Biogenic gas production from in-situ coal
Coal is the largest fossil energy resource in the world. However, it is estimated that 90% of the world’s coal cannot be mined due to technical and economic limitations, so the vast majority of the world’s coal is effectively stranded. Often, methane gas is trapped in deep coal deposits. This resource is commonly referred to as coal-bed methane (CBM). While CBM production is a 30-year old industry, turning this valuable resource into recoverable reserves is highly challenging due to numerous commercial and technical challenges.
An emerging technology, known as microbial enhanced CBM, or bioCBM, holds the potential to be a game-changer. A handful of US-based companies are pioneering an operationally simple process that generates new methane in coal, leading to significantly enhanced gas production and project economics. The low capital and operating cost of bioCBM, which ‘bolts-on’ to existing CBM projects, makes it a compelling business proposition.
BioCBM is inspired by the fact that naturally occurring anaerobic microbes in coal seams produce much of the natural gas in coal as a product of their life processes, which consume carbon and release methane. The CBM industry recognizes that gas produced from lower rank coals is almost exclusively biogenic in origin. Thermogenic methane production only becomes dominant in higher rank coals, which are harder and more energy-dense (see Figure 1).
Interest in understanding the role of biogenic organisms in CBM production is gaining momentum worldwide, particularly in the US, Canada and Australia. Researchers are working to not only understand the processes involved, but to control them in order to enhance gas production from coal.
Being able to control the production of gas from coal seams represents a paradigm shift in the way that a coal seam’s energy capacity is evaluated. Many of the natural constraints on gas production are removed, with gas production becoming a biological process rather than a simple exercise in extracting existing gas or coal. Given that deposits of coal are far more widespread than producible natural gas, this stands to benefit many countries that would ordinarily only recover energy from the 5-10% of coals that can be profitably mined. Anaerobic microbes are found throughout the natural world, from swamps, lake beds, and coal seams to the digestive tracts of most animals, including humans. Microbe populations (consortia) are highly complex, consisting of sub-sets of simple organisms that exist in symbiotic relationships. In coal seams, these organisms reside in biofilms on the surfaces of coal fractures and on coal particles. They can also flow through the natural groundwater system. The conversion of large chain organic molecules, such as coal, into gas is complex and occurs via multiple processes and pathways. In simple terms, the conversion involves two main steps: conversion of solids to simple liquids (organic acids) and conversion of those organic acids into gas (methanogenesis).
The first stage of coal breakdown (solid to liquids) is hydrolysis. It uses available groundwater and is the rate-limiting step in the bioconversion process. As the coal breaks down, complex coal molecules depolymerize into smaller chain compounds. The second stage of the bioconversion process transfers these organic molecules into methane and occurs predominantly outside of the groundwater system.
Most companies and groups seeking to develop bio-stimulation methods for gas production from coal seams use a fairly standard approach. Prominent among them are Luca Technologies, Ciris Energy, and Next Fuels, all based in the US. The standard approach samples and characterizes the natural microbe populations in coal seams and then develops a nutritional amendment program to stimulate the reproduction and activity of particular methane-producing species. This approach has been shown to work in lab and pilot tests. However, there are a number of fundamental limitations. These include the complex and expensive processes needed to analyze coal seam bacteria (including DNA sequencing, molecular analysis, and spectroscopic analysis/imaging); the extensive enrichment studies that are required to determine the impacts of nutritional amendment and design an appropriate program; the possibility of stimulating counterproductive bacteria; and, in some cases, the lack of suitable indigenous microbes to respond to stimulation.
The limitation of using indigenous microbes is becoming more apparent among some technology developers. In addition to the challenges mentioned above, there is the need to overcome the rate-limiting step of breaking down the solid coal (hydrolysis). Pioneering operators have sought out non-indigenous microbial consortia that are naturally suited to depolymerization. Such microbes can improve the bioavailability of intermediate liquid components for methanogens, speeding the conversion to methane. For example, C. quinii, C. acetobutylicum and C. magnum ferment carbohydrates to various fatty acids including acetate. Clostridium magnum and C. scatologenes are able to perform homacetogenic fermentation and create other useful molecules for methanogens.
Recently, the US Geological Survey tested non-indigenous microbe species in non-productive wells, taking methanogenic cultures derived from wetland sediment. Results showed that the introduction of non-indigenous species provided a clear advantage in coals that were naturally low in microbial activity.
One species of microbe, derived from the hindgut of the Zootermopsis sp. dampwood termite, was identified as having superior capabilities for coal conversion by ARCTECH, a 25 year old US-based company that has pioneered the application of non-indigenous microbes to convert coal to methane. Zootermopsis is specialized for digesting lignin, an organic polymer that comprises a quarter to a third of the total dry mass of wood. Lignin is the natural material most like coal. It is thought to be an important precursor of coal, so it is not surprising that organisms that biodegrade lignin anaerobically can also be bred to digest coal.
The identification of superior microbial consortia was accompanied by the development of new and innovative microbiological techniques that allowed subsets of the Zootermopsis microbe consortia to be adapted for specific coals. Recently, ARCTECH set up a new company, MicGAS, to commercialize their proprietary technologies, with an initial focus on stimulation of CBM wells and enhancement of gas production rates. At MicGAS, we adapt our proprietary microbial consortia (Mic1) to the specific coal seam by feeding it to the microbial consortia. Those microbes that survive and multiply rapidly are then separated and fed with a fresh batch of coal and nutrients in a process similar to filtration. After multiple adaptations, we have a tailored consortia of microbes that quickly produce gas with a high ratio of methane to carbon dioxide (see Figure 2).
BioCBM can enhance production from coal-bed methane fields that are marginal or in decline (see Figure 3). MicGAS bioCBM is designed to overcome all of the natural limitations of the amount of gas in place, gas pressure, and groundwater disposal, and it reduces the economic uncertainty imposed by reservoir conditions. Monitoring and numerical modeling of the in-situ process during the pilot test will be vital to successful commercialization.
This data will allow full field predictions to be made with reasonable confidence. In-situ performance will also depend on the contact efficiency between the coal and injected fluids (microbes and nutrients). We plan to monitor this closely in our pilot tests. If the coal does not have sufficient permeability and porosity, the volume of fluids in contact with the coal will be low, which may not lead to commercially viable gas production.
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