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International audienceThe results reported in this paper deal with the simulation of damage in cohesive geomaterials such as rocks or concrete subjected to dynamic loads. The practical objective is to stimulate the production of tight gas reservoirs with a technique that is an alternative to hydraulic fracturing. The principle is that when subjected to dynamic loads, cohesive materials such as concrete, rocks or ceramics exhibit distributed micro-cracking as opposed to localised cracking observed under static loads. Hence, a low permeability rock containing gas trapped into occluded pores can be fragmented with the help of dynamic loads, and gas can be extracted in a much more efficient way compared with hydraulic fracturing, where only large macro cracks are formed with very few connections between occluded pores. At the stage of laboratory development of this technique, compressive underwater shock waves have been used to increase the intrinsic permeability of concrete specimens. In a previous study, pressure waves generated by pulsed arc electrohydraulic discharges in water were used in order to induce micro-cracking and an increase of average permeability of concrete hollow cylinders subjected to confinement stresses (equivalent to geostatic stresses). We discuss here a 3-D anisotropic constitutive model aimed at describing the dynamic response of these specimens. It is based on rate-dependent continuum damage constitutive relations. Crack closure effects and damage-induced anisotropy are included in the model. The directional growth of damage is related to the directional growth of material intrinsic permeability. Numerical simulations of damage induced by shock waves show good agreement with the experiments for various confinement levels of the specimens. © 2013 John Wiley & Sons, Ltd

Today’s techniques to recover heavy oil are mining, when possible, or steam injection. For steam injection, once the oil is heated, its viscosity is reduced; it becomes mobile and can be extracted. In some cases, when steam injection isn’t efficient, other methods should be developed, one of them is based on electrical technology. One solution studied by Total to reduce viscosity when steam injection isn’t possible or efficient, is to use Radio Frequency heating. The electromagnetic waves have the capability to heat immediately deeper than steam. To improve efficiency of this process, we developed a downhole Radio Frequency generator composed of triodes. An antenna is added to form a complete assembly. Here, we will present the advantages of this original solution patented by Total. The optimization of this device concerns the impedance adaptation between generator and the antenna. But the impedance of this antenna depends also on the electrical characteristics of the reservoir. The reservoir’s impedance is mainly determined by the water in liquid phase. One critical parameter is the complex electric permittivity. This parameter impacts directly on the conversion from the electrical energy to heat, the dissipated power in the reservoir being related to the complex part of the permittivity. In this paper, we will show a solution to keep an optimum conversion ratio from high frequency electrical energy to heat. As the impedance varies with time and reservoir heating, the optimum radiating frequency of our system has to vary too. The choice of the central frequency could also depend on the presence of iron oxide in shale layer in the hydrocarbon reservoir. In this paper, we will choose a frequency around 10 MHz and we will show different spatial repartitions of the applied electrical field and the corresponding heated zone. The frequency band is a compromise between fast heating and depth of penetration: the lower the frequency, the deeper the electromagnetic waves penetrate in the reservoir.

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