BAKER HUGHES - Drilling Fluids Reference Manual

TABLE OF CONTENTS
Figure 13 - 4
Gas Hydrates in Seawater
(Phase Diagram)
Depth (km)
0
0.3
1
2
3
4
Methane Hydrate &
Sea Water - Stable
Methane Gas
& Sea W ater
- Stable
0 10 20 30
Temperature °C
Gas Hydrate Stability Curve
To the left is a curve representing the stability
of gas hydrates in sea water. Pressure and
temperature are two of the major factors
controlling where the hydrate (solid) or
methane gas will be stable. Whether or not gas
hydrate actually forms depends on the amount
of gas available.
IN-SITU HYDRATES
In-situ hydrates can exist at various depths below the seabed and may be encountered while drilling in
deepwater virtually anywhere in the world where there are sufficiently high pressures to favor hydrate
formation. Gas may exist below the in-situ hydrates.
Associated Problems
1. Maintaining competency of in-situ hydrates while drilling and cementing
2. Maintaining static well conditions while drilling and cementing in gas/water
3. Obtaining a competent cement job in a gas environment
Problem Resolution: It may be possible to drill the in-situ hydrate section without incurring hydrate
dissociation. Dissociation of the hydrates will lead to an enlarged hole section which will negatively
impact both the cement bond/hydraulic seal and lateral casing support. To maintain the in-situ
hydrates in a competent state, special attention needs to be paid to the annular temperature during
drilling, cementing and cement transitional period.
Drilling
• Simulations can be performed to predict fluid circulating temperatures inside the drill pipe
and in the annulus.
• If annular temperature does not exceed the hydrate equilibrium temperature, nothing more is
necessary.
• If annular temperature exceeds the hydrate equilibrium temperature, several options should
be explored:
• Increasing the mud weight, thus pressure in the annulus, if fracture pressure will allow
• Utilize lecithin to retard the rate of dissociation
• Reduce the target depth of the surface casing, so as to reduce the annular temperature
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 13-13