PROBLEMS ASSOCIATED IN CBM WELLSDrilling operation:Underbalanced Drilling (UBD) is a technique inwhich oil, gas or geothermal wells are drilled using pressures lower than thereservoir pressure. The result is an increase in rate of penetration (ROP),reduced formation damage and reduced drilling costs. Air drilling provides anefficient system in terms of operations costs and environmental safetybenefits.Two major problems occur at the time airdrilling i.
e. caving of the well and the waterinflux comes into the wellborefrom 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 drillingmethod is not successful and we switch to the mud drilling. In few of the wells the shale section couldnot be air drilled and in such cased drilling was carried out by KCl (shaleinhibitor), PHPA (polymer) mixed in the mud and the top section completed. Thecoal seams have been drilled with air/mist to avoid any formation damage.Progressive cavity pump:TheProgressive Cavity Pump (PCP) was invented by Rene Moineau in 1932. PCP’sbecame widely used as surface pumps, especially for the pumping of viscousmixtures.
The PCP is able to produce highly viscous sand layden fluids, makingit the primary artificial lift system for conventional heavy oil productionaround the world. KUDU along with PCM, have led the development of highcapacity, high head lift pumps as well as elastomers that are resistant tomedium and light crudes. PCP’s are also utilized for dewatering coal bedmethane and conventional gas wells.
Principlesof operation: PCP’s are positive displacement pumps, whichconsists of two key parts, the stator and the rotor. The stator remainsstationary and the rotor rotates. The PCP is described as a gear pump with asingle helical rotor, which rotates inside a double internal helical elastomerlined stator. The stator is run into the well on the bottom of the productiontubing, while the rotor is connected to the end of the rod string. When therotor is inserted into the stator, it creates a continuous seal line thatextends from the pump suction to discharge. This results in a series ofidentical but separate cavities that progress from the pump suction to thedischarge at the end of the pump, as the rotor turns eccentrically in thestator. One cavity opens as the other closes, creating a non-pulsating pumpingaction.
PCP delivers a constant flow that isproportional to the size of the cavity and rotation speed of the rotor.Rotation of the rod string at surface is powered by a hydraulic or direct drivehead. TROUBLESHOOTINGOF PCP’s: Elastomers swell: Elastomer swell can occur either when the elastomer isaffected by production fluids or treatment chemical that are incompatible withit (chemical swell) or due to an increase in temperature (thermal swell).Chemical swell is generally caused by the elastomer comingin contact with high API gravity crude oils or incompatible treating chemicals;both of which have a fairly high percentage of aromatics. Chemical swellgenerally permanent and nonreversible once theSource of the swelling has been removed.
Thermal swell on the other hand is purely due to anelevation in temperature resulting in the elastomer expanding due to thisincrease in temperature. This expansion also results in an increase in theinterference fit between the rotor and stator thereby in an increase inoperating torque as well. Thermal swelling can be predicted and is notpermanent.
Once the temperature decrease the thermal expression of theelastomer also decreases. Bond failure:A bond failure occurs when the bonding agent that keeps the elastomerin the stator tube fails.Thiscan occur at two interface is between the bonding agent and the elastomer. Inthis instance, there is no elastomer test on the stator tube but there isbonding agent.
A view of the backside of the elastomer will show it to besmooth. The stator tube will also be smooth with a gray or black color.