Coal bed methane is basically natural gas produced from coal beds. Conventionally, natural gas is produced from either gas wells or as associated gas with oil wells. 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.
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 USA, 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 sulphide at all. It is not like the world is just knowing about the presence of huge deposits of methane gas in coal beds, it has only become more attractive to natural gas producers.
Coal bed methane is a well 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 explosions. As much as possible, miners tried to avoid coal bed methane. The gas was nicknamed “marsh gas” because coal is normally found in marshy areas. As a matter of fact, wells used to be drilled from the surface into known coal beds in the subsurface, with the sole aim of venting the methane gas after which mining can commence.
Challenges of producing from coal bed methane
Producing from coal beds present a unique challenge. It is safe to say that it presents an unconventional challenge. In conventional gas wells, the gas rate is initially high but over time decline sets in and gas production decreases towards the end of the life of well. Near the end of life of conventional gas wells, water rate increases as gas rate decreases. The opposite is true with coal bed methane, the trapped gas in methane gas waits until later before production really increases. Initially, water rate is high while gas rate is low. Over time, water rate drops while gas rate increases. This special behavior means that considerable capital has to be invested at the onset to account for the large produced water volumes even though the well has not started yielding returns. Cost increases when we consider cost of produced water disposal.
Coal bed has dual porosity. One large porosity caused by fractures called cleats and another micro porosity within the coal matrix. The larger fracture pores is usually occupied with water. It is this water that rushes out first when a well is drilled. This water is significant as the methane gas is held tightly to the coal by the water. This is why over time as the water gets produced, pressure drops and more gas gets released. 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 through the entire matrix. This is the process of adsorption. So producing from coal beds basically releases the trapped methane gas by desorption.
Nearly every coal bed methane well, will require artificial lift at one point or the other in the life of the well. Popular conventional artificial lift techniques like electric submersible pumps, progressive cavity pumps (PCP), beam pumps and gas lifts. The cheapest of them all is gas lifting, especially at the initial stage of large volume of water production. Gas lifts can comfortably handle water rates 5 to 50 barrels of water per day. This dwarfs when compared to electric submersible pumps which can handle really high water volumes of over 1000 barrels per day. However, electric submersible pumps require constant electricity supply 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. These choices are available and decision of which one to use can be based on cost and efficiency.
Most coal bed methane fields are in undeveloped and remote areas. This presents infrastructural challenge to any company that ventures into coal bed methane production because there is a need to ensure the availability storage facilities, separators, flowlines and pipelines. In developed conventional fields, companies can simply make use of existing infrastructure. At the same time, infrastructure for removing impurities like CO2 and N2 need to be in place to meet with 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 bases, a coal bed holds more methane gases in its matrix than any conventional natural gas reservoir. This is favorable and encourages investment. Challenges are there to be overcome and moreover, the industry has already developed technology in conventional hydrocarbon production which has been used successfully with coal bed methane. To overcome water disposal challenges, disposal wells can be drilled or old wells utilized for disposal or surface evaporation of the produced water. Impurities like N2 and CO2 gases can be separated with molecular sieves, scrubbing with amines and the use of cryogenics. The gas is then dehydrated and compressed into pipelines for sales.