Summary
Coupled flow and geomechanics play an important role in the analysis of
gas-hydrate reservoirs under production. The stiffness of the rock skeleton and
the deformation of the reservoir, as well as porosity and permeability, are
directly influenced by (and interrelated with) changes in pressure and
temperature and in fluid- (water and gas) and solid- (hydrate and ice) phase
saturations. Fluid and solid phases may coexist, which, coupled with steep
temperature and pressure gradients, results in strong nonlinearities in the
coupled flow and mechanics processes, making the description of system behavior
in dissociating hydrate deposits exceptionally complicated.
In previous studies, the geological stability of hydrate-bearing sediments
was investigated using one-way coupled analysis, in which the changes in fluid
properties affect mechanics within the gas-hydrate reservoirs, but with no
feedback from geomechanics to fluid flow. In this paper, we develop and test a
rigorous two-way coupling between fluid flow and geomechanics, in which the
solutions from mechanics are reflected in the solution of the flow problem
through the adjustment of affected hydraulic properties. We employ the
fixed-stress split method, which results in a convergent sequential implicit
scheme.
In this study of several hydrate-reservoir cases, we find noticeable
differences between the results from one- and two-way couplings. The nature of
the elliptic boundary value problem of quasistatic mechanics results in
instantaneous compaction or dilation over the domain through loading from
reservoir-fluid production. This induces a pressure rise or drop at early times
(low-pressure diffusion), and consequently changes the effective stress
instantaneously, possibly causing geological instability. Additionally, the
pressure and temperature regime affects the various phase saturations, the rock
stiffness, porosity, and permeability, thus affecting the fluid-flow regime.
These changes are not captured accurately by the simpler one-way coupling. The
tightly coupled sequential approach we propose provides a rigorous, two-way
coupling model that captures the interrelationship between geomechanical and
flow properties and processes, accurately describes the system behavior, and
can be readily applied to large-scale problems of hydrate behavior in geologic
media.
© 2012. Society of Petroleum Engineers
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History
- Original manuscript received:
11 December 2010
- Meeting paper published:
22 February 2011
- Revised manuscript received:
10 July 2011
- Manuscript approved:
25 August 2011
- Published online:
7 June 2012
- Version of record:
11 June 2012
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