Jun Yoneda

A comparative analysis of the mechanical behavior of carbon dioxide and methane hydrate-bearing sediments

Abstract Understanding the mechanical behaviors of carbon dioxide/methane hydrate-bearing sediments is essential for assessing the feasibility of CO2 displacement recovery methods to produce methane from hydrate reservoirs. In this study, a series of drained triaxial compression tests were conducted on synthetic carbon dioxide hydrate-bearing sediments under various conditions. A comparative analysis was also made between carbon dioxide and methane hydrate-bearing sediments. The stress-strain curves, shear strength, and the effects of hydrate saturation, effective confining stress, and temperature on the mechanical behaviors were investigated. Our experimental results indicate that the newly formed carbon dioxide hydrate would keep the reservoir mechanically stable when CH4-CO2 gas exchange took place in a relatively short period of time and spatially well distributed in the pore space. Experiments of CO2 injection in methane hydrate-bearing sediments are necessary to confirm this hypothesis.

Bonding Strength by Methane Hydrate Formed among Sand Particles

The mechanical properties of methane hydrate‐bearing sand were investigated by low temperature and high confining pressure triaxial testing apparatus in the present study. The specimens were prepared by infiltrating the methane gas into partially saturated sand specimen under the given temperature and stress condition which is compatible with the phase equilibrium condition for the stability of methane hydrate. The tests were firstly performed to investigate the effect of temperature on the shear behaviour of the specimen. Then the effect of backpressure was investigated. The strength of methane hydrate bearing sand increased as the temperature decreased and the back pressure increased. The bonding strength due to methane hydrate was dependent on methane hydrate saturation, temperature and back pressure but independent of effective stress. Dissociation tests of methane hydrate were also performed by applying the temperature to the specimen at the various initial stress conditions. The marked development o...

Strengthening mechanism of cemented hydrate‐bearing sand at microscales

On the basis of hypothetical particle-level mechanisms, several constitutive models of hydrate-bearing sediments have been proposed previously for gas production. However, to the best of our knowledge, the microstructural large-strain behaviors of hydrate-bearing sediments have not been reported to date because of the experimental challenges posed by the high-pressure and low-temperature testing conditions. Herein, a novel microtriaxial testing apparatus was developed, and the mechanical large-strain behavior of hydrate-bearing sediments with various hydrate saturation values (Sh = 0%, 39%, and 62%) was analyzed using microfocus X-ray computed tomography. Patchy hydrates were observed in the sediments at Sh = 39%. The obtained stress-strain relationships indicated strengthening with increasing hydrate saturation and a brittle failure mode of the hydrate-bearing sand. Localized deformations were quantified via image processing at the submillimeter and micrometer scale. Shear planes and particle deformation and/or rotation were detected, and the shear band thickness decreased with increasing hydrate saturation.

Triaxial testing system for pressure core analysis using image processing technique

In this study, a newly developed innovative triaxial testing system to investigate strength, deformation behavior, and/or permeability of gas hydrate bearing-sediments in deep sea is described. Transport of the pressure core from the storage chamber to the interior of the sealing sleeve of a triaxial cell without depressurization was achieved. An image processing technique was used to capture the motion and local deformation of a specimen in a transparent acrylic triaxial pressure cell and digital photographs were obtained at each strain level during the compression test. The material strength was successfully measured and the failure mode was evaluated under high confining and pore water pressures.

Hydraulic fracturing in methane-hydrate-bearing sand

Knowledge of the fracturing behaviors in gas-hydrate-bearing sediments is essential to understand the accumulation mechanism of gas hydrates in fractured sediments and to apply hydraulic fracturing as a well stimulation method when considering gas recovery from gas hydrate reservoirs. We present an experimental study of hydraulic fracturing involving methane-hydrate-bearing sand formed in a triaxial pressure cell. The injection pressure rapidly increased after the start of distilled water injection from the core top through a small port, but suddenly decreased afterward. X-ray computed tomography revealed that laminar fractures, which were oriented in a plane perpendicular to the minimum principal stress, were generated after this pressure drop. The fracturing pressure was 2.9–3.9 MPa above the minimum principal stress. Although the host sediment was unconsolidated, the observed fracture behavior yielded a consolidated-rock-like fracturing mode, i.e., the tensile failure mode. It was affected by the low-permeable feature of hydrate-bearing sediments. The permeability was increased after fracturing and was maintained even after re-confining and closing the fractures. The results indicate that hydraulic fracturing is a promising well stimulation method for low-permeable gas hydrate reservoirs.