Page 62 - Air and Gas Drilling Manual

2-12 Air and Gas Drilling Manual
later for use in underbalanced drilling operations to recover oil and natural gas.
These recent applications have encouraged the development of rotating heads capable
of operating at higher pressures and temperatures. High pressure heads are used in
underbalanced drilling operations where light weight drilling mud (or other drilling
fluids) is used to drill through pressured oil or natural gas rock formations. The
light weight drilling mud (or other drilling fluids) allows oil and gas to flow into
the well bore as the drill bit advances into the rock formation. When these reservoir
fluids are circulated to the surface they impose high pressures on the wellhead
equipment. These higher pressure heads are capable of operating at pressures up to
approximately 1,500 psig (while rotating the drill string at about 100 rpm) and up to
approximately 3,000 psig (for the non-rotating drill string). The high temperature
rotating heads are generally used in geothermal drilling operations. Most of these
heads can operate with steam and hot water flows at temperatures up to about 500˚F.
High temperature rotating heads usually have a high pressure capability.
Rotating heads are also used in air and gas drilling operations where subsurface
high overpressured oil, natural gas, or geothermal fluids are not expected. These are
deep water well, deep monitoring well, deep mining borehole, and deep geotechnical
borehole drilling operations where double and triple drilling rigs are required. These
rotating heads are used to keep air or gas (in this case nitrogen) flow with entrained
rock cuttings from flowing to the rig floor (for direct circulation). But many of
these non-oil, natural gas, or geothermal recovery drilling operations utilize reverse
circulation. Reverse circulation requires that the compressed air or gas be injected
into the “outlet” of the rotating head to the annulus space of the borehole (see Figure
2-10). In this situation the rotating head still keeps the air or gas from flowing to
the rig floor.
Air and gas drilling operations using small single drilling rigs drill only
shallow (usually less than 1,000 ft in depth) water wells, monitoring wells, mining
boreholes, and geotechnical boreholes (see Figure 1-3). Some of the air drilling
operations on these small rigs use direct circulation, but most utilize reverse
circulation. These small single drilling rigs usually have hydraulic top head rotary
drives. The rig “floor” (the break-out platform) on these small rigs is protected from
cuttings returns by a rubber seal around the drill string and a flexible skirt around the
edge of the floor (skirt not shown in figure). When direct circulating, the air
returning up the annulus (with the entrained rock cuttings) is kept from coming
through the rig floor by the rubber seal around the drill string. The operator at the
control panel is protected only by the skirt around the edge of the floor. The drilling
cuttings are allowed to accumulate on the surface of the ground around the top of the
borehole where the skirt slows the air flow and allows the cuttings to be dropped
out.
Reverse circulation provides a useful way for dealing with the return flow of
compressed air and entrained rock cuttings from the borehole. The compressed air is
injected into the annulus of the borehole via a sealed fitting at the top of the
annulus, or a dual drill pipe annulus fitting. After circulating through the bit, the air
with entrained rock cuttings exits the borehole through the inside of the drill string,
then flows through the top head rotary drive, and then through the rotary hose. The
air with the cuttings can be diverted to a pit away from the rig with a hose
extension, or the hose extension run to a cyclone separator where cuttings samples