SEISMIC PREDICTION AND OPERATIONAL RESULTS FOR GULF OF MEXICO WELLS IN GAS-HYDRATE AREAS

Michael A. Smith*, William Kou, Adnan Ahmed, and Robert Kuzela
*Minerals Management Service, New Orleans, LA

Although more than 500 exploration 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. Seafloor hydrate mounds associated with active gas vents are commonly found at the edges of minibasins and, like chemosynthetic communities, are avoided during drilling operations. There is considerably less available information on the distribution, concentration, reservoir properties, and stability of subsurface hydrate. 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. At some Gulf of Mexico locations, a bottom-simulating reflector (BSR) may indicate the base of the hydrate stability zone. As geohazard assessment geophysicists become more aware of the potential of gas hydrate to cause seafloor or wellbore instability, detailed delineation of gas-hydrate zones and their relationship to free gas pockets will become more common.

Our understanding of gas-hydrate settings is further limited because the shallow section is drilled riserless as a large diameter hole with returns deposited on the seafloor. This drilling methodology makes it impossible to acquire samples of hydrated sediment and only a few Measurement While Drilling log curves are obtained. MMS and seven major oil and service companies in the Department of Energy Gulf of Mexico Gas Hydrates Joint Industry Project (JIP) have developed protocols, models, and laboratory tests designed to improve seismic imaging of gas hydrate zones. The JIP has selected locations near sampled seafloor hydrate mounds in the Atwater and Keathley Canyon areas for drilling, logging, and coring several 1,000- to 2,000-ft deep stratigraphic test wells through the hydrate deposits in Spring 2005. These wells will allow the first real calibration of geophysical data for characterizing buried gas hydrates and will provide critical data on the impact of hydrate drilling and production on seafloor stability.