Nowadays, coal bed methane has become a significant part of the worlds natural gas resource. EIA (energy information administration) data informs that US’s CBM reserves accounted for about 17.5 TCF in 2010. In addition, interest in other countries, especially Canada, Australia, China, and India, keeps growing. Indonesia, as a country having coal bed methane potential resources ranked number 6 in the world (according to consultancy Advanced Resources), should consider of this natural gas.
1. CBM as an unconventional gas reservoir
The energy industry classifies coal as an “unconventional gas reservoir” due to its highly cost to develop this gas production from it. However, many works have been continuously done to achieve the economically developing method for this. Another factor that can significantly increase of this interest is that methane as a “green house gas” is known to be more damaging to the environment than carbon dioxide (based on US environmental Protection Agency). Comparing with conventional reservoir of natural gas, CBM contains a few of heavy hydrocarbon such as propane and butane and has no condensate gas. CBM also has some percentage of carbon dioxide.
2. CBM reservoir characteristic and production mechanism
Methane gas is generated during the process of coalification, either by biogenic or thermo-genic routes. Once generated, the methane gets adsorbed on to internal surface of coal matrix or is compressed in the void space within coal. Gas storage capacity depends on the in situ pressure and adsorbed gas content in coal. The migration process of methane in coal is generally understood as three hydrodynamic mechanisms by taking the dual porosity nature of coal in to account desorption of gas from the internal coal surface, followed by diffusion through the coal matrix bounded by the cleat and lastly laminar flow through the naturally fracture network, known as the cleat system.
Compared with usual sandstone and conventional reservoir, CBM is different since the gas is stored inside stone through absorption process. Methane, which is almost in liquid condition, spreads around the pores of coal.
Most of CBM reservoirs, especially in USA or even in Indonesia, are water saturated initially. Therefore dewatering process, pumping this water out, is necessary to reduce the coal seam pressure. In the cleat system, there are three stages for flow in CBM reservoirs; single-phase water flow during dewatering, non-saturated single phase water flow, and gas and water two phase flow-gas flow. All of these stages are related to the changes of reservoir pressures and saturation of gas/water.
3. Engineering challenges of CBM
Considering those several unique features of CBM, which lead to a rather complex nature of reservoir formation, many evaluations and modeling absolutely are needed for optimized development of CBM. Geomechanics plays an important role here. Stress-strain relationship during depletion, failure mechanics of formation, compaction, mechanics of hydro fracturing, etc. are still in significant uncertainties for CBM reservoir engineering (cited from previous researchers). Even also dynamic changes in the reservoir stress field during gas production are still somewhat unclear.
Considering the hydraulic properties (aperture and permeability), water with micro bubbles flowing through fractured specimens (low permeable material) has mostly the lower values of those comparing with pure water only (Pardede and Osada, 2013). In this case, constant normal stresses were applied. From this simple approach, by flowing fluid through fractured specimen, different result was occurred. Absolutely, by developing and finding applicable experiment methods, many significant results might be gained and contribute to the development of gas production itself.
Indonesia, as the 6th rank in the world for the CBM potential resource, must be concern more and more especially for the production in order to reduce the dependency of oil source. Many researches and studies should be planned that Indonesia can joint with other countries having explored CBM as a new energy alternative such as Canada, USA, or Australia.
From several conference papers, proceeding, and author’s paper.