Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

CHAPTER 5.
GEOMECHANICAL SIMULATION OF CO 2 INJECTION
step the fluid flow and deformation are solved in an iterative manner, with data passed back and forth
between the simulations until a stable solution is found. This is illustrated in Figure 5.4. At each
time step the pore pressure and fluid properties are computed by the fluid flow simulator. These are
passed to the geomechanical simulator to compute deformation (using a number of sub-steps). The
changes in porosity are assessed for convergence, and if not yet converging, returned to the fluid flow
simulation to repeat the iteration. This method is more computationally expensive than the explicit
coupling, but produces more accurate results. Comparisons suggest that this method gives the same
result as fully coupled simulations (e.g., Longuemare et al., 2002), highlighting the accuracy of this
approach. The combination of accuracy with the ability to link successful commercial fluid flow and
geomechanical simulation packages means that the iterative coupling method is the method that I
will use. The computational speed of this approach has been greatly improved by Rockfield Software
through the development of a high speed message passing interface (MPI) to link between MORE and
ELFEN.
5.3.4 Workflow
Using the iteratively coupled MORE-ELFEN simulation, the workflow for developing simple geomechanical
models is as follows. The geomechanical mesh is constructed using the ELFEN pre-processing
GUI. The model must include both the reservoir and the over, under and side burdens, as these will
also be deformed by processes occurring in the reservoir. The GUI generates the input file for geomechanical
modelling. The cells are populated with mechanical properties as selected by the user based
on a large database of rock physics measurements. It is the geomechanical input file that controls
the simulation timesteps, convergence conditions, end times, and how often results are written to file.
The MORE input file is constructed by the user. The external dimensions of the reservoir in the
MORE model must match the reservoir as defined in the geomechanical input. However, they need
not have the same internal mesh, as ELFEN can interpolate between them. It is the MORE file that
defines the well locations and injection/production rates. Internal calculation timesteps are computed
by ELFEN are of a sub-day scale. However, to produce output files for each internal timestep would
lead to data overload. Therefore output files are generated by ELFEN at user-defined timesteps. In
my models I produce output files at 6 evenly spaced intervals during the simulations.
5.3.5 Simple representative models
In order to assess the sensitivity of the stress path parameters during injection to reservoir geometry
and material properties, I have developed a range of simple representative models, all consisting of
cuboid reservoirs contained within non-pay rocks (Figure 5.5). In order to decrease the computational
requirements, I simulate only a quarter of the system, relying on symmetry arguments to complete
the model.
The boundary conditions at the edges of the model are that there can be no movement perpendicular
to the vertical bounding planes at the edges of the model (i.e., no movement in the x direction of
the y − z plane) and no vertical movement at the base of the model. I include all of the overburden up
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