Hydro-mechanical effects of seismic events in crystalline rock barriers

Abstract. Under ideal conditions, owing to its extremely low matrix permeability, crystalline rock can constitute a suitable hydro-geological barrier. Mechanically, its high strength and stiffness provide advantages when constructing a repository and for long-term stability. However, crystalline rock usually occurs in a fractured form, which can drastically alter hydromechanical (HM) barrier functions due to increased permeability and decreased strength. Seismic events have the potential to alter these HM properties by activating faults, increasing their transmissibility, creating new fractures or altering network connectivity (De Rubeis et al., 2010). Therefore, it is of high importance to build computational models to allow assessment of the HM effects of seismic events in a Deep Geologic Repository (DGR) in crystalline rock, as illustrated in Fig. 1. For this purpose, we consider a DGR in Russia (Yeniseysky site) for high-level waste in crystalline rock (Proterozoic and Archaean gneiss complexes) that is located close to a potentially seismically active area (Jobmann, 2016). Here, we present a coupled HM simulation, using OpenGeoSys (Kolditz et al., 2012), of a large-scale, three-dimensional finite-element model of the Yeniseysky site to assess the consequences of seismically induced stress-field changes on the local stress field and the fluid flow. This research also provides an outlook of current model development geared towards a more detailed assessment of seismically induced hydro-mechanical processes in porous and fractured rocks.



MOTIVATION
Under ideal conditions, owing to its extremely low matrix permeability, crystalline rock can constitute a suitable hydro-geological barrier.Mechanically, its high strength and stiffness provide advantages when constructing a repository and for long-term stability.However, crystalline rock usually occurs in a fractured form which can drastically alter hydro-mechanical (HM) barrier functions due to increased permeability and decreased strength.Seismic events have the potential to alter these HM properties by activating faults, increasing their transmissibility, creating new fractures or altering network connectivity [1].Therefore it is of high importance to build computational models allowing to assess the HM effects of seismic events in a Deep Geologic Repository (DGR) in crystalline rock, as illustrated in Figure 1.The Noordbergum effect and the Mandel-Cryer effect are well-known examples of HM coupling [2] and we expect similar patterns in the model of an exemplaric storage site for nuclear waste.Particularly, we focus on the consequences of the time-dependent pressure and stress fluctuations after a change of the stresses in the ground.Such stress changes and stress redistributions may occure after earthquakes [3]- [5] and affect even distant areas.
Here, we present a coupled HM simulation, using OpenGeoSys [6], of a large-scale, three-dimensional finite-element model of the Yeniseysky Site (YS) in Russia to assess the consequences of seismically induced stress-field changes on the local stress field.
Fig. 1: Most relevant events for the integrity of a DGR in crystalline rock (i) Cap-rock failure due to seismic events, (ii) focused flow escapes from the aquifer, (iii) flow-induced fractures propagate generating a heterogeneous porosity-permeability field, (iv) Changing climate/permafrost impacts [7].

MODEL
As an example, we consider YS [8] for high-level waste in crystalline rock which is located close to a potentially seismically active area.

Geology and Hydrology
The site is located in Southern Siberia and lies on the southwestern edge of the Siberian shield.It consists of Archaean and Proterozoic gneiss complexes and is covered by Quaternary rocks.Hydrologically this region belongs to the catchment area of Yenisey river.The site itself is located on the watersheds between Yenisey and another river.The site is in a compression regime, the horizontal stresses in average σ h (z = −500 m) = 13.5 MPa are higher than they would be due to gravitation only [8].In a distance of 100 km to 1000 km earthquakes occured and seismic activity has been recorded in the last 200 years [9].

Physical Properties
The physical parameters, listed in Table 1, are estimated from borehole logs, geophysical measurements and laboratory experiments.

Field Equations
We use the simplified equation set (u-p form) for quasistatic, fully saturated, poroelastic media [10] Since the hydraulic conductivity distribution [11] is provided in higher resolution than the remaining parameters, we consider it as guiding for the others.In a first approximation we use a linear interpolation within their ranges.We assume that the higher the hydraulic conductivity the lower the elastic stiffness (k ↑ E ↓) and the higher the porosity (k ↑ φ ↑).

Boundary Conditions
As Figure 3 shows, we impose atmospheric pressure on top and impermable boundaries elsewhere (watersheds, impermable bedrock).The normal stresses applied at the lateral boundaries are linearly increasing with depth.The bottom is fixed in vertical direction, as is one side for each direction in space.Starting from an equilibrated initial state we simulate two load cases, similarly to observations elsewhere [5]: stress increase and stress rotation.In the former case, the stresses increases uniformly (factor 6/5), whereas in the latter case the stress field rotates at constant mean stress (factor 6 /5 in x-direction and 4 /5 in y-direction).
In both cases the stress change occurs in the time t = 20 × 10 5 s . . . 25 × 10 5 s.
x y stress increase stress rotation

Discretization
We use the open-source multi-physics finite-element software OpenGeoSys [6] to solve the coupled hydromechanical problem.The spatial discretization is done by hexahedral Taylor-Hood elements, as shown in Figure 4(a).The material parameters are assigned blockwise.As time discretization we have chosen an Euler-backward scheme.The discretization parameters are listed in Table 2 and the mesh is shown in Figure 4

RESULTS AND OUTLOOK
For the first load case stress increase we observe in Figure 5 that the effective deviatoric stresses, collected in the von-Mises stress q, increase as the total stresses do.Whereas the fluid (pressure p) carries the added total mean stress first and shifts it slowly to the solid (effective mean stress p ).A rotation of the horizontal stress, as in the second load case, does not change the total mean stress and so it changes neither the fluid pressure nor the effective mean stress.Still, there is a change in the deviatoric stresses.As future work, we are going to refine the scenarios and to analyze: fault-slip potentials, parameter dependencies and fully dynamic models.

Fig. 3 :
Fig. 3: Hydraulical and mechanical boundary conditions remain qualitatively the same in front and side view (a), whereas in top view there appears the change from the initial state to one of the final states (b) (b).

Fig.
Fig. Changes of fluid pressure, effective mean stress and effective deviatoric stress (von Mises) in the center of the model for load case stress increase.