According to Prashant Modi, in the development of any coal bed methane field, the key elements which must be considered are the thickness and permeability of the coal, and the gas content. A relatively large number of pilot well are usually required before the productivity levels of the reservoir (that is, the recoverable reserves of methane) can be predicted for the average well and for the field in its entirety.
The process of developing the coal bed methane field has to be done in accordance with a carefully considered plan, so as to maximise the profitability, gas production and field life. Firstly, Prashant Modi says that the reservoir’s geology, including the inter-bed formation has to be studied in great detail, so that the degree of fracturing, coal thickness and lateral extent of the reserves can be determined. Using this information, the reservoir’s approximate volume can then be calculated. As the field is developed, the temperature and gas content of the reservoir must be estimated; this is because coal bed methane fields are developed using core analysis programs, including pressure cores.
Another important aspect of this developmental stage is accurate, consistent testing of the wells. This data is then digitally stored, where it can be used to map out the methane reservoir, with these maps being updated as each additional well is drilled. Once these maps have been created, coal bed methane fields are then divided up into individual reservoir compartments, based on the variations in permeability, coal bed thickness or faults. More mapping is then conducted to get a better understanding of the characteristics of each of these sub-divisions of the original reservoir. These sub-divisions will often differ not only in permeability, but also in water and pressure levels.
Some coal bed methane extraction companies will use regional statigraphic studies to understand the unconformities, sequence boundaries and sedimentary differences in each sub-division- this information, Prashant Modi says, can be useful for predicting oxidized zones and coal cut-outs. Thermal modelling is another method of examining the reservoirs; this tells the company about the formation pressure, gas content, coal maturity and burial history.