FROZEN HEAT | Volume 2

largely overcome these problems by introducing modifications to the drilling procedures and equipment, including: • Chilling the drill mud to reduce thermal disturbance of the formation; • Managing the weight of the drill mud to achieve sufficient downhole pressure to stabilize the in situ gas hydrates,

while remaining below the pressures that might fracture downhole formations; • Using chemical additives (or avoiding dissociation-induc- ing inhibitors such as salts and alcohols) in the drill mud to maintain gas hydrate stability in the formation and pre- vent gas hydrate dissociation in the drill cuttings;

The major advances in understanding the drilling behaviour of in situ gas hydrates have come through field programs dedicated specifically to the study of gas hydrates. Substantial resources have been devoted to gas hydrate research and development over the past several decades (Collett et al. 2009), with more than 100 dedicated gas hydrate wells successfully drilled to date. Several Integrated Ocean Drilling Program (IODP) expeditions have investigated gas hydrate occurrences along active and passive continental margins. Multi-well exploration campaigns have been undertaken by national gas hydrate research programs in Canada, China, India, Japan, Korea, and the United States. In addition, dedicated research and development programs Box 3.1 Gas hydrate coring and drilling studies

conducted offshore Japan and in permafrost settings in Canada and Alaska have tested the effectiveness of gas hydrate drilling, coring, and cementing technologies. The Japanese program included a short horizontal well in gas-hydrate-bearing strata 350 metres below the sea floor (Takahashi and Tsuji 2005). At the Mallik site in the Canadian Arctic, a full-scale thermal production test was completed in 2002 (Dallimore and Collett 2005). Gas hydrate production by depressurization of the reservoir was tested there during successive winter programs in 2007 and 2008 (Dallimore et al. 2012). In Alaska in 2012, an advanced production test program involving carbon dioxide injection and pressure drawdown was completed (Schoderbek et al. 2012).

Selected gas-hydrates study areas

Prudhoe Bay area

Svalbard

Mallik test site

Messoyahka

Qilian Mountains

Cascadia Margin

Japan Sea

Ulleung Basin

Northern Gulf of Mexico

Blake Ridge

Eastern Nankai Trough

Shenhu Basin

Taiwan

Mexico

Indian Ocean

Costa Rica

Gumusut- Kakap

Peru

New Zealand

Figure TB-3.1: Notable gas hydrate field programs. This is a general representation of notable hydrate field programs that have been or are taking place around the world. The compilation is indicative and does not necessarily depict all hydrate field programs.

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