Analysis of the Characteristics of Pore Pressure Coefficient for Two Different Hydrate-Bearing Sediments under Triaxial Shear

Abstract

It is important to determine the volumetric change properties of hydrate reservoirs in the process of exploitation. The Skempton pore pressure coefficient A can characterize the process of volume change of hydrate-bearing sediments under undrained conditions during shearing. However, the interrelationship between A value responses and deformation behaviors remain elusive. In this study, effects of hydrate saturation and effective confining pressure on the characteristics of pore pressure coefficient A are explored systematically based on published triaxial undrained compression test data of hydrate-bearing sand and clay-silt sediments. Results show that there is a higher value of the coefficient A with increasing hydrate saturation at small strain stage during shearing. This effect becomes more obvious when the effective confining pressure increases for hydrate-bearing sand sediments rather than hydrate-bearing clayey-silt sediments. An increasing hydrate saturation leads to a reduction in A values at failure. Although A values at failure of sand sediments increase with increasing effective confining pressure, there are no same monotonic effects on clayey-silt specimens. A values of hydrate-bearing sand sediments firstly go beyond 1/3 and then become lower than 1/3 at failure even lower than 0, while that of hydrate-bearing clayey-silt sediments is always larger than 1/3 when the effective confining pressure is high (e.g., >1 MPa). However, when the effective confining pressure is small (e.g., 100 kPa), that behaves similar to hydrate-bearing sand sediments but always bigger than 0. How the A value changes with hydrate saturation and effective confining pressure is inherently controlled by the alternation of effective mean stress.