The Venelle-2 is the world’s hottest geothermal well, where downhole conditions considerably exceeded the critical temperature of geothermal brines, reaching more than 500°C at depth of 2.9 km.
The well was drilled in the Larderello geothermal field in Italy, the birthplace of geothermal power production, within the framework of the DECRAMBLE (Drilling in dEep, Super-CRitical AMBients of continentaL Europe) project running between 2015 and 2018. Throughout the project novel drilling and monitoring technologies, as well as proof of concept for reaching supercritical resources, were developed and tested. The project considerably increased knowledge of the deep chemical-physical conditions of the Larderello geothermal field and paved the way for the drilling of future deep geothermal wells worldwide. The well was reconstructed and deepened from an already existing 2.2 km dry well, already reaching temperatures of approximately 350°C, to reach supercritical conditions of the reservoir fluids at depth of 3 km. The selected Venelle-2 well is located in the vicinity of the exploration San Pompeo 2 well, which back in 1979 crossed the “K-horizon”, an important seismic marker located below currently exploited vapor dominated reservoirs.
Fig. 1. Random-line from 3D seismic in the area of Lago (Larderello) showing existing wells (Venelle 2 and San Pompeo 2), the main seismic markers (K and H) and target interpretation (black line) (Baccarin et al., 2019)
During drilling operations, novel technologies were applied to accommodate extremely high temperatures expected from the Venelle-2 well. The main improvements included a new type of temperature-resistant cement slurry, suitable for high temperatures up to 450 °C, as well as Water Base Mud weighted up with Ilmenite (Microdense) and sepiolite, acting as a suspending agent. Due to the extreme bottom hole temperatures and for mitigating the risks related to pore and fracture gradient uncertainties, it was necessary to implement the Managed Pressure Drilling (MPD) system. Such technology allows to continuously measure bottom hole pressures and flow rates to detect any smallest anomalies, which may lead to the drilling kick and blow-out. Such action allows keeping the pressure balance at the bottom hole and enables safe drilling in extreme conditions. TN125SS steel type was chosen for the tie-back liner section due to its elastic behavior. Although this steel grade is not full sour, it could be safely used in the presence of H2S for environmental temperatures higher than 80°C. A special type of PDC drill bit was used during drilling operations in the Venelle-2 well. The drill bit has a conical diamond element across the bit face, designed to significantly increase footage and ROP in tough-to-drill formations. Such bit types have no elastomer inner parts (all parts are metallic), so they can operate under extremely high downhole temperatures and guarantee a long working bit life. The potential risk from the presence of hazardous gases, with a composition not fully known required a dedicated service i.e. continuous monitoring of the air near e.g. tanks, BOP, rig floor and shale shakers as well as real-time analysis of drilling parameters. Due to a lack of logging tools that can withstand the extreme temperatures expected in the Venelle-2 well, a new logging tool that measures pressure and temperature, with a minimum of 6 hours of operating time and exposure to 450°C, was developed especially for the DESCRAMBLE project.
Fig. 2. Temperature data collected in the Venelle-2 well during DESCRAMBLE project (Bretani et al., 2018)
At the final depth of 2.9 km, extremely high temperature between 507°C and 517°C was measured. This value, associated with the geological conditions of the reservoir rocks, with a leak-off pressure of about 30 MPa, can in the near future, pave the way for an EGS system development in the area. The drilling activity ceased at depth of 2.9 km, and was finalized with a cement plug, enabling safe reopening of the well in the future. Bottom hole temperatures exceeding 500°C were considerably above the safety factors of the entire project and it was impossible to continue drilling to the planned depth of 3 km. Unfortunately, reservoir fluids were not observed during exploratory drilling of the Venelle-2 well, however, such fluids may exist at greater depths.
This concludes our final #GeothermalFactsandStats blog post of 2019! For more information and content follow us on LinkedIn, Twitter, Facebook, and Instagram.
Author: Michal Kruszewski
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