What is CBM?
Coal Bed Methane (CBM) is methane gas formed in coal seams or coal beds. CBM is one of the gas fractions of Coal Bed Gas (CBG), which is natural gas that produced in the coal seam. Gas in coal seams typically consists of methane (CH4), carbon dioxide (CO2), nitrogen (N), and water (H2O) (Thomas, 2002). CBM is often also called sweet gas because of the low content of hydrogen sulfide.
Unlike much natural gas from conventional reservoirs, coal bed methane contains very little heavier hydrocarbons such as propane or butane, and no natural gas condensate. CBG contained in the micropores of coal, CBG is in a near-liquid state, lining the inside of pores within the coal (the matrix).
CBM is generated either from a biological process as a result of microbial action (biogenic) or from a thermal process as a result of increasing heat with depth of the coal (thermogenic).
The gases (coal bed gas) produced from the decomposition of organic material by microorganisms commonly formed in peat bogs as an origin of coal. Biogenic gas can occur in two stages, in the early stages and the final stage of coalification process. In the early stages can be caused by the activity of organisms, from peat to subbituminous. While in the final stage is also caused by the activity of organisms, but after the coal is formed.
The gases are produced in the coalification process that has a higher rank, which is occurred in subbituminous A (high volatile bituminous coal – low volatile bituminous). Coalification process will produce more carbon-rich coal and then release some gases, such as CH4, CO2 and H2O.
Coalbed gasest formed in the coal seam are related to temperature and coal rank. Lower rank coal contains a lot of methane. Moreover, medium-volatile bituminous coal also contains a lot of methane. Meanwhile, high-volatile bituminous coal has a large content of CO2 than methane.
figure 1. coalification process
Coal is a porous sedimentary rock so the gas can be trapped in the pore. Coal has a deposite capacity that expressed in volume of gas per volume of coal. Unlike much natural gas from conventional reservoirs (sandstone), coalbed methane lining the inside of pores within the coal (called the matrix). The open fractures in the coal (called the cleats) can also contain free gas or can be saturated with water. Coalbed methane contains very little heavier hydrocarbons such as propane or butane, and no natural gas condensate.
As a reservoir, coal has a matrix consisting of micropores and fractures (face & butt cleats) as shown.
Figure 4. Coal microcleats.
Coal has some micro-fracture (microcleats) with size of 2mm – 25mm, when we see in more detail (microscopic scale) on the microcleats there are some micropores with size of 0.01mm – 20mm. The gases is adsorbed on the coal so to release it we can use desorb methods.
With a smaller pore diameter, there will be a larger surface area of the matrix. So there’re more and more amount of gas that can be absorbed. This make the deposit mechanism on CBM gas will be bigger than a conventional reservoir. As an illustration, a porous matrix with 1 cm3 volume will produce 6 cm2 surface areas, while porous matrix with four times smaller will produce 12 cm2 surface areas, and so on.
Coal has a small porosity and the smaller pore in the coal matrix causes the matrix become larger so that more gas adsorption occurs. In the conventional gas production, gas fraction will go right out in the early stages of production. Along the gas produced decrease, the water produced will increase. Otherwise, in the early stages of CBM production, as the number of CBM wells increase, the amount of water produced will also increase. CBM stays in the coal seam due to hydrostatic pressure. So to extract the CBM involves pumping available water from the seam in order to reduce the water pressure that holds the gas in the seam. CBM has very low solubility in water and readily separates as pressure decreases, allowing it to be piped out of the well separately from the water. Water moving from the coal seam to the well bore encourages gas migration toward the well.
CBM producers try not to dewater the coal seam, but rather seek to decrease the water pressure (or head of water) in the coal seam to just above the top of the seam. However, sometimes the water level drops into the coal seam.
Factors that affect the CBM reservoir
When the magnitude of the pressure increase, the absorption capacity will also increase, but when approaching a certain limit the speed will reduced. If the pressure reduced, the gas release rate will be decreasing (desorption). Therefore, by increasing the depth of the well, the coalbed gas content will be higher.
As the higher of the temperature increase, the adsorption capacity will be decrease in other word increase gas desorption. Temperature is very important to determine the lower limit of the gas, which is depends on the geothermal gradient.
c. Mineral Matter
Coalbed gas is only bound to the organic fraction of the coal. Meanwhile, there is a volatile in various forms that are usually called mineral matter (ash and sulfur). The higher content of mineral matter in the coal, the smaller gas absorption capacity.
Basically, the water content (moisture) in coal has properties that similar to the mineral matter, in relation to the absorption capacity. So the higher content of water in the coal, then the smaller gas absorption capacity.
e. Coal Rank
If the coal rank increases, the gas deposit capacity will also increase. So that the higher coal rank most probably has the capacity of gas deposit. Methane will formed in a high speed when coalification process move from high-volatile bituminous to low-volatile bituminous.
f. Composition of Coal Maceral
The amount of the gas in the coal is also affected by the composition of the coal maceral. Exinite or liptinite (organic matter type II) which contains hydrogen will produce more methane and then followed by vitrinite (organic matter type III).
g. Fracture (Joint in The Coal)
Joint in a coal or commonly named cleats, has a very big impact in the capacity of the gas produced. Although coal has a large porosity but it permeability layer determined by the fracture system (cleats). Most fracture can be found in the medium until low-volatile bituminous. These cleats can be affected by: coal rank, coal seam thickness, lithotype, regional stress, and HGI (Hardgrove Index).
CBM Potential in Indonesia
CBM potential in Indonesia has been reviewed by several experts, including Scott H. Stevens and Wahyudi Soetoto in 2000. They have conducted a study on 10 coal basins, there’re Barito Basin, Berau Basin, Kutai Basin, North Tarakan Basin, Pasir Asem-asem Basin, Central Sumatra Basin, South Sumatra Basin, Bengkulu Basin, Jatibarang, and Sengkang Basin. They estimate the potential of CBM in 10 basins with prospects area about 74,000 km2 is about 336 Tcf.
In 2003, Advanced Resources International, Inc. (ARII) Arlington, Virginia, USA, conducted a study on 11 coal basins. ARII estimate the potential of CBM in Indonesia (11 basins) is about 453.3 Tcf.
That enormous CBM potential is almost equivalent to the natural gas potential of Indonesia which is 507 Tcf with proven reserves 112 Tcf. CBM products are projected to meet the energy need of Indonesia and support the National Energy Policy (Kebijakan Energi Nasional / KEN) that written in President’s Decision (Keputusan Presiden) No. 5 2006 with a focus on improving new and renewable energy resources as well as to reduce the use of petroleum and natural gas as an energy resources in in Indonesia.
Figure 7. Coal Bed Methane Resources in Indonesia
Hafizhan Abidin Setyowiyoto
Undergraduate, Department of Geological Engineering, Gadjah Mada University
Jalan Grafika 2, Yogyakarta (55281)
Sukhyar R., et al. 2012. “Potensi dan Pengembangan CBM Indonesia”. Jakarta: Badan Geologi, Kementerian Energi dan Sumber Daya Mineral.