Mitigating Operational Risks with Geomechanical Modelling: a Geothermal Case Study
Unlike other renewable energy sources, the appraisal, development and exploitation of geothermal resources requires understanding the subsurface environment. The presence, quality, and accessibility of suitable heat sources is associated with large uncertainties, whose impact needs to be reduced to make the project economically viable. Geothermal site modelling is a way to reduce these uncertainties, and geomechanical modelling in particular addresses some key risks during the exploration, development and operation of a site: drilling instability, fracture and fault behaviour, and induced micro-seismicity. We have built an integrated geological, thermo-hydraulic, and geomechanical model of the Rittershoffen geothermal site in France, to demonstrate the feasibility of this workflow with the subsurface data acquired in a typical geothermal project. The site consists of a well doublet targeting fractured sandstone and granite at a depth of over 2 km in a faulted environment, and produces 25MW of heat for an industrial site 15 km away. Geological modelling comprised the interpretation of wireline well logs, the inversion of 2D seismic data to understand the rock property distribution, and a discrete fracture model based on tectonic constraints. The distribution of the features interpreted in image logs was analysed to identify the dominant tectonic regimes, using a geomechanically based solution; then a 3D model of the natural fracture network was built, taking into account the geomechanical drivers (paleo-stress state) and the proximity to the main fault. Dynamic fluid and heat flow simulations were then used to forecast the long-term behaviour of the field. Finally, these forecasts were coupled to a geomechanical simulator to obtain the distribution of stress and deformations in space and in time, allowing the quantification of drilling-related risks, such as wellbore stability, and operational risks such as ground deformation, fault reactivation, and induced micro-seismicity. The geomechanical model was used to predict the occurrence of wellbore instability events (such as break-outs, tensile fractures, and losses) in the injector well and compare them with the events observed during drilling. A fair match was observed, confirming the predictive value of the model. 3D geomechanical modelling results confirmed that the cooling of the rock in the vicinity of the injector well is likely to cause plastic strain, resulting in the likely reactivation of fractures and fault segments. The stress and strain tensor could be used to generate synthetic micro-seismic events; their distribution was qualitatively matched with micro-seismic measurements acquired during the stimulation of the injector well, allowing a calibration of the geomechanical model. This integrated study proved that modelling can be a useful tool to predict and mitigate the risks related to the development and operation of the geothermal site, potentially improving its economic and social aspects.
AAPG Datapages/Search and Discovery Article #90345 © 2018 AAPG European Region, Geothermal Cross Over Technology Workshop, Part II, Utrecht, The Netherlands, April 17-18, 2018