Shun Uchida

Explicitly Coupled Thermal Flow Mechanical Formulation for Gas-Hydrate Sediments

This paper presents an explicit time-marching formulation for the solution of the coupled thermal flow mechanical behavior of gas- hydrate sediment. The formulation considers the soil skeleton as a deformable elastoplastic continuum, with an emphasis on the effect of hydrate (and its dissociation) on the stress-strain behavior of the soil. In the formulation, the hydrate is assumed to deform with the soil and may dissociate into gas and water. The formulation is explicitly coupled, such that the changes in temperature because of energy How and hydrate dissociation affect the skeleton stresses and fluid (water and gas) pressures. This, in return, affects the mechanical behavior. A simulation of a vertical well within a layered soil is presented. It is shown that the heterogeneity of hydrate saturation causes different rates of dissociation in the layers. The difference alters the overall gas production and also the mechanical-deformation pattern, which leads to loading/ unloading shearing along the interfaces between the layers. Copyright

Numerical Study on Eastern Nankai Trough gas Hydrate Production Test

Copyright 2014, Offshore Technology Conference. The fully coupled methane hydrate model developed in Cambridge was adopted in this numerical study on gas production trial at the Eastern Nankai Trough, Japan 2013. Based on the provided experimental data of hydrate soil core samples, the soil parameters at Eastern Nankai Trough were successfully calibrated. With calibrated soil parameters and site geometry, a 50 days gas production trail was numerically simulated using the fully coupled simulator CMHGS (Cambridge Methane Hydrate Geomechanics Simulator). The geomechanical behavior of hydrate bearing sediments and production history results under 3 different depressurization strategies were explored and discussed. With the latest gas production site data, several input parameters for the numerical study were calibrated and an updated numerical simulation of the gas production test was carried out. The comparison of gas production history and vertical displacement along the wellbore between the updated simulation and the previous simulation results suggest large discrepancy, which highlights the importance of parametric study for numerical simulation of the gas hydrate production test. Therefore, parameter sensitivity of production history and vertical displacement were investigated and concluded the relative permeability curve; temperature profile, sea water salinity and permeability anisotropy all influence production results and mechanical responses.

Role of critical state framework in understanding geomechanical behavior of methane hydrate‐bearing sediments

A proper understanding of geomechanical behavior of methane hydrate-bearing sediments is crucial for sustainable future gas production. There are a number of triaxial experiments conducted over synthetic and natural methane hydrate (MH)-bearing sediments, and several soil constitutive models have been proposed to describe their behavior. However, the generality of a sophisticated model is questioned if it is tested only for a limited number of cases. Furthermore, it is difficult to experimentally determine the associated parameters if their physical meanings and significance are not elucidated. The objective of this paper is to demonstrate that a simple extension of the critical state framework is sufficient to capture the geomechanical behavior of MH-bearing soils from various sources around the world, while the significance of each parameter is quantified through variance-based global sensitivity analyses. Our results show that the influence of hydrates can be largely represented by one hydrate-dependent parameter, pcd′, which controls the expansion of the initial yield surface. This is validated through comparisons with shearing and volumetric response of MH-bearing soils tested at various institutes under different confining stresses and with varying degrees of hydrate saturation. Our study suggests that the behavior of MH-bearing soils can be reasonably predicted based on pcd′ and the conventional critical state parameters of the host sediments that can be obtained through typical geotechnical testing procedures.

In Situ Profiling of Soil Stiffness Parameters Using High-Resolution Fiber-Optic Distributed Sensing

AbstractThis paper explores the possibility of using high-resolution fiber-optic distributed sensing for in situ geotechnical estimation of soil shear modulus distribution with depth. It is shown that a recursive analysis of an elastic problem together with a measured vertical strain can assist in evaluating the sought stiffness values. It is suggested that high-resolution fiber-optic distributed sensing can provide the necessary strain for the proposed process. The approach was demonstrated in a field trial, entailing a stratified soil profile including a thin sand layer encapsulated between two clay layers. Results of the suggested profiling method are compared against data from a geophysical survey and against correlations with conventional in situ testing. Excellent agreement is exhibited between the different methods, indicating aptitude and viability of the idea for implementation as a geotechnical investigative tool.

