Influence of Coal Shrinkage and Swelling on Coalbed-Methane Production and CO2 Sequestration
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 102767, "Influence of Shrinkage and Swelling of Coal on Production of Coalbed Methane and Sequestration of Carbon Dioxide," by H.J. Siriwardane and D.H. Smith, Natl. Energy Technology Laboratory/DOE; F. Gorucu, SPE, Advanced Resources International; and T. Ertekin, SPE, Pennsylvania State University, prepared for the 2006 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 24–27 September.
The potential for enhanced methane (CH4) production and geologic sequestration of carbon dioxide (CO2) in unmineable coal seams needs to be evaluated before large-scale sequestration projects are undertaken. Coal is known to competitively adsorb CO2 and CH4 gases. Laboratory experiments indicate that coal swells when a gas is adsorbed and shrinks when a gas is desorbed. The swelling and shrinkage may change the coal-cleat porosity and permeability. A 3D swelling and shrinkage model (SSM) was developed that is based on equations that account for coupled fluid-pressure/-deformation behavior of a porous medium that undergoes swelling and shrinkage.

Introduction
Greenhouse-gas emissions are increasing. Anthropogenic CO2 emissions can contribute to global warming, and attempts are being made to identify methods to reduce them. Geologic sequestration has been identified as one of the methods for CO2 storage. One of the promising methods is to store CO2 in unmineable coal seams. Coal seams can hold large amounts of CO2 compared to the amounts of CH4 gas that they contain. However, many technical and safety issues need to be investigated before such efforts can be undertaken.
Several previous studies have addressed important aspects of CO2 sequestration in coal seams. One of the potential problems of CO2 injection into a coal seam is swelling of the coal. While more laboratory-research work is needed to resolve the exact amount of coal swelling, it is suspected that coal swelling will cause a reduction in permeability, which in turn may reduce injection volumes during large-scale injection operations.
As part of a research and field-demonstration project of CO2 sequestration in deep unmineable coalbeds, a field study involving the injection of CO2 for enhanced coalbed-CH4 recovery was performed in the San Juan basin. The influence of coal swelling and shrinkage on reservoir performance of the reported field project was investigated by use of a new SSM. The full-length paper contains results of several reservoir simulations of the reported field study. Results from the present study are compared with previous studies published in the literature.

Methodology
A theoretical model for stress-dependent permeability caused by matrix shrinkage under uniaxial-strain conditions has been presented previously in the literature.
It is anticipated that when CO2 is injected into a coal seam while CH4 is produced from the same layer, the seam will undergo simultaneous swelling and shrinkage at different locations, depending on the gas composition and pressure at each location. There have been numerous research investigations on reservoir modeling of gas flow in coal seams. In many earlier investigations, the influence of swelling and shrinkage of the reservoir matrix on reservoir permeability was not considered.