GNGTS 2013 - Atti del 32° Convegno Nazionale

Modelling Reservoir Stimulation in enhanced geothermal system A. Troiano 1 , M.G. Di Giuseppe 1 , A. Monetti 2 , D. Patella 2 , C. Troise 1 , G. De Natale 1 1 Istituto Nazionale di Geofisica e Vulcanologia - Osservatorio Vesuviano, Napoli, Italy 2 Dipartimento di Fisica. Università degli Studi di Napoli ‘Federico II’, Italy Introduction. Geothermal systems represent a large resource that can provide, with a rea- sonable investment, a very high and cost-competitive power generating capacity. Considering also the very low environmental impact, their development represents, in the next decades, an enormous perspective (MIT Report, 2006). Despite this unquestionable potential, geother- mal exploitation has always been perceived as limited, mainly because of the dependance from strict site-related conditions, mainly correlated to the reservoir rock’s permeability, the amount of fluid saturation and, first of all, a convenient temperature-depth relationship. How- ever, many of such limitations are overcome with the Enhanced Geothermal System (EGS, Majer et al. , 2007), where massive fluid injection is performed to enlarge the natural frac- ture system of the basement rock. The permeability of the surrounding rocks results highly increased by pres- surized fluids circulation and geothermal resource, in such way, become accessible in areas where exploitation, otherwise, could be not advantageous or even possible. Numerical procedures have already been presented in lit- erature reproducing the thermodynamic evolution of the system where fluids are injected (Troiano et al. , 2013). In such a way, changes of Coulomb stress can be com- puted from Pressure and Temperature changes; the cor- relation between computed Coulomb stress changes and observed induced seismicity patterns has been shown to be very effective for the Soultz-sous-forets case (Troiano et al. , 2013) thus validating the procedure. We upgrade this kind of procedures to obtain an evaluation of the permeability enhancement obtained in the stimulation process. Assuming a conceptual model linking induced strain and permeability modification, we can estimate the induced permeability change during the water injection. In this way we adapt the medium behavior to mechanical changes and we could possibly evaluate the effectiveness of stimulation process in enhancing a geothermal reser- voir permeability. In addition to the enhancement permeability with evaluation of induced seismicity, another kind of phenomena has been be studied. Stimulated fluid flow in geothermal reservoirs can produce surface Self-Potential (SP) anomalies of several mV. A commonly accepted interpretation involves the activation of electro-kinetic processes. SP anomalies observed above the Soultz-sous- Forets geothermal reservoir while injecting cold fresh water have been modeled, imposing a source related to the fluid flow induced by the well stimulation process. In particular, the retrieved changes of pressure due to well stimulation in the EGS system has been utilized as source term to evaluate the electric currents generating the potential anomalies, using Comsol Multiphysics. Fig. 1 – Top: sketch of the simulation volume. Blue plane, Earth surface; red plane, injection plane. Bottom: pressure and temperature initial conditions are in- dicated. Initial pressure (blue) and tem- perature (red) conditions as a function of depth. 286 GNGTS 2013 S essione 1.3