Thakur / Schatzel / Aminian Coal Bed Methane

From Prospect to Pipeline
1. Auflage 2014
ISBN: 978-0-12-801089-1
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 440 Seiten

Coal Bed Methane: From Prospect to Pipeline is the proceedings of the 25th anniversary of the North American Coal Bed Methane Forum. It provides the latest advancements in the production of coal bed methane covering a variety of topics, from exploration to gas processing, for commercial utilization. Additionally, it presents the origin of gas in coal, reservoir engineering, control of methane in coal mines, production techniques, water management, and gas processing. The vast coal resources in the United States continue to produce tremendous amounts of natural gas, contributing to a diverse range energy assets. Following a rapid advancement and subsequent plateau in technological developments, this book captures the full life cycle of a well and offers petroleum geologists and engineers a single source of a broad range of coal bed methane applications. This book addresses crucial technical topics, including exploration and evaluation of coal bed reservoirs; hydraulic fracturing of CBM wells; coal seam degasification; and production engineering and processing, among others. It also covers legal issues, permitting, and economic analysis of CBM projects. - Edited by a team of coal bed methane experts from industry, academia and government who have more than 75 years of combined experience in the field - Authored by well-recognized members of the gas and coal industry, universities, US government departments, such as the Department of Energy and the National Institute of Occupational Safety and Health (NIOSH) - More than 200 figures, photographs, and illustrations aid in the understanding of the fundamental concepts - Presents the full scope of improvements in US energy independence, coal mine safety, and greenhouse gas emissions

Autoren/Hrsg.

Thakur, Pramod

Schatzel, Steven J.

Aminian, Kashy

Leseproben

Chapter 2

The Origin of Coalbed Methane

LingGao1MariaMastalerz2ArndtSchimmelmann1     1Department of Geological Sciences, Indiana University, Bloomington, IN, USA     2Indiana Geological Survey, Indiana University, Bloomington, IN, USA

Abstract

Coalbed methane (CBM), is an economically important unconventional fossil energy resource attracting much scientific and industrial interest today. This review outlines the generation of CBM via biogenic, thermogenic, mixed, and possibly catalytic origins and summarizes compositional and isotopic characteristics that are used to distinguish between those different origins. We compare critical geological, geochemical, and hydrological factors controlling distribution and producibility of thermogenic and biogenic coalbed gases. This review often refers to examples of the most productive CBM basins in the US, namely the dominantly thermogenic gas-producing San Juan Basin and the biogenic gas-producing Powder River Basin.

Knowledge of the origin of CBM and its underlying geological controls in a sedimentary basin is critical for developing a successful and cost-effective CBM exploration strategy. Therefore, multidisciplinary analytical methods for characterizing the origin and distribution of coalbed gas are gaining importance in CBM exploration.

Keywords

Biogenic gasCatalytic gasCBMCoalbed methaneMethanogenesisThermogenic gas

Introduction

Coalbed methane (CBM) occurs as unconventional natural gas in coal seams. During the past 20years, CBM has emerged as an important energy resource in the United States (Figure 2.1) and presently accounts for about 9% of total US natural gas production. Because its combustion releases no toxins, produces no ash, and emits less carbon dioxide per unit of energy than combustion of coal, oil, or even wood (U.S. Environmental Protection Agency), it is expected that CBM will remain an environmentally friendly component in our energy portfolio over the next decades.

Based on the large volume of coal in the US, conservative estimates suggest that there are more than 700×1012cubic feet (i.e., 700Tcf or 19.8×1012m3) of coalbed gas in US coal seams (Rice, 1997), of which 100Tcf may be economically recoverable with today’s technology—roughly equivalent to 5year total natural gas use in the US at present consumption.

The recognition of coalbed gas as a largely untapped energy source in the 1980s (Rice and Claypool, 1981; Rice, 1983; Dugan and Williams, 1988) triggered many studies toward CBM exploration and production (Rice, 1993; Scott et al., 1994; Whiticar, 1994; Ayers, 2002; Faiz and Hendry, 2006; Str?po? et al., 2007; Flores et al., 2008). Several coal basins in North America are already producing prolific CBM, for example, the Powder River Basin (Montgomery, 1999; Flores, 2004), San Juan Basin (Scott et al., 1994), and Black Warrior Basin (Pashin and McIntyre, 2003). Other basins such as the Illinois Basin (Tedesco, 2003; Faiz and Hendry, 2006), Forest City Basin (Tedesco, 1992; McIntosh et al., 2008), and Michigan Basin (Martini et al., 2003, 2008) are considered to be prospective sources of CBM in the future (Figure 2.2).

