BAKER HUGHES - Drilling Fluids Reference Manual

PRESSURE PREDICTION AND CONTROL
Low Formation Fracture Gradients
Fracture gradients decrease as the water depth increases. This is a function of the reduction in
overburden pressure because the sediments are replaced by water. As a comparison, the
calculated fracture gradient at 3000 ft on land in the Gulf Coast is 13.6 lb m /gal in normally
pressured formations. At the same depth in 1000 ft of water the fracture gradient is 11.3
lb m /gal. This lower fracture gradient requires more casing strings to be run offshore and closer
attention to maximum allowable casing pressures.
Shallow Gas
The presence of shallow gas pockets in offshore sediments poses special problems to the
drilling operations. As discussed earlier, seismic methods are being used to detect these zones.
If shallow gas zones are detected in the area to be drilled, one method of dealing with the
problem is to drill small diameter pilot holes into the sands. The sands are penetrated and the
gas depleted from the zone through use of diverters at the surface.
If the shallow gas is not detected, it is essential that proper methods of diverting the gas at the
surface be available. Quick action is necessary to prevent the hole from being completely
unloaded of fluid. Another method used to handle shallow gas is to divert the flow to the sea
floor. Before this is done, careful analysis of the prevailing currents and winds in the area must
be conducted.
Kick Detection
The movement of the rig and large volumes of fluid in the riser causes kick detection to be
more difficult on floaters. It is important that the rig crew pay close attention to all abnormal
pressure and kick indicators, regardless of how small the change.
New developments in measurement-while-drilling (MWD) technology are aimed at detection
of gas influxes on a real-time basis. Currently, the information obtained from MWD tools
(resistivity and gamma ray data) lags behind the bit depth by the distance the tool is above the
bit, usually about 30 ft.
A technique has been developed which involves measuring the annular acoustic responses of
the MWD tool. A pressure transducer is installed on the bell nipple and continually monitors
the amplitude and character of the MWD pulses as they travel through the fluid. If gas from the
formation enters the wellbore, the physical properties of the fluid are altered, thereby changing
the way the acoustic signals travel through the fluid. If these changes can be measured and
detected at the surface, this would mean earlier detection of gas influxes.
Choke Line Friction
The riser choke lines, manifolding, stack port, and expansion lines all exert a pressure loss on
the well that varies according to choke line size and increases with water depth. The riser choke
line pressure loss is found by comparing the pressure loss while circulating through the riser
with the pressure loss while circulating through the kill line. The total back pressure on the
annulus when circulating through the choke line equals the casing pressure gauge reading plus
the choke line pressure loss. At the end of the kill procedure, additional pressure equal to the
total choke line friction will be imposed against the annulus. In situations where fracture
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 12-39