Abstract:
Shock waves generated in water by Pulsed Arc Electrohydraulic Discharges (PAED) have, over the past years, offered new perspectives for the stimulation of hydrocarbon reservoirs, aimed at increasing their production. This contribution addresses the implementation of PAED techniques with two objectives: first, the development of an alternative to classical hydraulic fracturing, and second, the stimulation of existing fractures, aimed at removing debris and particles that may clog drains and decrease the production of oil or gas.
With regard to the development of an alternative to hydraulic fracturing that would be more effective for tight formations and potentially less dangerous for the environment, the principle is to induce a shock wave in rock masses generated by PAED. The dynamic load generates distributed damage instead of a localized fracture whose propagation may be difficult to control. In the context of tight rock masses with gas contained in the occluded porosity, it is expected that distributed cracking will be more efficient at connecting these pores, although the volume of rock affected remains confined nearby the well. Experiments reproducing PAED fracturing in reservoir conditions are described. The influences of the amplitude of the shock wave and of the number of shocks applied to laboratory specimens on damage and on the intrinsic permeability of the material are illustrated. Then, a computational model that simulates the entire process is discussed. We focus on the constitutive model for rock masses, based on orthotropic damage with crack closure effects, coupled to an orthotropic description of the evolution of permeability under loads. Rate effects on the damage growth are also included. Finally, numerical simulations of laboratory experiments and PAED fracturing under in situ conditions are discussed.
As far as the stimulation of existing fractures is concerned, the issue is to flush out the various particles that may be packed within the propped fracture and may induce a decrease of permeability of the fracture viewed as a drain for hydrocarbon. Applications go beyond unconventional gas production and cover conventional oil and gas production as well, thus making hydrocarbon production more effective. The shock wave generated inside the borehole by PAED is converted into surface waves travelling on the fracture surface. These waves induce fast variations of pressure that may potentially destabilize flocculates and put in suspension particles that have clogged the drain. In order to check this basic principle, an experimental set up has been developed in which a small portion of fracture is clogged and then unclogged by applying a dynamic load. Experiments illustrate the clogging-unclogging effects as a function of the opening of the propped fracture and of the density of fine that are introduced in order to promote clogging. These results should further help at understanding the basic parameters that govern the clogging-unclogging processes and therefore understanding what could be the best conditions of applicability of the method.
References:Pijaudier-Cabot, G., La Borderie, C. Reess, T., Chen, W., Maurel, O., Rey-Bethbeder, F., de Ferron, A., Electrohydraulic fracturing of rocks, ISTE-Wiley, 2016.
Candela, T., Brodsky, E.E., Marone, C., Elsworth, D., laboratory evidence for particle mobilization as a mechanism for permeability enhancement via dynamic stressing, Earth Planet. Sci. Lett., vol. 392, pp. 279-291, Apr. 2014.
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