D4.4 – Role of temperature changes

This deliverable 4.4 consists in a description of the conceptual models and results obtained by the partners involved in sub task 4.2 of work package 4, referred to as ‘Role of temperature changes’. Different modelling approaches and laboratory experiments have been performed to investigate the role of temperature evolution as a key factor influencing induced seismicity. Some of he results are shown appropriate for further discussions in mitigation strategies developed in work package 6. Following the general description of works, the role of temperature is studied throughout this deliverable in terms of: • Investigation of the interaction of far-field stress and induced poro-elastic / thermo-elastic stress in selected sites using coupled models that allow combining various scales (TNO) • Pressure/thermal stress distribution and evolution in a long term exploitation regime of fractured rock masses (ARMI), based on a hydro-thermo-mechanical re-analysis of CSMA experiments at Rosemanowes site (UK) However contributions by partner GFZ and GEOW, initially planned as follows, were not delivered. • Thermal impact of a cold-fluid injection in a critically stressed rock specimen on acoustic emissions and on rock failure (GFZ), • Identification of possible thermo-elastic events (review of field data) and determination of possible associated diffusive parameters. Using a fully coupled model, the time frame of the thermal effect should be investigated with different fracture geometries. The possibility of thermally induced shear displacements will be evaluated (GEOW), Reason given by GFZ is based on experimental constraints. They decided to concentrate on a series of experiments performed at room temperature rather than including temperature experiments as initially suggested. In the course of the project it turned out that experiments at sufficiently high temperatures to enhance stress corrosion kinetics required a reduction in sample size and excluded observation of the failure processes by use of advanced AE analysis. The comparison of AE location and signal analysis with microstructure studies was an indispensable tool to fully understand processes involved in fault reactivation by fluid injection. As the principal processes were not expected to change at low temperatures achievable with large samples under AE monitoring we achieved better results by focussing on room temperature tests. GEOW reviewed the Soultz data in order to try to identify possible thermally induced events. They firstly derive an analytical estimate of temperature front location during an injection in an equivalent porous media after an injection period of known duration. The experimental time/distance distribution of observed seismic events was compared with the possible location of the thermal front. The figures that were obtained for deep well GPK2, GPK3 and GPK4 stimulations respectively, show that the temperature front is extremely slower than the pressure front that can be correlated to the envelope of seismicity. Indeed, no large magnitude events, and very few seismic events can be associated to thermal effects in the reservoir (at least on the Soultz dataset, which is constituted of only short time stimulations). According to these preliminary results, it was decided not to focus on temperature effects longer in the framework of the WP4 with the tools of GEOWATT. It was decided to investigate more in detail the role of large structures in large magnitude seismic events. The efforts are incorporated in the task 4.3 (role of faults and fractures), and the results will be presented in deliverable 4.5.FP