Probing 3D fracture displacements under injection in deep vertical boreholes - Comparison of fracture hydromechanical response at two hard rock sites with differing stress regimes
Abstract
By using the same tool in deep vertical boreholes with mechanically similar rock types but differing stress regimes, we observed distinct borehole interactions with the stress fields when subject to pressurization.
The SIMFIP injection tool was used at three closely spaced locations within the 2.5 km deep COSC-1 borehole in Sweden in gneiss at ~500 meters depth with three different fracture scenarios. One scenario had intact rock that was then hydrofractured. Another had a steeply dipping mineralized fracture that was thought to be opened upon pressurization. A third one investigated a known flowing foliation-parallel fracture set under pressurization and opening. Each had distinctive characteristics in the tool response but shared trends in overall measurements with strong borehole axial displacements related to the opening of flat fractures under a reverse stress regime. The same tool was then deployed at SURF in S. Dakota in a ~50 m deep vertical hole drilled from a drift ~1250 meters below surface and a normal stress regime: this borehole intersects mostly amphibolite that has been cut by a Tertiary rhyolite intrusion causing vertical variations in the stress magnitudes. In profiling the borehole at the SURF site, we encountered a previously opened, preexisting fracture. It was re-pressurized and its movement measured. We also injected into a featureless section thereby creating a hydrofracture. Response in these two sections of the SURF hole markedly differed but shared some radial displacement. Compared to Sweden, these displacements highlighted opening and shearing of steeply dipping fractures in reasonable accordance with the normal stress regime. Vertical shearing initiates on borehole's natural defects such as foliation or natural fractures at a much lower pressure than the maximum pressure reached during injections. While there were commonalties of responses within sites, overall character of the responses between the two test sites was vastly different, possibly due to differences in depth, stress magnitudes and stress heterogeneity. These measurements open perspectives to better understanding why, depending on sites, flow initially concentrates at preexisting fractures, but then eventually drives hydraulic fractures. This is crucial in defining efficient stimulation protocols in EGS wells.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMMR013..06C
- Keywords:
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- 1858 Rocks: chemical properties;
- HYDROLOGY;
- 1859 Rocks: physical properties;
- HYDROLOGY;
- 3653 Fluid flow;
- MINERALOGY AND PETROLOGY;
- 8045 Role of fluids;
- STRUCTURAL GEOLOGY