Page 389 - Air and Gas Drilling Manual

8-72 Air and Gas Drilling Manual
5. If the gas is dry and there are no sparks and no black smoke nor wet
samples at the surface, drill 5 to 10 ft and then raise and lower the drill
string to avoid a pressure increase due to mud rings. Continue drilling at
5 to 10 ft increments until the wet gas condition does not exist.
There is a new technology that allows underbalanced drilling operations to be
carried out using inert atmosphere. This new technology has allowed for the
development of an industrial sized filter system that strips most of the oxygen from
the compressed air output of the primary compressor [21]. The compressed inert
atmosphere gas either flows directly to the top of the drill string or via a booster
compressor to the drill string.
8.6 Drilling and Completion Problems
There are important drilling completion problems that are unique to air and gas
drilling operations. These require unique solutions.
8.6.1 Sloughing Shales
Since the drilling circulation fluid is not heavy, there is a constant threat of
caving and sloughing of the openhole borehole wall. Air and gas drilling operations
will have drilling penetration rates that can be twice that of mud drilling operations.
This faster drilling penetration rate is an important feature since openhole integrity is
very dependent upon the length of time the hole remains open and unsupported by
cement and casing.
When drilling with air and gas the shale sequences of rock formations are
usually the most susceptible to caving or sloughing. This is due mainly to bedding
layered texture of shale and the generally weak bonding between these layers. Thus,
when these shales are penetrated with a drill bit, the openhole wall surfaces of the
exposed shale formations tend to break off and the large fragments fall into the
annulus space between the openhole wall and the drill collar and drill pipe outside
surfaces. This sloughing of shale formations can be temporarily controlled by
injecting additional additives into water being injected into the circulation air or gas
(in addition to those given in Table 8.2). Table 8.3 gives the formula for these
additional additives. This formula has been successfully used in the San Juan Basin.
Table 8-3: Typical approximate additive weights or volumes per 20 bbls of water for
controlling sloughing shales (actual commercial product may vary).

Additives Weights or Volumes per 20 bbls of Water
Foamer 8.5 gallons
Bentonite 40 lbs
CMC 2 lbs
Corn Starch 5 lbs
Soda Ash 1 quart
8.7.2 Casing and Cementing
When drilling with air or gas, the borehole will be basically dry when the
borehole is cased and cemented. The well is not filled with treated water and the