Rôle des fluides dans le comportement hydromécanique des roches fracturées hétérogènes : Caractérisation in situ et modélisation numérique

17 pagesNational audienceLes processus de couplages hydromécaniques dans un massif rocheux fracturé sont étudiés à travers des expérimentations in situ et des simulations numériques. L'approche expérimentale consiste à mesurer simultanément la pression de fluide et le déplacement mécanique en différents points d'un réservoir carbonaté tout en contrôlant les conditions aux limites hydrauliques. Ces mesures sont analysées par modélisation couplée hydromécanique. A l'échelle du massif, mesures et modèles montrent que le couplage hydromécanique est contrôlé par un comportement hydraulique de double perméabilité de fractures associé à un comportement mécanique de double rigidité de fractures. A l'échelle de la fracture unique, des mesures dynamiques par capteurs à fibre optique réalisées lors d'un pulse de pression montrent une réponse pression/déplacement présentant une boucle caractéristique dont l'évolution est différente entre les phases d'augmentation et de chute de pression. A partir de ces données in situ, les paramètres hydromécaniques des fractures et de la matrice rocheuse sont rétro analysés par les modèles numériques. Ces modélisations montrent que la sensibilité de la réponse hydromécanique de la fracture pressurisée est fortement dépendante de la raideur normale et de l'ouverture hydraulique de la fracture, de la raideur de la matrice rocheuse et de la géomé trie du réseau de fractures

Coupling between hydrogeology and deformation of mountainous rock slopes: Insights from La Clapière area (southern Alps, France)

International audienceMeteoric infiltration influence on large mountainous rock slopes stability is investigated by comparing hydrogeologic and gravitational structures from detailed mapping of the 'La Clapière' slope. The slope infiltrated waters are trapped in a perched aquifer that is contained in deposits inside tensile cracks of the upper part of the slope. Flow rates of 0.4 to 0.8 l s−1 from the perched aquifer to the landslide cause landslide accelerations. Numerical modeling shows that a 0.75 l s−1 infiltration yield increases conditions for toppling with failure through tilting of large rock volumes from the perched aquifer bottom down to the foot of the slope

3D simulations of an injection test done into an unsaturated porous and fractured limestone

We have developed a numerical model to represent the effect of injection test in unsaturated porous and fractured rock mass. The test was conducted at the LSBB (Laboratoire Souterrain à Bas Bruit) site close to Rustrel, Vaucluse, France in the field of the French ANR project called 'HPPP-CO2'. The results underline the impact of fractures on the hydro-mechanical response of the rock mass. Indeed fractures allow a faster dissipation of the water pressures and stress variations induced by the water injection. Back analysis lead us to also estimate the rock mass intrinsic permeability and compressibility of the injected laye

Seismicity triggered by fluid injection–induced aseismic slip

Anthropogenic fluid injections are known to induce earthquakes. The mechanisms involved are poorly understood, and our ability to assess the seismic hazard associated with geothermal energy or unconventional hydrocarbon production remains limited. We directly measure fault slip and seismicity induced by fluid injection into a natural fault. We observe highly dilatant and slow [~4 micrometers per second (µm/s)] aseismic slip associated with a 20-fold increase of permeability, which transitions to faster slip (~10 µm/s) associated with reduced dilatancy and micro-earthquakes. Most aseismic slip occurs within the fluid-pressurized zone and obeys a rate-strengthening friction law µ = 0.67 + 0.045ln (v/v_0) with v_0 = 0.1 µm/s. Fluid injection primarily triggers aseismic slip in this experiment, with micro-earthquakes being an indirect effect mediated by aseismic creep

Hydromechanical interactions in a fractured carbonate reservoir inferred from hydraulic and mechanical measurements

10 pagesInternational audienceHydromechanical coupled processes in a shallow fractured carbonate reservoir rock were investigated through field experiments coupled with analytical and numerical analyses. The experiments consist of hydraulic loading/unloading of a water reservoir in which fluid flow occurs mainly inside a heterogeneous fracture network made up of vertical faults and bedding planes. Hydromechanical response of the reservoir was measured using six pressure–normal displacement sensors located on discontinuities and two surface tiltmeters. A dual hydraulic behavior was characterized for low-permeability bedding planes well connected to highpermeability faults. Displacement responses show high-variability, nonlinear changes, sometimes with high-frequency oscillations, and a large scattering of magnitudes. Initial normal stiffnesses and effective normal stresses along fault planes were estimated in the field by interpreting pressure–normal displacement relations with a nonlinear function between effective normal stress and normal displacement. Two-dimensional discontinuum modeling with transient fluid flow was performed to fit measurements during hydraulic loading tests. Results show that the hydromechanical behavior of the reservoir is restored if a high stiffness contrast is allocated between low- and high-permeability discontinuities. Thus, a dual-permeability network of discontinuities will likely also be a contrasting stiffness network, in which the deformation of major flow-conducting discontinuities is significantly influenced by the stiffness of the surrounding less-permeable discontinuities

Location of largest earthquake slip and fast rupture controlled by along-strike change in fault structural maturity due to fault growth

