Hydromechanical 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 dualpermeability 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.

Modelling the discontinuity network of fractured reservoirs may be addressed (1) by purely stochastic means, (2) with a fractal approach, or (3) using mechanical parameters describing the spatial organisation of fracture systems. Our paper presents an approach where the geometrical properties of the fracture networks are incorporated in the form of both statistical and mechanical rules. This type of approach is particularly suitable to model stratified fractured rock masses comprising two orthogonal families of joints and a family of sedimentary discontinuities. Their geometrical arrangement is governed by two kinds of rules based on (1) statistical parameters such as the mean, standard deviation of joint length and of bed thickness, both determined by field observations, and (2) geometrical parameters that result from genetic processes inferred from field observations and analogue experiments on the nucleation and propagation mechanisms of joints. Using these parameters, we generate realistic networks in terms of the relative position of joints that control the overall network connectivity: the model enables all combinations of joint spacing and vertical persistence for orthogonal patterns ranging from ladder type to grid type patterns. It also integrates the concept of mechanical "saturation" of a bed, thereby permitting the generation of both "saturated" and "unsaturated" networks.

The influence of geometrical and structural properties of layered fractured rocks on their permeability tensor is investigated for an elementary volume at the meter scale. Various synthetic networks were generated with a three-dimensional model that incorporates a pseudo-random process controlled by mechanical rules of fracturing. The permeability is supposed to be a function of the discontinuities aperture in an impermeable matrix. Steady state, saturated laminar flow is assumed within discontinuities of constant aperture (parallel joint walls). For 81 networks associated to different sets of numerical parameters, the geometrical and structural properties are computed from 200 statistically equivalent simulations and averaged to asses the mean geometrical and structural properties. In a similar way, the mean permeability tensor associated to each network is calculated. Then, the effect of the geometrical and structural properties of the network on the permeability tensor values is studied. We show that some geometrical and structural properties may have non-negligible effects: The bedding perpendicular joint spacing and the bedding plane stopping capacity can induce more than one order of magnitude change in the permeability values, while other investigated properties (mean and standard deviation of the bedding perpendicular joints length distribution) have a very small influence. These results were compared with the permeability tensor 0022-1694/$ -see front matter ª a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j h y d r o linferred from one analytical model which provided permeability values of the same order of magnitude as those inferred from the numerical model. This model can thus be considered within the hierarchical procedure proposed to characterize rock mass permeability tensors in the field. ª