HR: 0830h
AN: S41D-05    [Abstracts]
TI: Numerical Modelling of the Triggering of Microseismicity in Enhanced Geothermal Systems Using a Discrete Fracture Network Approach
AU: * bruel, d
EM: dominique.bruel@ensmp.fr
AF: Ecole des Mines de Paris, 35 rue St Honoré, Fontainebleau, 77300 France
AU: Baujard, c
EM: clement.baujard@ensmp.fr
AF: Ecole des Mines de Paris, 35 rue St Honoré, Fontainebleau, 77300 France
AB: Heat extraction from deep engineered fractured formations is currently under investigation at the Soultz sous Forêts site with the support of the European Commission. The challenge is to develop a reservoir at great depth and to circulate a fluid in order to recover heat and produce electricity. The pilot project evolved toward a three well system at 5 km in depth with temperatures close to 200 C. Massive hydraulic tests performed to develop the reservoir have shown from the recorded micro-seismic signature, that fractures can easily be re-activated. The discussion now focusses on the hydraulic significance of the shear failure mechanism, considered as the source of the accoustic emissions.
To improve our understanding of these coupled hydrau-mechanical processes, a numerical model was presented [1], based on a 3D random description of fracture networks. Local flow rules along equivalent 1D channels connecting the fractures can account for (i) a normal closure versus effective stress law together with (ii) a dilatant behaviour during shearing motion when a Mohr-Coulomb failure criterion is met. The purpose of the present work is to simulate injection tests in some synthetic fracture network using power law distributions for the fracture size, and to analyse the spatio-temporal growth of the sheared zones. Assuming that this process is analogue to the triggering of the microseismicity, we then perform an evaluation of the so called SBRC reservoir characterisation method [2] stating that the spatial position of the triggering front in an homogeneous isotropic poroelastic medium with a hydraulic diffusivity D_h is at time t given by √4 π D_h t. We conclude to its validity, although it is found sensitive to the hypothesis of critically stressed pre-existing fractures. The connectivity of the sub-set of subcritically oriented fractures plays a major role in the succes of a stimulation treatment and controls an equivalent macro-cohesion behaviour at the reservoir scale. The examples also confirm that the migration rate of the seismic front is not affected by the irreversible changes in hydraulic parameters experienced by the fractures in between the injection zone and the failure front.
The DFN numerical procedure is shown appropriate to discuss the occurence of delayed seismic events, triggered in the far field after the injection phase has terminated. This direct approach seems well suited to predict the change in permeability resulting from a stimulation treatment and gives pressure estimates to properly stimulate a network made of several fracture sets, accounting from a given anisotropic stress tensor.
[1] Bruel, D., (2002) Impact of induced thermal stress during circulation tests in an engineered fractured geothermal reservoir. Oil & Gas Science and Technology - Rev. IFP, 57, no. 5, 459-470
[2] Shapiro, S.A., Royer, J. and Audigane, P., (1999), Large scale in situ permeability tensor of rocks from induced microseismicity. Geophysical Journal International, 137, 207-213.
DE: 5104 Fracture and flow
DE: 5114 Permeability and porosity
DE: 7209 Earthquake dynamics and mechanics
DE: 7212 Earthquake ground motions and engineering
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2005 Joint Assembly