ΔSMe are positive with small values, the ΔSTr and ΔSMi are positive and negative before and after the second coalification jump; thus, the pore connectivity is improved. These results indicate that Sc-CO2 extraction not only increases the numbers of micropores, but also enlarges the pore diameter size; these changes in the pore structure are influenced by the second coalification. The changes in the pore structure by Sc-CO2 extraction provide more spaces for gas storage and may improve the pore throats for gas migration.

Highlights

The burning of fossil fuel is a primary reason for the increase of greenhouse gas CO2 concentration in the atmosphere and the exacerbation of global warming
In this work, we focus on changes in the pore structure and the connectivity of five undeformed coals before and after the second coalification jump when they are exposed to Supercritical CO2 (Sc-CO2) environments that are typical conditions of CO2 -ECBM
Experiments in this study show that small organic matters filling in pores can be dissolved and flushed away by Sc-CO2 fluid, which directly results in changes in the pore structure of coal

Summary

Introduction

The burning of fossil fuel is a primary reason for the increase of greenhouse gas CO2 concentration in the atmosphere and the exacerbation of global warming. CO2 Enhanced Coalbed Methane recovery (CO2 -ECBM) is a technique that combines CO2 sequestration and enhanced coalbed methane recovery. The implementation of this technique and its commercial value have been studied via different projects over the past 20 years [5,6]. According to the results of previous studies [7,8], coal structure is characterized as a dual pore system that includes the primary porosity consisting of microspores, transitional pores, and mesopores in the coal matrix, and the second porosity composed of non-uniformly distributed macropores and microfractures. Previous studies have shown that there are various physical and geochemical interactions between CO2 and coals, such as gas adsorption on the coal

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