FIGURE 2.1Annual coalbed methane production in the United States, rising from 91Bcf in 1989 to 1966Bcf in 2008. 1×109m3=35.289×109cubic feet=35.289Bcf.Data from U.S. Department of Energy, Energy Information Administration (2007).

FIGURE 2.2Basins producing coalbed methane in the lower 48 states in the US.From U.S. Department of Energy, Energy Information Administration (2007).

The objectives of this paper are to (1) characterize geochemical and microbial origins of coalbed gases, (2) assess geological and environmental factors controlling coalbed gas composition and generation, (3) evaluate petrological, chemical, and isotopic proxies that have been used to distinguish between types of coalbed gas origin, and (4) discuss implications of gas origin and generation for exploration and production.

Origins of CBM

CBM is technically defined as natural gas that can be recovered from coal seams. Most CBM has been produced in situ by microbial, thermal, or possibly catalytic degradation of organic material present in coal, although some allochthonous gas components may have migrated into coal seams from other strata. CBM is mainly composed of methane (CH4) with variable additions of carbon dioxide (CO2), elemental nitrogen (N2), and heavier hydrocarbons, such as ethane (C2H6), and traces of propane (C3H8), and butanes (C4H10).

In general, coalbed gas is traditionally assumed to be either of biogenic, thermogenic, or mixed origin. Biogenic coalbed gas is generated by the breakdown of coal organic matter by methanogenic consortia of microorganisms at low temperature (usually less than 150°F or 56°C) (Scott et al., 1994). Many shallow and thermally immature coal seams contain biogenic CBM, such as the Powder River Basin. In contrast, thermogenic gas is produced from coal organic matter by chemical degradation and thermal cracking mainly above 100°C, above the thermal threshold where microbial methanogenic activity becomes biochemically impossible (Hunt, 1979; Rice and Claypool, 1981). Thermogenic gas generation commences at the high-volatile bituminous coal rank with vitrinite reflectance values Ro between 0.6% and 0.8% (Scott et al., 1994). Thermogenic methane production from coal reaches a maximum at Ro?1.2% and becomes negligible at Ro?3.0% after the organic hydrogen pool in coal has been severely depleted and remaining organic carbon is largely present in condensed aromatic structures. Therefore, higher rank coal is expected to have generated more thermogenic CBM than relatively lower rank coal, which would translate into a higher CBM content if the gas has not been lost.

Knowledge of the origin of CBM is important to formulate an effective and successful CBM exploration strategy (Scott et al., 1994; Martini et al., 2008). A predominantly microbial gas play should target shallow coal seams at lower temperature close to basin margins, where the organic matter is likely less mature and larger fractures allow for faster gas extraction. In contrast, predominantly thermogenic coalbed gas accumulations likely occur in deeper, more thermally mature coal seams (Rice, 1993; Str?po? et al., 2007; McIntosh et al., 2008) with a more restricted network of open fractures. Basins with coal seams hosting mixed (i.e., thermogenic and biogenic) CBM require complex exploration and production strategies depending on local geological and hydrological conditions. Knowledge of the origin of coalbed gas has become a prerequisite for successful CBM exploration (Rice, 1993; Scott et al., 1994; Whiticar, 1999; Faiz and Hendry, 2006; Flores et al., 2008; Dai et al., 2009).

Methods for Characterizing the Origin of CBM

Originally, coal rank was used to diagnose the origins of associated coalbed gases. Coals of low rank with vitrinite reflectance values, Ro<0.3% were considered to host biogenic gas whereas higher rank coals were assumed to contain thermogenic gases (Claypool and Kaplan, 1974). However, with the recognition of secondary biogenic gases (Rice, 1993; Scott et al., 1994), which overwhelmingly contribute to biogenic gas in coals with elevated vitrinite reflectance (Ro?0.6–0.8%) (Russell, 1990; Scott et al., 1994), the chemical gas composition and the ratio of the abundance of CH4 relative to higher hydrocarbons (i.e., “gas dryness”) became indispensable proxies for evaluating the origin of coalbed gas (Stahl, 1973; Bernard, 1978; Schoell, 1980, 1983; Whiticar, 1996) (Table 2.1). However, these chemical compositional indices may vary...