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A GLOBAL OUTLOOK ON METHANE GAS HYDRATES
63
Drilling through gas-hydrate-bearing strata involves a variety of
potential technical and environmental challenges. For example,
some early exploration wells in the Arctic experienced shallow
gas flows and borehole stability problems, including abnormal
hole erosion and/or tight hole conditions (Collett and Dallimore
2002). The problems were linked mainly to the accepted drill-
ing practices in the 1970s and 1980s, which could cause signifi-
cant thermal and/or mechanical disturbance of the gas-hydrate-
bearing strata. This could potentially result in the release of free
gas and a significant reduction in sediment strength (Figure
3.2). Similar problems have been encountered in other settings
(Borowski and Paull 1997; Nimblett
et al.
2005). Industry has
3.3
DRILLING A GAS HYDRATE
PRODUCTIONWELL
Gas hydrate drilling and production problems
Gas release
Gas leakage
Gas
leakage
Collapsed casing
Production facilities
Open
borehole
Cased
borehole
Cased
borehole
Production of hot
hydrocarbons
Gas hydrate
Hydrate
Hydrate
Hydrate
Hydrate
Gas hydrate
Free
gas
Free
gas
Free
gas
Free
gas
Free gas
Free gas
Figure 3.2:
Potential drilling and production problems: The figure shows typical gas-hydrate-related drilling and production problems
encountered during drilling programs in the Arctic (from Collett and Dallimore 2002). Gas release scenario (left): over-pressured free gas
is encountered unexpectedly beneath a gas hydrate layer. Gas leakage scenario (centre): possible disturbance of gas hydrate by drilling
that has dissociated gas hydrate and caused free gas migration outside of the drill casing. Collapsed casing scenario (right): possible
disturbance of gas hydrate caused by conventional production of warm hydrocarbon at depth.