Strain-Softening Model for Hydrate-Bearing Sands

Abstract

The extraction of methane gas from hydrate-bearing sediments has garnered increasing global interest in recent years. Understanding the sediment response to potential production scenarios is vital for both accurate reservoir response simulation as well as the development of field extraction methodologies. Gas hydrate has an icelike structure, and when present within sediments it will significantly alter their geomechanical behavior. Of particular interest in this paper is the observed strain-softening response of hydrate-bearing sands during drained shearing. Experimental results indicate that the strain softening during shearing is related to deviatoric behavior. A new analytical strain-softening model is proposed in which a direct relationship between volumetric expansion and reduction in hydrate saturation is developed. In the proposed model, the apparent cohesion of hydrate-bearing sediment reduces with increasing plastic shear strain. To validate the model, it was implemented into a standard numerical code, and the results were compared with those obtained from an advanced hydrate-soil numerical simulator. The model is also compared with experimental results from drained shear tests conducted on hydrate-bearing sands. The deviatoric stress results of the analytical model and numerical simulation were found to be almost identical, indicating the validity of the simple analytical formulation. In addition, the analytical model showed good agreement with the experimental results.