Multistage Triaxial Tests on Laboratory‐Formed Methane Hydrate‐Bearing Sediments

Abstract

AbstractA sound understanding of the geomechanical behavior of hydrate‐bearing sediments (HBSs) is essential not only for assessing reservoir and wellbore stability during methane gas production but also for projecting the impact of global warming on the stability of geological settings that contain hydrates. This study experimentally investigated the geomechanical responses of laboratory‐formed methane‐HBS to triaxial shearing under drained conditions. The hydrate pore habit formed in the sediments is noncontact‐cementing, which is different from contact‐cementing or grain‐coating hydrate pore habits formed when the excessive gas method is used. The multistage triaxial test method, which shears a single specimen under different confinements, was employed to examine its applicability in measuring geomechanical properties of the HBS. Conventional single‐stage triaxial tests were also performed to serve as baselines. The results show that both the strength and stiffness of HBS increase with increased hydrate saturation and confining stress. Shear dilatancy increases with increased hydrate saturation and decreased confining stress. The peak strength and peak/postpeak shear dilation measured using the multistage tests are comparable to those using the single‐stage tests. However, the stiffness measured in later stress stages in the multistage tests was enhanced by the stress‐strain history of earlier stages. Therefore, the multistage test offers an efficient way of measuring the strength and volumetric response of the HBS with a much smaller number of specimens than the single‐stage test. This benefits the geomechanical characterization of the HBS, as obtaining the pressure cores is extremely costly and preparing lab samples is usually sophisticated and time consuming.