BAKER HUGHES - Drilling Fluids Reference Manual

PRESSURE PREDICTION AND CONTROL
gradients are low, it may be necessary to take special precautions to avoid breaking down a
formation.
Riser Collapse / Displacement / Trapped Gas
After formation gas has been circulated from the wellbore through the choke lines and the well
is dead, a small volume of gas can be trapped in the sub-sea BOP between the choke line and
uppermost closed BOP. If this trapped gas is not removed before opening the stack, its
expansion can deplete the riser of fluid and cause another kick, collapse of the riser, or both. It
is critical that before the BOPs are opened, (1) the riser has been displaced with kill fluid and
(2) trapped gas has been removed from the BOP stack. A technique that has been proven
successful is discussed by Shaunessy, et al.
Gas Hydrates
Gas hydrates are an ice-like mixture of natural gas and water. Their formation is a function of
pressure and temperature. At low temperatures (30°F to 40°F), hydrate formation can occur at
low pressures (40 to 400 psi). In the deepwater depths being drilled in the Gulf Coast,
temperatures at the sea floor can be as low as 40°F. There have been several instances while
killing kicks where the formation of gas hydrates has plugged choke lines and BOP stacks,
making well control difficult or impossible.
The suppression of hydrate formation can be accomplished by increasing the chloride
concentration of the drilling fluid. Fluid systems with chlorides in the 150,000 mg/L (≈ 21% by
wt.) range will suppress hydrate formation by 26°F. These fluid systems have been used to
prevent the formation of gas hydrates. A discussion of the components of these systems can be
found in Chapter 1, Fundamentals of Drilling Fluids.
Other potential hydrate suppressors are methanol, glycol, and oil. Problems with the use of
these additives relate to their toxicity levels and their use may be prohibitive because of
handling and disposal requirements. Another option is to insulate and heat the subsea well
control equipment but, at present, the technology to do this has not been developed.
Commonly Used Equations in Pressure Control
Hydrostatic Pressure (psi)
Pressure Gradient (psi/ft)
Hydrostatic Pressure = 0.052 x MW 1 x TVD
Pressure Gradient = MW 1 x 0.052
Pressure Expressed as Equivalent Fluid (Mud) Weight (EMW)
EMW =
0.052P s
× TVD
+ MW 1
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 12-40