Coalbed methane
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The process by which plant material is converted to coal generates large quantites of methane gas, which is often stored within the coal. The presence of this gas is very well known due to underground coal mining, where it presents a serious safety risk. This is coalbed methane, often referred to as CBM. It is fairly different from a typical sandstone gas reservoir, as the methane is stored within the coal by a process called adsorption. The methane is in a near-liquid state, 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.
Estimated methane reserves vary, but are generally agreed to be huge. A 1997 estimate from the U.S. Geological Survey predicts more than 700 trillion cubic feet (20,000 km³) of methane within the US. At a natural gas price of US$6.05 per million Btu (US$5.73/GJ), that volume is worth US$4.37 trillion. At least 100 trillion cubic feet (2,800 km³) of it is economically viable to produce.
High natural gas prices are making CBM economically viable where it previously may not have been. Coalbed methane wells produce at low gas rates, typically maxing out around 300 million cubic feet per day (100 m³/s), and can have large inital costs. The production profiles of CBM wells are typically characterized by a "negative decline" in the gas rate as water is pumped away and gas begins to desorp and flow. A dry CBM well does not look very different from a standard well, except that the gas rates are lower and decline at a much slower rate.
The methane desorption process follows a curve (of gas content vs. reservoir pressure) called a Langmuir isotherm. The isotherm can be analytically described by a maximum gas content (at infinite pressure), and the pressure at which half that gas exists within the coal. These parameters (called the Langmuir volume and Langmuir pressure, respectively) are properties of the coal, and vary widely. A coal in Alabama and a coal in Colorado may have radically different Langmuir parameters, despite similar other coal properties.
As production occurs from a coal reservoir, the changes in pressure are believed to cause changes in the porosity and permeability of the coal. This is commonly known as matrix shrinkage/swelling. As the gas is desorbed, the pressure exerted by the gas inside the pores decreases, causing them to shrink in size and restricting gas flow through the coal. As the pores shrink, the overall matrix shrinks as well, which may eventually increase the space the gas can travel through (the cleats), increasing gas flow.
There are a few pieces of software which can simulate CBM reservoirs for the purpose of forecasting or determining reserves:
- EXODUS Reservoir Simulator (http://www.petrostudies.com/exodus.htm)
- FAST CBM Reservoir Toolkit (http://www.fekete.com/software/cbm/description.htm)
- ECLIPSE CBM (http://www.sis.slb.com/content/software/simulation/eclipse_options/cbm.asp)
- 3M Coalbed Methane Reservoir Model (http://oil-gas.state.co.us/Library/sanjuanbasin/3m_project/3MCBMReport.htm) (public domain software)