Earthquake slip distributions are asymmetric along strike, but the reasons for the asymmetry are unknown. We address this question by establishing empirical relations between earthquake slip profiles and fault properties. We analyze the slip distributions of 27 large continental earthquakes in the context of available information on their causative faults, in particular on the directions of their long-term lengthening. We find that the largest slips during each earthquake systematically occurred on that half of the ruptured fault sections most distant from the long-term fault propagating tips, i.e., on the most mature half of the broken fault sections. Meanwhile, slip decreased linearly over most of the rupture length in the direction of long-term fault propagation, i.e., of decreasing structural maturity along strike. We suggest that this earthquake slip asymmetry is governed by along-strike changes in fault properties, including fault zone compliance and fault strength, induced by the evolution of off-fault damage, fault segmentation, and fault planarity with increasing structural maturity. We also find higher rupture speeds in more mature rupture sections, consistent with predicted effects of low-velocity damage zones on rupture dynamics. Since the direction(s) of long-term fault propagation can be determined from geological evidence, it might be possible to anticipate in which direction earthquake slip, once nucleated, may increase, accelerate, and possibly lead to a large earthquake. Our results could thus contribute to earthquake hazard assessment and Earthquake Early Warning

Hydromechanical modeling of pulse tests that measure both fluid pressure and fracture-normal displacement of the Coaraze Laboratory site, France

21International audienceIn situ fracture mechanical deformation and fluid flow interactions are investigated through a series of hydraulic pulse injection tests, using specialized borehole equipment that can simultaneously measure fluid pressure and fracture displacements. The tests were conducted in two horizontal boreholes spaced one meter apart vertically and intersecting a near-vertical highly permeable fault located within a shallow fractured carbonate rock. The field data were evaluated by conducting a series of coupled hydromechanical numerical analyses, using both distinct-element and finite-element modeling techniques and both two- and three-dimensional model representations that can incorporate various complexities in fracture network geometry. One unique feature of these pulse injection experiments is that the entire test cycle, both the initial pressure increase and subsequent pressure fall-off, is carefully monitored and used for the evaluation of the in situ hydromechanical behavior. Field test data are evaluated by plotting fracture normal displacement as a function of fluid pressure, measured at the same borehole. The resulting normal displacement-versus-pressure curves show a characteristic loop, in which the paths for loading (pressure increase) and unloading (pressure decrease) are different. By matching this characteristic loop behavior, the fracture normal stiffness and an equivalent stiffness (Young's modulus) of the surrounding rock mass can be back-calculated. Evaluation of the field tests by coupled numerical hydromechanical modeling shows that initial fracture hydraulic aperture and normal stiffness vary by a factor of 2 to 3 for the two monitoring points within the same fracture plane. Moreover, the analyses show that hydraulic aperture and the normal stiffness of the pulse-tested fracture, the stiffness of surrounding rock matrix, and the properties and geometry of the surrounding fracture network significantly affect coupled hydromechanical responses during the pulse injection test. More specifically, the pressure-increase path of the normal displacement-versus-pressure curve is highly dependent on the hydromechanical parameters of the tested fracture and the stiffness of the matrix near the injection point, whereas the pressure-decrease path is highly influenced by mechanical processes within a larger portion of the surrounding fractured rock

Caractérisation hydromécaniques des fractures in situ. Développement d'une sonde amovible d'auscultation et amélioration des méthodes d'interprétation par simulations numériques 3D

National audienceA removable device allowing the in situ fracture monitoring through the realization of simultaneous measurements of pressure and mechanical displacement was developed. Measurements are carried out using fiber optic sensors that prove to be of one order of magnitude more accurate than conventional measurements with vibrating wire sensors. The higher frequency of measurements (120 Hz) makes it possible to record with much more accuracy the temporal variations of the measured parameters. This device was used at the Coaraze Laboratory site, a small calcareous fractured rock mass located in southerneast France. The experiments consisted in injection and pumping of water volumes (while controlling the hydraulic pressure or the flowrate) at the intersection between a horizontal borehole and the studied fault. The instrumental device proved to be relevant to in situ characterize the fracture hydromechanical behavior. Numerical simulations made possible to correctly reproduce the in situ experiments and to détermine the hydraulic and mechanical characteristics of the fractures (normal stiffness, hydraulic aperture).Un dispositif amovible d'auscultation in situ du comportement hydromécanique des fractures permettant la réalisation de mesures simultanées de pression et de déplacement a été mis au point. Les mesures sont réalisées à l'aide de capteurs à fibre optique qui se révèlent être d'un ordre de grandeur plus précis que les mesures par capteurs à cordes vibrantes. La fréquence des mesures est également bien supérieure (120 Hz), ce qui permet d'enregistrer avec beaucoup plus de finesse les variations temporelles des paramètres mesurés. Ce dispositif a été testé sur le site expérimental de Coaraze, petit massif calcaire fracturé situé au Sud-Est de la France. Les expérimentations ont consisté à injecter ou pomper un certain volume d'eau (en contrôlant la pression ou le débit) au niveau de l'intersection d'un forage et de la faille que l'on souhaite caractériser. Le dispositif instrumental s'est révélé pertinent pour caractériser in situ le comportement hydromécanique des fractures. Les simulations hydromécaniques ont permis de reproduire correctement les expérimentations et de déterminer par calage les caractéristiques hydromécaniques des fractures (raideur normale, ouverture hydraulique)