Abstract

Density is an important coal property that determines the potential of gas resources in CBM reservoir. This paper aims to investigate coal density and structure variation during primary CBM and CO2-ECBM experiments. A coal core sample from Alberta Mannville formation with the rank of SubB was used to conduct the core flooding experiments covering the stages of inert gas flow, methane production, methane displacement by CO2 and inert gas flow after CO2 desorption. The x-ray CT experiments were carried out parallel to the core flooding experiment to provide x-ray images of coal core saturated with different gases at different stress conditions. The x-ray techniques were used for visualization and mapping of larger fractures and mineral streaks, as well as identification of flow paths. The coal density and density distribution changed with the gas adsorptive capacity and the stress condition were obtained.

The results show that net stress, gas adsorption capacity, and the production history are all key factors affecting coal core structure, leading coal density and density distribution variations. Hence, the core flow path, which contributes to the coal permeability, changes with those factors during CBM/ECBM processes. The results from this study provide laboratory coal characterization techniques using x-ray imaging analysis.

Introduction

Coalbed methane (CBM) is a new energy source and has the potential to contribute a significant portion of Canadian natural gas production. Coal reservoir characterization is one of the important steps to successfully develop CBM reservoirs. Coal seams are heterogeneous in terms of lithotypes and morphologies. This creates a challenge in understanding the subsurface behavior of the coal and injected gas during primary and enhanced gas recovery processes. As an organic rock, coal structure is easily deformed by the net stress imposed on it. Besides, adsorption of gases causes the coal matrix to swell and desorption of gases causes the coal matrix to shrink(1, 2). Therefore, coal experiences many changes in stress conditions uring the production life of the CBM reservoir. Because the network of natural fractures and cleats in a coal determines to a large extent the mechanical properties of the coal. Coal is very soft and has low elastic modulus compared to the rock formation. The stress and time dependent deformation of the coal porous structure is expected to change the behavior of the most important properties of the coal, such as porosity and permeability, which in turn change the reservoir production profiles. Coal physical properties such as density are therefore dynamically changed with the coal structure deformation.

Coal bulk densities are highly useful for determination of the ash content of the coal. Because of the excellent correlation between core ash and gas content and the excellent correlation between core ash and open-hole bulk density data, it is possible to accurately estimate the gas content from the high resolution bulk density log data(3). Coal bulk density is also used in the estimation of the total gas content in a given drainage area(4).

This content is only available via PDF.
You can access this article if you purchase or spend a download.