Six Kilometers to Heat: Drilling, Characterizing and Stimulating the OTN-III EGS Well in Finland

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, OTN­III, which in 2019 is being followed by an equally deep, nearby production well. In 2015 St1 cored a 2.015 km OTN­I pilot hole to assess drilling and temperature conditions. Entirely in basement, it was extensively logged, establishing a gradient of ~18oC /km (at bottom, OTN­3 is ~120oC.) In 2016 air hammer drilling of production and injection wells began. A 1.8 km, 24­level seismometer chain was installed in OTN­1 for Drill Bit Seismic while OTN­2 was drilled to 3.325 km. OTN­3 was air hammered to 4.2 km, then water hammered to 4.5 km. OTN­III was then mud drilled to TD, deviating ~45o from 4.9 km, normal to the WNW­ESW stress maximum. Well logs show rock properties at OTN have power­law 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 meter­scale, 0.1oC incursions, several decameter­long 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 open­hole 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 seismic­activity controlled clean water injection. Including hours­-to-­days 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 3­tier Traffic Light System, none exceeding an M2.1 Red­light. Development at the project continued with a VSP for seismic velocity control, followed in 2019 by deepening of the OTN­II and stimulation to form an EGS doublet.