Coal Bed Methane
“Coalification” is the transformation of plant matter into coal under particular conditions of pressure, temperature and time. Once coal has been formed, further molecular changes lead to the formation of vast quantities of thermogenic methane. This methane (natural gas) is variously known as Coal Bed Methane, Coal Seam Gas or Coal Bed Gas. Other gases that accompany methane include ethane, carbon dioxide, nitrogen, hydrogen and helium.
The methane is stored in micropores within the coal. The micropores give coal an enormous surface area. One tonne of coal has a surface area of around 90 million square metres. This leads to something of a paradox: that coal, a rock with very low porosity, can store such a vast amount of gas.
Fractures (“cleats”) form in the coal, which allow the transport of excess methane throughout the coal bed. These fractures also host large quantities of formation water. It is pressure from this water that keeps the methane adsorbed in the micropores.
Methane in coal mines is a killer, many thousands of men have been killed over the centuries by methane explosions. Hence “the canary in the coal mine”. Canaries were taken into underground coal mines well into the twentieth century as an early warning system to detect hazardous gases such as methane and carbon dioxide. They would succumb early, giving the men time to escape.
Today, coal mines are ventilated by drawing vast quantities of air through the mine. This is expensive, it requires a large amount of energy, and polluting, as it vents the gases to the atmosphere.
Coal bed methane is extracted by drilling, which is often followed by hydraulic fracturing. Read about these technologies here. Well depths are typically in the range of 200 to 1,000 metres and may be completed horizontally.
Once complete, formation water is pumped from the well and, when hydrostatic pressure has been sufficiently reduced, the gas begins to flow. The gas and water are separated at the surface and the gas then passes to a processing plant. Coal seam gas contains water, coal fines and other contaminants that must first be removed to produce a saleable product.
Coal bed methane wells initially produce large quantities of formation water. As pressure drops water production drops rapidly and gas production rises. Wells can have a long life, of the order of 20 years. Production will decline steadily over time, but wells can be re-fracced to extend their life.
Water production and its disposal costs can make or break marginal projects. Initial water production rates and decline rates vary widely. Initial rates can range from 10 to over 1,000 barrels of water per day (“BWPD”). Production can decline to as low as 10 BWPD for most of a well’s life.
The chemical composition of coal bed water also varies greatly. The pH of production water can be mildly acidic or alkaline, it may contain substantial bicarbonate and/or chloride and/or sulphate. A variety of metals may be dissolved along with hydrocarbons.
Some water is potable but generally it has to be purified prior disposal by either surface discharge or well injection. In Australia, until recently, it was common for formation water to be pumped into evaporation ponds. This left behind salts and other compounds that then needed to be disposed of. This practice has been curtailed because of concerns about groundwater contamination. In the United States it has been common practice for the water to be treated in ponds before being released to surface drainage or re-injected back into the ground.
In Australia, it is expected to become common practice to purify the water (typically through reverse osmosis). The purified water may then be used by local communities or to recharge distressed aquifers.
Who produces Coal Bed Methane
Coal bed methane was first produced in Kansas, in the United States, in 1920 and the first coal bed methane well to be fractured was by Halliburton in 1954. By 1992 the United States was producing a substantial quantity of natural gas from around 5,500 wells.
The major producers today are Australia, Canada, China, India, Kazakhstan, Russian Federation and the US. Canada, China and Australia expect to ramp up production over the next few years. Australia plans to convert coal bed methane to liquefied natural gas (“LNG”) for export. Many other countries have geology appropriate for coal bed methane extraction and a number have plans for future production, particularly in Africa and Asia.
The Pros and Cons
Methane is the cleanest-burning fossil fuel, a significant benefit when it replaces coal or oil for electricity generation. Extraction of coal bed methane can improve the safety of coal mining. It can also dramatically reduce methane vented to the atmosphere by coal mines. Drilling for coal bed methane is a relatively benign activity when compared to drilling for oil and shale gas or mining oil sands.
The primary concerns are environmental. The development of coal bed methane projects requires extensive surface disturbance for well sites, roads, pipelines etc. This can continue for some time as more wells are drilled at increasing density.
Aquifer contamination by methane and other contaminants from coal bed methane development is often cited as a concern. However methane can come from a variety of sources. For example: the United States Geological Survey notes that reports from the 1800’s refer to gas in water wells, streams and fields after heavy rains.
Production water and chemicals and fluids used in drilling are potential contaminants of the land surface and water ways.
Production of coal bed methane is well established in several countries, with many others looking to join the production ranks. The investor primarily needs to ensure that methane can be extracted from the coal in question, this is mission critical. Secondly, in some countries there is a real risk that environmental concerns will prevent the development of coal bed methane.