Coal bed methane is basically natural gas produced from coal beds. Conventionally, natural gas is produced either from gas wells or as associated gas with oil wells. In contrast, coal bed methane is an unconventional hydrocarbon source. The methane bed serves as the source rock and the reservoir at the same time, unlike with conventional hydrocarbons where the source rock is different from the reservoir rock.
Background on Coal Bed Methane
In recent times, coal bed methane wells have become an alternative source of natural gas production in certain regions of the world like Australia, the United States, Mexico and Canada. There are unique advantages to the methane gas obtained from coal beds; it is a sweet gas with little or no hydrogen sulfide at all. Although the presence of huge deposits of methane gas in coal beds has been known for some time, it has only recently become more attractive for natural gas producers.
Coal bed methane is a known cause of mining explosions. Of course methane gas is highly combustible, so striking a methane bed while mining can easily ignite the gas and cause an explosion. Consequently, miners normally try to avoid coal bed methane.
The gas was nicknamed “marsh gas” because coal is normally found in marshy areas. In the past, wells were drilled from the surface into known coal beds with the sole aim of venting the methane gas, after which mining could commence.
Challenges of Producing from Coal Bed Methane
Producing from coal beds presents a unique and unconventional challenge. In conventional gas wells, the gas rate is initially high, but over time declines so that gas production decreases towards the end of the life of the well. Near the end of life of conventional gas wells, the water rate increases as the gas rate decreases. (For more on this topic, read Understanding Reservoir Drive Mechanisms.)
The opposite is true with coal bed methane – the trapped gas in methane gas waits until later before production really increases. Initially, the water rate is high while the gas rate is low. Over time, the water rate drops while the gas rate increases. This special behavior means that considerable capital must be invested at the onset to account for the large produced water volumes even though the well has not started yielding returns. The cost increases when we consider the expense of water disposal.
Porosity in Coal Beds
A coal bed has dual porosity. A large porosity that is caused by fractures is known as a cleat; the other is a micro porosity within the coal matrix. The larger fracture pores are usually occupied with water. It is this water that rushes out first when a well is drilled. The water is significant because the methane gas is held tightly to the coal by the water, which is why over time as more water is produced the pressure drops and more gas is released.
Understanding Adsorption and Desorption
Methane is trapped in coal through adsorption. This is different from how conventional hydrocarbons are trapped in oil and gas reservoirs. There is no impermeable trap to hold the methane within the coal like conventional reservoirs, so the methane gas is adsorbed to the surface of the coal. In other words, the surface of the coal captures generated methane gas and distributes it throughout the entire matrix. This is the process of adsorption. Thus, producing from coal beds basically releases the trapped methane gas by desorption.
Coal Bed Methane Wells Usually Require Artificial Lift
Nearly every coal bed methane well will require artificial lift at one point or another in its lifetime. (Get the lowdown on artificial lift in the article Artificial Lift: When is it Needed?) Popular conventional artificial lift techniques are electric submersible pumps (ESP), progressive cavity pumps (PCP), beam pumps and gas lifts. The least expensive is gas lifting, especially at the initial stage when there is a large volume of water production.
Gas lifts can comfortably handle water rates of 5 to 50 barrels of water per day, which is tiny compared to electric submersible pumps that can handle very high water volumes of over 1000 barrels per day. However, electric submersible pumps require constant electricity and may not be able to handle fines or produced sand. PCPs are great with sand though, and beam pumps can handle some sand production to an extent. The decision of which one to use can be based on cost and efficiency.
Infrastructure Requirements for Coal Bed Methane Fields
Most coal bed methane fields are in undeveloped and remote areas. This presents an infrastructure challenge to any company that ventures into coal bed methane production because there is a need to ensure the availability of storage facilities, separators, flowlines and pipelines. In developed conventional fields companies can simply make use of existing infrastructure. At the same time, infrastructure to remove impurities such as carbon dioxide (CO2) and dinitrogen (N2) need to be in place to meet pipeline requirements.
In spite of all these challenges, the industry has continued to pursue the development of coal bed methane fields. On a pound for pound basis, a coal bed holds more methane gases in its matrix than any conventional natural gas reservoir. This is favorable and encourages investment.
Challenges exist to be overcome, and moreover, the industry has already developed technology in conventional hydrocarbon production that has been used successfully with coal bed methane. To overcome water disposal challenges, disposal wells can be drilled or old wells can be used. Surface evaporation of the produced water is another option. Impurities such as N2 and CO2 gases can be separated with molecular sieves or scrubbed with amines and the use of cryogenics. The gas is then dehydrated and compressed into pipelines for retail sales.