African Mining July 2022 | Page 39

OPERATIONS •

seam is then dewatered to decrease the in-seam water pressure by pumping the contained water from the well . The decrease in pressure allows methane to desorb from the coal . The methane gas is then able to flow as a gas up the well to the surface . The captured methane is compressed on surface and piped to market or power stations .

The process is designed in such a way as to prevent the methane being dissolved into the pumped water , but to allow the released methane to flow up the installed casing to the compressor station . The main risk , and hence the need for monitoring , is to prevent the water level to be pumped too low during dewatering , resulting in the methane traveling up the piping into the water line causing the well to become " gassy ". Although it is possible for the methane to be recovered in a water-gas separator at the surface , pumping water and gas is inefficient and can cause pump wear and breakdown .

The potential of methane gas content is determined by two main methods . The first is a direct method , which measures the volume of methane released from a coal sample sealed into a desorption canister . The second is an indirect method based on empirical correlations or laboratory derived sorption isotherm methane storage capacity data .

Challenges There are however a number of environmental concerns associated with CBM extraction . A large volume of pumped water , or saline brine , is produced from coalbed methane wells during dewatering . Acceptable disposal methods of the produced brine are a major cost factor for economic methane production . The freshwater component may be discharged on the surface , but pumped brine is generally injected back into the host rock environment at a depth where the salinity of the injected brine is less than connate fluids of the host rock . The alternative is evaporation of the saline water for recovery of potentially saleable solid residues . This may be a viable option in the Southern African region due to having high evaporation rates . thin or of poor or have inconsistent quality with the potential of reducing the volume of methane , a greenhouse gas , which may be released into the atmosphere .

CBM is considered economically viable due to its large internal surface area , which is able to store six to seven times more gas than the equivalent rock volume of a conventional gas reservoir . The contained methane gas content generally increases with coal rank , depth of burial and with reservoir pressure . Fractures within coalbeds are usually filled with water . Deeper coal beds contain less water but tend to be more saline .

Removing water from the coal bed reduces pressure and releases methane . The methane in the coal seam is released when the coal bed is depressurised . Contained coalbed methane is one of the risk factors in underground coal mines and apart from coal dust , considered to be the cause of underground explosions . CBM extraction is therefore mainly carried out prior to mining to increase the safety of mining coal beds .

The extraction Process CBM is extracted by drilling wells into the coal seam . The coal

CBM is considered economically viable due to its large internal surface area , which is able to store six to seven times more gas than the equivalent rock volume of a conventional gas reservoir .

The full potential and economic viability of CBM extraction must still to be determined in the SADC region . The CBM extraction method would need to be developed to make provision for the various coal types hosted mainly in the Karoo formations scattered throughout the region . This would also include investigating the most effective extraction method and the sustainability of these operations within the energy transformation landscape . •