Page 216 - Air and gas Drilling Field Guide 3rd Edition

8.4 Water Injection and Formation Water Influx 207

appropriate volumetric flow rate of fresh water to the well to eliminate stickiness
is a field trial and error process.
The actual volumetric flow rate of injected fresh water into the standpipe dur-
ing a drilling operation will be very dependent on the type of rock being drilled.
Certain rock formations when drilled will create more rock flour, which will in
turn create more well stickiness problems (i.e., mud rings). The worst formations
are dry shale, limestone, and dolomites.
The typical field procedure used to attain the proper water injection flow rate
is to watch the surface air standpipe pressure gauge as water injection progresses.
Stickiness will usually manifest itself as mud rings and will be seen on the stand-
pipe pressure gauge as sharp increases in pressure. The pressure increases can be
2
as little as 5 or 10 psi (3 to 7 N/cm ) and as high as 20 or 30 psi (15 to 20 N/
2
cm ). As the water flow rate is being injected into the well (and the drill string
is periodically raised and rotated at a higher speed to break up the mud rings),
the injected flow rate is increased until the standpipe pressure drops back to near
normal. The pressure gauge will always read a slightly higher pressure than
before water injection, as the column of air and water in the annulus will be
heavier than the previous air column. For the example well described in Illustra-
tive Example 8.3, the typical fresh water injection to the standpipe might be
approximately1 to 2 bbl/hr.
Once the appropriate fresh water injection flow rate has been field determined,
the drilling operation can go forward and the well can be advanced into rock for-
mations that have formation water influxes. This brings up the problem of how
much formation water a typical air drilled well can carry. Figure 8-12 shows results
from the Mathcad solution for Illustrative Example 8.3 and shows the critical annu-
lus minimum kinetic energy value as a function of depth for two possible formation
water influx volumetric flow rates. For a 10-bbl/hr influx, Figure 8-12 shows that
the annulus minimum kinetic energy drops as the open hole section is drilled from
7000 ft (2134 m)to 10,000 ft(3048 m). However, the situation is worst for a
20-bbl/hr influx (the annulus minimum kinetic energy is lower all along the open
hole section). In fact, for the influx of 20 bbl/hr, the minimum kinetic energy drops
3 3
below 3.0 lb/ft/ft (143.7 N-m/m ) at a drilling depth of approximately 9200 ft
(2804 m). This means that cuttings removal as drilling approaches this depth will
be impaired, resulting in a variety of bottom hole problems.

8.4.3 Suppression of Hydrocarbon Ignition
The next higher level of water injection volumetric flow rate is the volume
needed to suppress the ignition of downhole explosions and fires due to the mix-
ture of circulation air with produced oil, natural gas, or coal dust and fragments as
the drill bit is advanced. When drilling into rock formations that are coal seams,
or reservoir rock containing oil or natural gas, the steel drill bit action on the rock
cutting face can easily cause a spark. If the circulation gas is air, then the three
ingredients for downhole ignition are present (i.e., hydrocarbons, a spark, and
an oxygen source). Increasing the water injection volumetric flow rate (with addi-
tives) to the borehole and creating unstable foam at the bottom of the well can