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Cohesive Strength of Gas-Hydrate-Bearing Marine Sediments

Ann Elizabeth Cook1, David Goldberg1, and R.L. Kleinberg2
1 Columbia University, Palisades, NY
2 Schlumberger Doll Research, Ridgefield

We examine the relationship between gas hydrate saturation and the cohesive strength of marine sediments in a variety of continental margin settings. The cohesive strength (cohesion) is a fundamental physical property controlling sediment resistance to compressive failure. The cohesion (Co) can be related to the dynamic Young's modulus (ED), where: Co = 1.5*10-3ED. The dynamic Young's modulus is computed using in situ Vp, Vs, and bulk density borehole logs. The Co profiles are compared to estimates of the in situ hydrate saturation, Sh, calculated using electrical resistivity logs and the modified Archie formula: Sh = 1 - (aRw/Rpm)1/n. We will present results of these comparisons from data collected during: Ocean Drilling Program Legs 204 (at Cascadia margin) and 164 (at Blake Ridge); the JIP gas hydrate drilling project in the Gulf of Mexico; and Mallik permafrost wells. In general, Co steadily increases downhole as sediments compact due to overburden. In the marine sediment environments, cohesion ranges from 500-2000kPa above the BSR, with a baseline gradient between 5 and 10 kPa/m. Preliminary results at Cascadia margin show that in sediments with Sh > 15%, Co increases dramatically, at least 200kPa greater than the general trend of the downhole gradient. This suggests that Co is affected directly by Sh, and may be related to the rate of change in Sh (e.g. gradual or sharp) as a function of depth. Further study on the relationship between Co and Sh may provide information on the growth habit of gas hydrates in sediment pore spaces.