Reservoir Stimulation Experiments at the Grimsel Test Site: Stress Measurements using Hydraulic fracturing, Hydraulic Tests on Pre-existing Fractures and Overcoring

Abstract:

A decameter-scale in-situ hydraulic stimulation and circulation experiment has been planned in the Deep Underground rock Laboratory (DUG Lab) at the Grimsel Test Site, Switzerland. The general objective of this experiment is to improve our understanding of the pressure, temperature and stress changes in the rock mass due to hydraulic stimulation. In this context, the main goal is to investigate the effect of hydro-shearing on the local stress variation as well as transient and permanent permeability changes with comprehensive thermo-hydro-mechanical (THM) and acoustic emission monitoring. This experiment is designed such that stimulation processes are recorded in a dataset that is unique in THM coupled processes and induced seismicity research.

In preparation to the hydro-shearing experiments, the experimental rock volume has been studied in detail using geological tunnel mapping, optical televiewer in existing boreholes, hydraulic tests, geophysical imaging and review of the extensive literature on experiments at the Grimsel Test Site. The geophysical investigations include reflection and transmission ground penetrating radar (GPR) and seismic measurements between the tunnels to image shear zones and reveal heterogeneity of the rock mass. The orientation and magnitude of the principal stresses of the rock volume and its surroundings has been analyzed using hydraulic fracturing, hydraulic tests on pre-existing fractures and overcoring. The hydraulic fracturing tests for stress measurements were monitored using a 32-channel acoustic emission monitoring system and a regional seismic monitoring network. Here, we present the results of the pre-investigations and stress measurements, and give an outlook for the hydro-shearing experiments planned for spring 2016.