Thermo-hydro-mechanical Sand Production Model in Hydrate-bearing Sediments

A better understanding of the behavior of hydrate-bearing sediments during gas extraction is a vital step towards realization of commercially viable gas production for the future. In 2007, the world first trial of gas production from hydrate-bearing sediments by depressurization method was conducted at the Mallik gas hydrate site, located in the Mackenzie Delta of Northwest Territories, Canada. However, the operation encountered a large amount of sand migration into the well, a phenomenon known as sand production, and thus was terminated after 24 hours. This incident highlights the importance of development of hydro-mechanical sand production model within hydrate-bearing sediments and understanding of the behavior of hydrate-bearing sediments with the effect of sand production during gas extraction. This extended abstract provides a formulation for the sand production including grain flow and hydraulic dispersion effect. A formulation is fully-coupled such that the sand production affects fluid pressures, saturations and temperature. In addition, the effective stress reduction due to grain detachment is incorporated. This results in further deformation of hydrate-bearing sediments, which may need to be considered for stability of the wellbore.

Thermo-Hydro-Chemo-Mechanical Formulation for CH4-CO2 Hydrate Conversion Based on Hydrate Formation and Dissociation in Hydrate-Bearing Sediments

Gas production from gas hydrate-bearing sediments has been attracting global interests because of its potential to meet growing energy demand. Methane (CH4) gas can be extracted from CH4 hydrates by depressurization, thermal stimulation or chemical activation. However, it has never been produced on a commercial scale and the past field trials faced premature termination due to the technical difficulties such as excessive sand flow into the well, a phenomenon known as sand production. One exception is the trial at the Ignik Sikumi, Alaska in 2012, which was conducted by chemical activation followed by depressurization. During the trial, initial sand production ceased after two weeks while CH4 gas production continued for five weeks. The mitigation of sand production is deemed attributed to mechanical or hydraulic effects through formation of CO2-rich gas hydrates. This incident has highlighted the favorable effect of CO2 hydrate formation and needs to incorporate the chemo-processes into existing thermo-hydro-mechanical formulations. This paper presents an analytical formulation to capture the coupled thermo-hydro-chemo-mechanical behavior of gas hydrate-bearing sediments during gas production via CO2 injection. The key features of the formulation include hydrate formation and dissociation, gas dissolution and multiphase flow for both CH4 and CO2, facilitating CH4-CO2 hydrate conversion.

Geomechanical Effect of Hydrate Dissociation-induced Stress Relaxation

The geomechanical behaviour of gas hydrate-bearing sediments is unique. Since gas hydrate exists as a solid in pores, it effectively densifies the host sand and bonds surrounding grains together. As a result, hydrate-bearing sediments exhibit stiffer, stronger and more dilatant behaviour than hydrate-free sediments. The uniqueness of hydrate-bearing sediments becomes more prominent during gas production. Unlike conventional oil and natural gas, gas production from hydrate-bearing sediments involves phase change of the gas hydrate from solid to gaseous. This implies not only that the aforementioned characteristics diminish accordingly to the remaining hydrate in pores, but also that the solid (i.e. hydrate) that has been carrying the effective stresses disappears, resulting in release of the effective stresses. The release of the effective stresses upon hydrate dissociation, referred to as hydrate dissociation-induced stress relaxation, causes stress redistribution as well as plastic deformation. Neglecting the stress relaxation term could therefore lead to inaccurate deformation prediction. This paper presents the formulation for hydrate dissociation-induced stress relaxation and demonstrates the importance of the term for an accurate wellbore deformation prediction.

