Although more than 1,000 exploration and development wells have been drilled through the zone of hydrate stability in the deepwater Gulf of Mexico, gas hydrate has not been documented as a serious drilling hazard. Acquisition parameters and survey design of 3D exploration seismic are not optimized for shallow sediments and do not allow accurate characterization of naturally occurring hydrate. Deep stratigraphic test wells drilled through hydrate deposits in spring 2005 will allow calibration of geophysical data and provide information of the impact of hydrate drilling and production on seafloor stability.
There has been considerable debate in recent years about drilling safety, long-term wellbore integrity, and foundation stability for subsea facilities during deepwater petroleum operations in gas-hydrate areas. With more than 1,000 Gulf of Mexico exploration and development wells drilled through the zone of hydrate stability, however, there are still no documented reports of significant drilling problems associated with gas hydrates. In most of these wells the shallow hydrate-bearing section to at least 2,000 feet below the seafloor is drilled riserless, and Logging While Drilling curves for the large-diameter shallow hole are the only available information.
Geohazard assessment of the entire lease block and hazards evaluation of the drillsite and associated anchor and chain positions must be completed, reviewed, and approved before drilling. In a few cases, anomalous amplitudes, seismic blanking, shingling, or a bottom-simulating reflector (BSR) indicate hydrated sediment on conventional or high-resolution seismic data, but to date shallow water flow (SWF) is the only deepwater geohazard with an important economic impact in this province. While SWF sands generally occur at drilling depths similar to gas hydrates, any association of these occurrences remains unproven.
Four pairs of 1,000- to 2,000-foot dedicated gas-hydrate wells with additional coring on an active gas-hydrate mound are planned by the Department of Energy Gulf of Mexico Gas Hydrate Joint Industry Project (JIP) at two locations this spring. Four wells in Keathley Canyon Block 151 will be drilled through gas hydrate above a BSR with a complete log suite and up to 30 percent core recovery. A similar drilling program in Atwater Blocks 13 and 14 will provide samples and information on the flanks of an active seafloor hydrate mound. Results of JIP hydrate drilling and other preliminary studies will yield critical ground truth data on the distribution and hazard potential of Gulf of Mexico hydrates.
To date, drilling through gas hydrates has not been considered a high-risk situation. The shallow section in the zone of hydrate stability in deep water, however, is drilled riserless as a large diameter hole with returns deposited on the seafloor. Fracture gradients in these sediments are low and there is a narrow margin between minimum and maximum mud weights that can be used. The deepwater conductor hole section is typically drilled using seawater and gel sweeps until wellbore instability or shallow water flow is encountered. Weighted waterbase drilling mud is then used to kill any flow or overcome hole stability problems.