| Abstract |
In 2018 the Finnish company St1 objective Deep Heat Ltd completed the 1st phase of a 6.1 km deep EGS development on Aalto University’s Otaniemi campus. The I aim is to pilot test EGS district heating for the city of Espoo. This talk presents a summary of the project’s drilling, characterization, and stimulation of its 6.4 km MD injection well, OTNÂIII, which in 2019 is being followed by an equally deep, nearby production well. In 2015 St1 cored a 2.015 km OTNÂI pilot hole to assess drilling and temperature conditions. Entirely in basement, it was extensively logged, establishing a gradient of ~18oC /km (at bottom, OTNÂ3 is ~120oC.) In 2016 air hammer drilling of production and injection wells began. A 1.8 km, 24Âlevel seismometer chain was installed in OTNÂ1 for Drill Bit Seismic while OTNÂ2 was drilled to 3.325 km. OTNÂ3 was air hammered to 4.2 km, then water hammered to 4.5 km. OTNÂIII was then mud drilled to TD, deviating ~45o from 4.9 km, normal to the WNWÂESW stress maximum. Well logs show rock properties at OTN have powerÂlaw length scales and lognormal populations meaning a few large features dominate small local ones –be they electrical, elastic, or thermal one. Temperature profiles reveal numerous meterÂscale, 0.1oC incursions, several decameterÂlong isothermals below 4000 m, and a remarkable 260 m long isothermal between 4770 and 5030 m. After reaching a measured depth of 6.4 km, the last openÂhole 1260 m was completed with a stimulation assembly. The stimulation assembly consisted of 5 Stages of ball operated sleeves, separated by packers roughly 200 m apart. The Stages in the assembly were sequentially pumped in a 5 Phase program of seismicÂactivity controlled clean water injection. Including hoursÂ-to-Âdays long rest periods, the stimulation lasted 47 days. It appears that due to either limited packer seal or near-well fractures all Stages and Phases were in partial communication with each other. Further, based on engineering data it appears that the Stage 2 sleeve did not operate properly and Phase 2 pumping is more closely associate with Stage 3. Thousands of microearthquakes were detected by a 3Âtier Traffic Light System, none exceeding an M2.1 RedÂlight. Development at the project continued with a VSP for seismic velocity control, followed in 2019 by deepening of the OTNÂII and stimulation to form an EGS doublet. |