Coalbed methane is anunconventional natural gas and a very good source of energy, which areextracted from the coal beds or coal seams. It is formed during the process of coalificationand the transformation of plant into the coal. CBM is considered as importantenergy resource with reserves and production. Various method of recovery makesCBM stable source of energy.
It can be recovered from underground coal before,during and after mining operations. For extraction and development of CBM orCBM resources vertical and horizontal wells are used, which require drillinginto the coal seam or coal beds and the water present in the seam is removed toreduce hydrostatics pressure and released adsorbed gas out of the coal. CBM production behaviour iscomplex and difficult to predict in the early stages of recovery.
Coal bedmethane (CBM) recovery is associated with production of large quantity of groundwater.The coal seams are depressurized by pumping of water for regular and consistentgas production. Usually, CBM operators need to pump >10 m3 of water per dayfrom one well, which depends on the aquifer characteristics, drainage and rechargepattern.
WhyCBM? CBM can fulfil the national goals in India: Provide a clean burning fuel. Improvesafety of coal mining. Decrease methane vented to the atmosphere that mightcause global warming. Most importantlyprovides a means to use an abundant coal resource that is too deep to mine. Itminimizes the environmental impacts due to coal mining as methane is not lostto the environment anymore. It is used for other purposes like steel manufacturing,fuel for industries, fertilizer manufacture, ceramics glass textiles etc.
Indianscenario of CBMIndia is a coal rich nationand has significant volumes of coal bed methane reserves. India has the 3rdlargest proven coal reserves and is the 4th producer in the world. In India,CBM is being produced from the Gondwana coalfields of Raniganj, Jharia andSohagpur. Total coal resources of India are approximately 300B. The miningindustry is over two centuries old. The prognosticated CBM resources are around4.6TCM. Moreover, the estimated CBM resources hold significant prospects forcommercial recovery of natural gas.
Deeper reserves from which recovery of CMBseems less feasible, technologically as well as economically, may be the primetargets for enhanced recovery techniques, for example the coal seams which arefound at greater depths and are below the limit of mining are consideredsuitable for these techniques. India stepped into CBM exploration andproduction decades after the US success. Methane is native to coal and formsduring the coalification process. It has traditionally been recognized as anevil companion of coal bringing in major safety problems in underground mininguntil recently when it changed from a mining hazard to an effective energyresource. In recent times CBM is receiving increased attention as a valuablefuel source. Coal is the source as well asreservoir to significant quantities of methane, a potentially economicresource. The huge reserves of coal in India may be categorized in two types,based on their period of formation namely Gondwana coal and Tertiary coal.
Nearly 99% of the total coal reserves in India belong to Gondwana basins andare characteristically different from the rest of the tertiary coal. The majorfocus of research and development in CBM sector has been based mainly onGondwana coals.PROBLEMSASSOCIATED IN CBM WELLSDrilling operation:UnderbalancedDrilling (UBD) is a technique in which oil, gas or geothermal wells are drilledusing pressures lower than the reservoir pressure. The result is an increase inrate of penetration (ROP), reduced formation damage and reduced drilling costs.Air drilling provides an efficient system in terms of operations costs andenvironmental safety benefits.Two major problems occur at the time air drilling i.
e. caving of thewell and the waterinflux comes into the wellbore from the formation becauseØ Formation pressurecontrol is minimal and, therefore, drilling is limited to geological regionswhere reservoir pore pressures are low. Ø There is little or nofluid pressure to support the borehole wall and prevent sloughing.
Ø There is limitedability to cope with significant volumes of water entering the annulus fromwater producing formations. In most of the wells (about 80%) air drilling method is not successfuland we switch to the mud drilling. In few of the wells the shale section could not be air drilled and insuch cased drilling was carried out by KCl (shale inhibitor), PHPA (polymer)mixed in the mud and the top section completed. The coal seams have beendrilled with air/mist to avoid any formation damage.
Progressive cavity pump:The Progressive CavityPump (PCP) was invented by Rene Moineau in 1932. PCP’s became widely used assurface pumps, especially for the pumping of viscous mixtures. The PCP is ableto produce highly viscous sand layden fluids, making it the primary artificiallift system for conventional heavy oil production around the world. KUDU alongwith PCM, have led the development of high capacity, high head lift pumps aswell as elastomers that are resistant to medium and light crudes.
PCP’s arealso utilized for dewatering coal bed methane and conventional gas wells.