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Prediction of stress and strain for the seabed and production well during methane
hydrate exploitation in turbidite reservoir
Prédiction de stress et déformation pour le fond de la mer et de puits pendant l’exploitation
d'hydrate de méthane dans le réservoir du turbidité
Yoneda J.
National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan
ABSTRACT: During MH production, there are concerns about the settlement of the seabed and the possibility that negative friction
will occur along production well due to change in effective stress induced by water movement due to depressurization, dissociation of
MH, methane gas generation and thermal change, which are all inter-connected. The authors have developed a multi phase coupled
simulator using finite element method named COTHMA. Stresses and deformation of methane hydrate vicinity production well and
the deep seabed ground were predicted which simulating NANKAI trough where is planned as Japan's first offshore production test
area.
RÉSUMÉ : Pendant la production de gaz d'hydrate de méthane, il peut y avoir des phénomènes de tassement du fond marin et de
frottement négatif autour du puits. Ces problématiques sont dues à des changements de la contrainte effective induits par un ensemble
de phénomènes couplés : écoulements d'eau dus à la dépressurisation, dissociation des hydrates de méthane, génération de méthane et
changements thermiques. Les auteurs ont développé un outil de simulation numérique multiphasique par éléments finis, nommé
COTHMA. Les contraintes et les déformations au voisinage du puit de production et du fond marin en profondeur ont été simulées
dans le cadre du site de NANKAI qui est prévu pour être le premier site de test de production off-shore du Japon.
KEYWORDS: methane hydrate, effective stress, deformation, finite element, multi phase
1 INTRODUCTION
Recent investigations have indicated that methane hydrate (MH)
could become a potential future energy resource. In the MH
extraction project, a well is drilled into the sea floor from a
marine platform. Then, fluids in the well are either heated or
depressurized to induce MH dissociation and the dissolution of
methane gas is collected in-situ. During MH production, there
are concerns about the settlement of the seabed and the
possibility that negative friction will occur along production
well due to change in effective stress induced by water
movement due to depressurization, dissociation of MH,
methane gas generation and thermal change, which are all inter-
connected. The authors have developed a multi phase coupled
simulator
using
finite
element
method
named
COTHMA(Coupled thermo-hydro-mechanical analysis with
dissociation and formation of methane hydrate in deformation
of multiphase porous media). Reliability of the simulator is
being established by predicting experimental model tests and
performing parametric study (Sakamoto, 2010). In this paper,
predict stresses and deformation of methane hydrate vicinity
production well and the deep seabed ground which simulating
NANKAI trough where is planned as Japan's first offshore
production test area.
In this study, production well was constructed in the simple
geological model of NANKAI trough for evaluating the strain
of the seabed and stress which apply to the well by
depressurization. Joint elements were used for the interface
between different material, casing-cement-soil. And stress
changing have been evaluated during methane hydrate
production by calculating stresses of soil and cement along the
water depth.
2. CIRCUMFERENCE ENVIRONMENT OF THE WELL
2.1. Geological property of MH reservoir
Methane hydrate reservoir which is targeted in Japan's first
offshore production test is the sediment called turbidite. It has
changed from sand to mud gradually in 50cm and hundreds of
layer have overlapped with it. Methane hydrate bearing in this
sand layer. These methane hydrate reservoir exist under
hundreds meter overburden. In this research, the seabed ground
is targeted where hydrate reservoir has the mud layer for the cap
lock.
2.2. Composition of MH Production well
The production well will be constructed into the shallow
sediments in methane hydrate exploitation besides the oil
engineering. However, the production well must have bearing
capacity for heavy production equipment and itself. In drilling
process, firstly, the strata are drilled with muddy water.
Secondary, casing pipe is putted into borehole. Then, muddy
water between strata and casing pipe replace with cement slurry
from bottom of the casing pipe to the top of the well. Finally,
bearing capacity of this production well increase with cement
hardening.
3. SIMULATION METHOD
Basic Features of Multiphase coupled analysis is as follows.
・
Analysis of complex processes on multi-phase (Solids,
liquid and gases).
・
Disregard the flow of solid phases (soil, hydrate and ice).
・
Treatment of ice solidification/melting.
・
Treatment of MH dissociation/re-formation.
・
Consider about mass change of each phase due to methane
hydrate dissociation and re-formation.
Prediction des contraintes et des déformations pour le fond de la mer et pour le puits pendant
l’exploitation d’hydrates de méthane dans un réservoir de turbidite
