A 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. Key Points: • Multistage triaxial tests are conducted on noncontact-cementing, hydrate-bearing sediments under drained conditions • The strength, dilatancy, and stiffness of the hydrate-bearing sediments are measured • The multistage test can be a viable and efficient method to measure the strength and dilatancy of the hydrate-bearing sediments
Establishing the geomechanical stability of marine sediments in the vicinity of a production well is one of the key design considerations in planning offshore gas production from marine hydrate reservoirs. This paper presents an assessment of the sediment stability at India's National Gas Hydrate Program, Expedition 2 (NGHP-02) Site 16 Area B offshore eastern India, for which gas production is to be carried out by depressurization. One important feature of the study is that extensive calibration of constitutive model parameters has been conducted based on laboratory test data from pressured core samples. From analysis perspective, the site is challenging because the hydrate reservoir consists of thin layers of hydrate-bearing sands interbedded with mud. Moreover, depressurization at the depth of a reservoir more than 2750 m below sea surface will lead to a pore pressure drop, and accordingly an effective confining stress increase as high as 25 MPa. In dealing with thin interbedded hydrate-bearing strata, meshing requirements for flow and geomechanical analysis are quite different from those for reservoirs with thicker massive layers, An axisymmetric model and one-way coupling simulations were thus adopted for this study, in which the geomechanical study utilizes pore pressure and hydrate saturation output from the flow study, but the flow study does not takes the porosity changes from the geomechanical analysis. Instead, the reduction of porosity due to sediment deformation in the flow study is based on a pressure-dependent pore compressibility relationship derived from geomechanical modeling. The rationality is validated through back computing the pore compressibility from the geomechanical deformation results. The study shows that large compression in the reservoir will result in movement of the sediments from above and below, as well as laterally in smaller magnitudes; and the sediment is deemed stable during the gas production period.
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