Role of Temperature Change in Micro Seismic Activity During Fluid Injections in Faulted and Fractured Zones. Part 1: Updating the Thermal Modelling in a DFN Model Using a Double Media Approach

Abstract

According to the general agreement that carbon emissions use should be strongly controlled, governments are now committed to diversify the primary fossil energy resource and particularly use more renewable resources. This is even more vital for countries that depend on imported oil products. Developing electricity production obtained from geothermal fields is therefore a priority at many places in the world. A step forward in this domain has been achieved with the Enhanced Geothermal System (EGS) concept, which was demonstrated at a pilot plant scale at the Soultz sous Forêts site (Genter et al., 2009). In such reservoirs, developed in deep hard rocks, the overall flow pattern is controlled by the pre-existing network of geological discontinuities. Access to the reservoir can be improved thanks to hydraulic stimulation techniques resulting in better connections between boreholes and these natural pathways. Controlling the efficiency of stimulation techniques upon the natural structures forming the bulk part of the reservoir is still a matter of research, as results obtained at various sites are highly variable. In all cases, raising the pore pressure, thereby reducing the shear strength of fractures zones and promoting shear failure and some increase in fracture permeability, is accompanied by micro seismic activity and acoustic emissions (AE). These signals are recorded and analysed. The spatial localization of rupture sources often delineates planar structures where pressure perturbations were propagated. It is therefore confirmed that flow occurs in a 3D network of 2D structures and that specific quantitative models are required to capture the coupled hydraulic-mechanical processes at issue. During the past decade, attention was first given to the engineering of the reservoir using various combinations of hydraulic tests, and to the evaluation of the hydraulic improvement of the pathways developed the fractured system, using tracer tests (Sanjuan et al., 2006). Recent works are re-focussing at physical processes, such as the determination of fracture/matrix transfer area, (K. Pruess and C. Doughty, 2010) or at comparisons of tracer and thermal transport in fractured reservoirs (Juliusson et Horne, 2010) to investigate the extent to which tracer return can be used to predict thermal breakthrough. Very few new programmes were dedicated to long term thermal behaviour, assuming that a reservoir with sufficient size would last long enough for industrial purposes. The phenomenon of large induced seismic events (LISE) then received considerable interest after the generation in 2003 of a Magnitude 2.9 earthquake at Soultz sous Forêts (France). In 2006, a M 3.4 earthquake was generated during the hydraulic development of a new EGS reservoir, near the Swiss city of Basel, that stopped the project.