Seismology and Drilling, Characterizing and Hydraulic Stimulation of the OtN-3 EGS Well in Finland

Date: 4/26/2019

Time: 06:00 PM

Room: Fifth Avenue

Using hammer drilling for the 1st 4500 m of an Enhanced Geothermal System well, OTN-3, the Finnish company St1 Deep Heat Ltd completed this well in May 2018 to 6400 m measured depth. This district heating EGS well is on Aalto University’s Otaniemi campus, a few km from downtown Helsinki. The development included the installation of a 3-tier seismic monitoring and Traffic Light System. This talk sets up the seismological background to the project’s drilling, characterization, and hydraulic stimulation, describing the seismological aspect of each stage and operation. Follow-on talks will discuss the TLS and how monitoring of the stimulation was used to limit the induced seismicity to less than an M2.1 event.

In 2015 St1 cored a 2 km OTN-1 pilot hole and logged it, establishing a gradient of ~18oC /km. A 1.8 km, 24-level seismometer chain was installed in OTN-I for Hammer-Drill Seismic Profiling while OTN-2 was drilled to 3.325 km. This profiling effort reveled several geological structures and estimated seismic velocities for use in the stimulation event location effort. During its drilling, OTN-3 intersected and cut through the OTN-1 array. OTN-3 was then mud drilled to 6400 m md. It was deviated ~45o from 4.9 km, normal to the WNW-ESW stress maximum. Temperature profiles revealed numerous m-scale, 0.1oC incursions, decameter-long isothermals, and a remarkable 260 m long isothermal between 4770 and 5030 m.

The bottom of OTN-3’s 1260 open-hole section m was completed with an 800 m, 5 stage stimulation assembly. These stages were sequentially pressured during clean water injection program, including hours-to-days long rest periods over 49 days. The net injection was 18,160 cubic meter. Thousands of microearthquakes were detected by the network, none exceeding the M2.1 Red-light. The main public notice resulted not so much from the ground motion of the induced events, but from the thunder-like acoustic signals for events larger than ~M1.

Presenting Author: Peter E. A. Malin

Authors