Page 402 - Air and Gas Drilling Manual

9-6 Air and Gas Drilling Manual
incompressible drilling fluid is injected into the drill string and the compressed gas
injected into the annulus. This aerated drilling technique is also used to reduce the
drilling fluid pressures in the openhole of the annulus. In the early years of aerated
drilling, the technique was used to eliminate the threat of formation fracture (and the
eventual loss of drilling fluid). More recently, this technique has been used for
limited underbalanced drilling operations (usually in the geometry of parasite casing
drilling, or through completion drilling).
Figure 9-3 shows the schematic of parasite tubing string aerated drilling. This
configuration is carried out by attaching a small diameter tubing string to the outside
of the casing string that is installed just above the openhole. The casing string with
the parasite tubing string is placed and cemented into the well in the normal fashion.
Care must be taken to keep the tubing flow line open and clear while the cement is
placed around the casing. After the cement has set, the incompressible drilling fluid
can be direct circulated down the inside of the drill string to the drill bit, through the
drill bit, and up the annulus to the surface. As the incompressible drilling fluid is
circulating, the compressed gas is injected into the top of the tubing string. The
compressed gas exits the bottom of the tubing string into the annulus. In this
manner, the incompressible drilling fluid is aerated in the annulus from the depth
where the tubing string exits into the annulus to the surface.

Figure 9-3: Schematic of annulus injection aerated flow (parasite tubing string example).
An alternative annulus injection technique to that of using the parasite tubing
string is to utilize an annulus created by hanging a temporary casing string off the
inside of the last cemented casing string. This alternative technique would be carried
out in much the same manner as the parasite tubing string technique described
above.