Fracture and stress characterization in crystalline rocks - Fracture displacement measurements associated with water injection in the COSC-1 borehole, Sweden
Abstract
Fluid flow in crystalline rocks is typically controlled by transmissive fractures. The 2.5 km-deep near-vertical COSC-1 borehole in Sweden intersects a thick sequence of high-grade metamorphic rocks. Previous characterization revealed two dominant fracture sets: a gently dipping set subparallel to foliation, and a steeply dipping set. