PROBLEMS ASSOCIATED IN CBM WELLS
Underbalanced Drilling (UBD) is a technique in
which oil, gas or geothermal wells are drilled using pressures lower than the
reservoir pressure. The result is an increase in rate of penetration (ROP),
reduced formation damage and reduced drilling costs. Air drilling provides an
efficient system in terms of operations costs and environmental safety
Two major problems occur at the time air
drilling i.e. caving of the well and the waterinflux comes into the wellbore
from the formation because
Ø Formation pressure
control is minimal and, therefore, drilling is limited to geological regions
where reservoir pore pressures are low.
Ø There is little or no
fluid pressure to support the borehole wall and prevent sloughing.
Ø There is limited
ability to cope with significant volumes of water entering the annulus from
water producing formations.
In most of the wells (about 80%) air drilling
method is not successful and we switch to the mud drilling.
In few of the wells the shale section could
not be air drilled and in such cased drilling was carried out by KCl (shale
inhibitor), PHPA (polymer) mixed in the mud and the top section completed. The
coal seams have been drilled with air/mist to avoid any formation damage.
Progressive cavity pump:
Progressive Cavity Pump (PCP) was invented by Rene Moineau in 1932. PCP’s
became widely used as surface pumps, especially for the pumping of viscous
mixtures. The PCP is able to produce highly viscous sand layden fluids, making
it the primary artificial lift system for conventional heavy oil production
around the world. KUDU along with PCM, have led the development of high
capacity, high head lift pumps as well as elastomers that are resistant to
medium and light crudes. PCP’s are also utilized for dewatering coal bed
methane and conventional gas wells.
PCP’s are positive displacement pumps, which
consists of two key parts, the stator and the rotor. The stator remains
stationary and the rotor rotates. The PCP is described as a gear pump with a
single helical rotor, which rotates inside a double internal helical elastomer
lined stator. The stator is run into the well on the bottom of the production
tubing, while the rotor is connected to the end of the rod string. When the
rotor is inserted into the stator, it creates a continuous seal line that
extends from the pump suction to discharge. This results in a series of
identical but separate cavities that progress from the pump suction to the
discharge at the end of the pump, as the rotor turns eccentrically in the
stator. One cavity opens as the other closes, creating a non-pulsating pumping
PCP delivers a constant flow that is
proportional 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 drive
Elastomer swell can occur either when the elastomer is
affected by production fluids or treatment chemical that are incompatible with
it (chemical swell) or due to an increase in temperature (thermal swell).
Chemical swell is generally caused by the elastomer coming
in contact with high API gravity crude oils or incompatible treating chemicals;
both of which have a fairly high percentage of aromatics. Chemical swell
generally permanent and nonreversible once the
Source of the swelling has been removed.
Thermal swell on the other hand is purely due to an
elevation in temperature resulting in the elastomer expanding due to this
increase in temperature. This expansion also results in an increase in the
interference fit between the rotor and stator thereby in an increase in
operating torque as well. Thermal swelling can be predicted and is not
permanent. Once the temperature decrease the thermal expression of the
elastomer also decreases.
A bond failure occurs when the bonding agent that keeps the elastomer
in the stator tube fails.
can occur at two interface is between the bonding agent and the elastomer. In
this instance, there is no elastomer test on the stator tube but there is
bonding agent. A view of the backside of the elastomer will show it to be
smooth. The stator tube will also be smooth with a gray or black color.