We modify the Modified Cam-Clay (MCC) model for large stress ranges encountered in geological applications. The MCC model assumes that the friction angle (ϕ) and the slope of the compression curve (λ) of a mudrock are constant and thus predicts constant values for the lateral effective stress ratio under uniaxial, vertical strain (K0) and undrained strength ratio (S_u/(σ_v^’ )). However, experimental work shows that λ, ϕ, and S_u/(σ_v^’ ) decrease and K0 increases substantially with stress over large stress ranges (e.g., up to 100 MPa). We incorporate the stress dependency of λ and ϕ into the MCC model and use the new model to predict S_u/(σ_v^’ ) and K0 ratios. The modified model, with only one additional parameter, successfully predicts the stress dependency of these ratios. We encode the modified model and use it for finite-element analysis of a salt basin in the deepwater Gulf of Mexico. The stresses that the new model predicts around salt differ significantly from those predicted using the original model. We incorporate the stress dependency of the friction angle into the analytical models developed for critical tapers, wellbore drilling, and the stability of submarine channel levees. We show that the decrease of the friction angle with stress 1) results in a concave surface for critical wedges, 2) shifts the drilling window to higher mud weights and makes it narrower for a vertical wellbore, and 3) causes deep-seated failure of submarine channel levees at lower angles. Our study could improve in situ stress and pore pressure estimation, wellbore drilling, and quantitative understanding of geological processes.

The flow of gas through shallow marine sediments is an important component of the global carbon cycle and affects methane release to the ocean and atmosphere as well as submarine slope stability. Seafloor methane venting is often linked to dissociating hydrates or gas migration from a deep source, and subsurface evidence of gas-driven tensile fracturing is abundant. However, the physical links among hydrate dissociation, gas flow, and fracturing has not been rigorously investigated. We used mercury intrusion data to model the capillary drainage curves of shallow marine muds as a function of clay content and porosity. We combined these with estimates of in situ tensile strength to determine the critical gas saturation at which the pressure of the gas phase would exceed the pressure required to generate tensile fractures. Our work demonstrates that tensile fracturing is more likely as clay content increases due to decreased pore sizes and increased capillary pressure, but tends to be restricted to the shallowest portion of the sediment column (<130 m below seafloor) except when the clay-sized fraction exceeds 50%. Dissociating hydrate may supply sufficient quantities of gas to cause fracturing, but this is only likely near the updip limit of the hydrate stability zone, where release of methane bubbles from discrete vents is to be expected due to the combination of weak sediments and significant gas expansion. Gas-driven tensile fracturing is probably a common occurrence near the seafloor, does not require much gas, and is not necessarily an indication of hydrate dissociation.

In the last 20 years, there has been an international effort to develop approaches to experimentally measure the petrophysical and geomechanical properties of hydrate-bearing core samples. The measurements are extremely challenging because sub-sampling, sample preparation, and testing must be conducted at high pressure and low temperature. Despite these challenges, multiple laboratories are now measuring the geotechnical properties of hydrate-bearing sediments. However, there have been relatively few attempts to validate these measurements. We developed experimental protocols to accurately conduct zero-lateral strain tests at effective stresses up to 20 MPa using a pressure core triaxial device. We directly measure displacement during compression through periodic instantaneous undrained loading. To evaluate the accuracy of our measurement system, we conducted a benchmark study to compare properties obtained in our pressure core test chamber against classical geotechnical devices. We prepared a Boston Blue Clay specimen through re-sedimentation. Comprehensive properties databases favor the use of this material for comparison analyses. A compression test to 20 MPa accurately reproduced the compression, lateral stress, and permeability behavior demonstrated in previous testing programs. This experimental procedure provides a convenient framework for future validation studies in a broad range of pressure core laboratory devices.