The progress of certain geological activities like the geological storage of CO2, the enhanced geothermal energy or the hydraulic fracturing operations has revealed the lack of knowledge in the field of induced seismicity and the propagation of the fractures. In fact, sometimes, after shut-in seismic events take place and the pattern of fracture
growth is unclear. In order to perform these activities in an effective and safe manner it is necessary to discover what parameters are
key in the process of fracture propagation and induced seismicity. This is the main goal of this work (developed as a part of the EU - FracRisk project) which consists on the development of a new thermo-hydro-mechanical code with a new method to solve shear and tensile failure of fractures. This new method is focused in hydraulic fracturing operations which take place in hard, fragile rocks where it is possible to assume an elastic matrix and to impose irreversible displacements in fractures when rock failure occurs. Hence, the formulation used to simulate shear and tensile failure is based on the analytical solution proposed by Okada (1992) and it is part of an iterative process. The analytical solution allows to avoid numerical issues and to calculate the failure in a straightforward manner, reproducing the possible avalanche of consecutive slip events. In conclusion, the aim of this work is to investigate the most relevant parameters in the thermo-hydro-mechanical coupling process that take place in rock failure and induced seismicity by means of the new code and the new failure method that has been developed.
Non-isothermal fluid injection into a geological formation causes alterations of the pressure and temperature fields, which affect the mechanical stability of the reservoir. This coupled thermo-hydro-mechanical behavior has become a matter of special interest because of public concern about induced seismicity. The response is complex and its evaluation often requires numerical modeling. Nevertheless, analytical solutions are useful in improving our understanding of interactions, identifying the controlling parameters, testing codes and in providing a rapid assessment of the system response to an alteration. We present analytical expressions for hydraulic and thermal driven displacements and stresses for unidirectional and radial geometries. To obtain them, an easy-to-use solution to the transient advection-conduction heat transfer problem is developed, whereas the fluid flow is assumed at steady-state. The validity is verified by comparison with numerical simulations and yields fairly accurate results. The solution is then used to illustrate some features of the poroelastic and thermoelastic response and, in particular, the sensitivity of stresses to the outer mechanical boundary conditions.
Coupled thermo-hydro-mechanical modeling is essential for CO2 storage because of (1) large amounts of CO2 will be injected, which will cause large pressure buildups and might compromise the mechanical stability of the caprock seal, (2) the most efficient technique to inject CO2 is the cold injection, which induces thermal stress changes in the reservoir and seal. These stress variations can cause mechanical failure in the caprock and can also trigger induced earthquakes. To properly assess these effects, numerical models that take into account the short and long-term thermo-hydro-mechanical coupling are an important tool. For this purpose, there is a growing need of codes that couple these processes efficiently and accurately. This work involves the development of an open-source, finite element code written in C ++ for correctly modeling the effects of thermo-hydro-mechanical coupling in the field of CO2 storage and in others fields related to these processes (geothermal energy systems, fracking, nuclear waste disposal, etc.), and capable to simulate induced seismicity. In order to be able to simulate earthquakes, a new lower dimensional interface element will be implemented in the code to represent preexisting fractures, where pressure continuity will be imposed across the fractures.