Page 390 - Air and Gas Drilling Manual

Chapter 8: Air, Gas, and Unstable Foam Drilling 8-73
casing is floated into the well (making use of buoyancy) as is done in mud drilling
operations. This presents some special problems for air and gas drilling operations.
When an openhole section of a gas drilling well is to be cased, the casing with a
casing shoe on the bottom is lowered into the dry well. A pre-flush of about 20
bbls of CMC (carboxymethyl-cellulose) treated water must be pumped inside the
casing just prior to pumping the cement. A diaphragm (bottom) plug is run ahead
of the CMC treated water pre-flush, another diaphragm (bottom) plug run behind the
CMC treated water pre-flush and ahead of the cement, and a final top plug run
behind the cement. Fresh water is pumped directly behind the top plug and fills the
inside of the casing to the surface. The CMC treated water pre-flush seals the surface
of the dry borehole walls prior to the cement and precludes the cement from
hydrating as it flows from the inside of the casing to the annulus between the
openhole and the outside of the casing. This CMC treated water pre-flush is
essential. If the pre-flush is not used the initial cement flowing through the casing
shoe to the untreated openhole of the borehole will immediately set-up and disallow
the remaining cement to flow to the annulus.
Once the casing and cementing operations are properly carried out and the
cement successfully sets up in the annulus, it is necessary to remove the water from
inside the casing in order to return to gas drilling operations (drill out the cement at
the casing shoe and continue drilling ahead). There are several safe operational
procedures that can be used to removing water from the inside of the casing after a
successful cementing operation.
Aerated Fluid Procedure
The aerated fluid procedure is as follows [22]:
1. Run the drill string made up with the appropriate bottomhole assembly
and drill bit to a depth a few tens of feet above the last cement plug.
2. Start the mud pump running as slowly as possible, to pump water at a
rate of 1.5 to 2.0 bbl/min. This reduces fluid friction resistance to the
moving fluids in the circulation system.
3. Bring one compressor and booster on line to aerate the water being
pumped to the top of the drill string. The air rate to the well should be
about 100 to 150 acfm per barrel of water. If the air volumetric flow rate
is too high, the standpipe pressure will exceed the pressure rating of the
compressor and the compressor will shut down. Therefore, the
compressor must be slowed down until air is mixed with the water
going into the drill string.
4. As the fluid column in the annulus (between the inside of the casing and
the outside of the drill string) is aerated, the standpipe pressure will
drop. Additional compressors can be added (i.e., increasing air
volumetric flow rate) to further lighten the fluid column and unload the
water from the casing.
5. After the hole has been unloaded, the water injection pumps should be
kept in operation to clean the borehole.
6. At this point, begin air or mist drilling. Drill out the cement at the
bottom of the casing and drill an additional 20 ft to 100 ft to allow any
sloughing walls of the borehole to clean up.
7. Once the hole has been stabilized, stop drilling and blow the hole with