In Situ and Laboratory Mechanical Characterization Using High-Resolution Fiber Optic Distributed Sensing

Distributed Sensing DTU Orbit (13/12/2018) In Situ and Laboratory Mechanical Characterization Using High-Resolution Fiber Optic Distributed Sensing This paper explores the potential use of high-resolution fiber optic distributed sensing technology for in situ moduli profiling and in laboratory element testing. In recent times, strain measurement using fiber optics has been employed in innovative civil engineering applications such as in the health monitoring of ageing infrastructures. Through recent developments, in particular Rayleigh backscatter optical frequency domain reflectometry technique, the fiber optic sensing technology is nowadays capable of providing continuous distributed strain measurement with a higher spatial resolution of the order of millimeters. As a result, the technology can potentially serve as a viable alternative to conventional strain gauges (i.e. high-spatial resolution yet localized measurement devices) or seismic geophysical measurement (i.e. distributed yet lowspatial resolution). This paper provides two examples of its applicability to both in situ and laboratory mechanical characterization.

Numerical Analysis of Wellbore Behaviour during Methane Gas Recovery from Hydrate Bearing Sediments

Copyright 2014, Offshore Technology Conference. Japan Oil, Gas, and Metals National Corporation (JOGMEC) conducted the first offshore methane hydrate production trial on March 2013, in the eastern Nankai Trough, Japan. Field work for the gas production trial began in early 2012. Coring and logging operations were conducted to prepare for the gas hydrate production trial. On March 12th, 2013, JOGMEC succeeded in extracting methane gas from the hydrate-bearing sediments by the depressurization method and approximately 120, 000 cubic meters of methane gas were produced totally. The production finally terminated due to the increase in sand production. It indicates the importance of evaluation the state of wellbore in order to produce methane gas safety and efficiently. It calls for an analysis to predict the stress changes and plastic strain evolution of wellbore during the life of well. Hence, a wellbore model, specifically designed for methane gas production, is necessary to assess the potential risks associated with the construction and production stage. This paper describes a finite element approach, which mainly takes into account of (a) all the typical processes associated with wellbore construction and gas production and (b) the interaction between cement-casing-formation. Based on this approach, a finite element model, which includes the layered soil profile, cement, casing, gravel and sand screen, was developed in ABAQUS to simulate the gas production trial at East Nankai Trough in March, 2013. The Methane Hydrate Critical State (MHCS) model (Uchida et al., 2012), which was developed to simulate the geomechanical response of the methane hydrate bearing sediment, was incorporated into this finite element model. It presents results of coupled ground deformation and pore fluid flow analysis of a gas production wellbore from a methane hydrate formation. The study focused on the short- and long-term mechanical behaviour of the critical physical components at and near the wellbore by simulating the construction processes of the wellbore as well as the production stage. The interaction between cement-casing-formation was examined in detail.

Numerical Study on Hydrate Bearing Sediments during Gas Production

The fully coupled methane hydrate model developed in Cambridge was adopted in this numerical study on gas production trial at the Eastern Nankai Trough, Japan 2013. Based on the latest experimental data of hydrate soil core samples, the clay parameters at Eastern Nankai site were successfully calibrated. With updated clay parameters and site geometry, a 50 days gas production trail was numerically simulated in FLAC2D. The geomechanical behaviour of hydrate bearing sediments under 3 different depressurization strategies were explored and discussed. The results from both axisymmetrical and plane-strain models suggest, the slope of the seabed only affects mechanical properties while no significant impact on the dissociation, temperature and pore pressure. For mechanical deformation after PT recovery, there are large settlements above the perforation zone and small uplift underneath the production zone. To validate the fully coupled model, numerical simulation with finer mesh in the hydrate production zone was carried out. The simulation results suggest good agreement between our model and JOE’s results on history matching of gas and water production during trial. Parameter sensitivity of gas production is also investigated and concluded the sea water salinity is a dominant factor for gas production.