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 © 2013 Society of Petorlleum Engineers.

Direct numerical simulation of turbulent flows over superhydrophobic surfaces with gas pockets using linearized boundary conditions

Superhydrophobic surfaces are shown to be effective for surface drag reduction under laminar regime by both experiments and simulations (see for example, Ou and Rothstein, Phys. Fluids 17:103606, 2005). However, such drag reduction for fully developed turbulent flow maintaining the Cassie-Baxter state remains an open problem due to high shear rates and flow unsteadiness of turbulent boundary layer. Our work aims to develop an understanding of mechanisms leading to interface breaking and loss of gas pockets due to interactions with turbulent boundary layers. We take advantage of direct numerical simulation of turbulence with slip and no-slip patterned boundary conditions mimicking the superhydrophobic surface. In addition, we capture the dynamics of gas-water interface, by deriving a proper linearized boundary condition taking into account the surface tension of the interface and kinematic matching of interface deformation and normal velocity conditions on the wall. We will show results from our simulations predicting the dynamical behavior of gas pocket interfaces over a wide range of dimensionless surface tensions.

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.