Cohesive Strength of Gas-Hydrate-Bearing Marine Sediments

Ann Elizabeth Cook¹, David Goldberg¹, and R.L. Kleinberg^2
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 (C_o) can be related to the dynamic Young's modulus (E_D), where: C_o = 1.5*10^-3E_D. The dynamic Young's modulus is computed using in situ Vp, Vs, and bulk density borehole logs. The C_o profiles are compared to estimates of the in situ hydrate saturation, S_h, calculated using electrical resistivity logs and the modified Archie formula: S_h = 1 - (aR_w/Rp^m)^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, C_o 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 S_h > 15%, C_o increases dramatically, at least 200kPa greater than the general trend of the downhole gradient. This suggests that C_o is affected directly by S_h, and may be related to the rate of change in S_h (e.g. gradual or sharp) as a function of depth. Further study on the relationship between C_o and S_h may provide information on the growth habit of gas hydrates in sediment pore spaces.