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 can characterize the process of volume change of hydrate-bearing sediments under undrained conditions during shearing. However, the interrelationship between 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 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 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.

Undrained Shear Properties of Shallow Clayey-Silty Sediments in the Shenhu Area of South China Sea

“…In general, the pore pressure response rarely causes dramatically post-peak behavior in the slightly OC specimen (i.e., OCR = 2), and its effective path would essentially not move above the CSL; as a result of that, a specimen with OCR = 2 would not exhibit significant dilative behavior [9]. Furthermore, the negative pore pressure response only appears until the accumulated dilatancy is greater than the total contractive deformation in the sediment during undrained shearing [44]. Mayne and Stewart [22] suggested that there is a critical value of OCR greater than ~4 dictating whether a negative pore pressure is generated in the OC specimen.…”

Mechanical Properties of Gas Hydrate-Bearing Sediments Influenced by Multiple Factors: A Comprehensive Review of Triaxial Tests

“…It was found that the stress–strain curve of HBS was strain-hardening during the shearing process and generated excess pore water pressures (Figure ). , Under undrained conditions, the pore volume of the specimen remains unchanged during the shear, and the water in pore space cannot be drained, making it difficult to change the hydrate distribution mode from pore-filling to load-bearing types. , Thus, undrained conditions limit the contribution of hydrate particles to the strength of HBS. For HBS specimens, the pore pressure increases with the increase of axial strain first, and gradually decreases after reaching the peak strength .…”

Discrete Element Simulation of the Macro-Meso Mechanical Behaviors of Gas-Hydrate-Bearing Sediments under Dynamic Loading

“…The mechanical properties of hydrate sediments are governed by the hydrate distribution in addition to factors such as net confining pressure, saturation, and mid-major stress coefficient [30]. Hydrate in hydrate sediments exists in three main forms [31], as shown in Figure 3: (i) pore-filling (Figure 4a); (ii) acting as a sediment soil skeleton (Figure 4b), and (iii) cemented between soil particles in the form of colloidal material (Figure 4c).…”