Numerical approaches to model and monitor geomechanical reservoir integrity
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Abstract
The emergence of new ideas to obtain energy from the earth subsurface and to store radioactive waste on it impels us to further investigate the response of geological media to forces and temperature variations.
For instance, injection of water into wells at high pressures to create fractures and increase permeability of rocks is a widely employed technique to enhance geothermal systems or gas reservoirs. Therefore, increasing our understanding about how such actions influence and are affected by the local stress state of the reservoir and how fractures propagate through it is necessary to efficiently develop extraction projects and, hopefully, to avoid undesired side effects.
Also, placing nuclear waste in repositories below the surface monitored by acoustic transmission of seismic signals is currently under investigation. Whereby, an optimal knowledge about how the heterogeneity of the medium and its changes are reflected on the seismic waves is necessary for the correct interpretation of the collected signals.
The physics involving these geological processes can be approximately expressed in a mathematical way through the laws of physics and the constitutive relationship of materials deriving in a set of partial differential equation whose complexity depends on the adopted physical model and the rheology of materials to be modelled. As, in most cases, the governing equations are not susceptible to analytical solutions, efficient numerical methods and software are required to simulate realistic problems.
We present, by a compilation of current theories, a general mathematical model describing the physics of poro-visco-elasto-plastic geological media.
Moreover, on the one hand, we present a computational massively-parallel 3D code to model fluid injection and crack propagation in poro-visco-elasto-plastic rheologies. We reproduce existing 2D benchmarks and give some examples of 3D cases. The results show the importance of the initial stress conditions on the development of failure zones and give a generalization of failure patterns generated from a local increase of pore pressure.
Furthermore, we perform simulations of hydraulic fracturing for Well KM-8, UK, showing the significant influence of the permeability of rocks and its changes after fracturing on the development of failure areas.
On the other hand, we use the wave propagation software Sofi2D to simulate the seismic monitoring of a deep circular backfilled tunnel with the aim of increasing our ability to understand path effects and, therefore, to infer the situation inside a nuclear waste repository over time. We apply our results on the Full-Scale Emplacement Experiment at the Mont Terri underground rock laboratory.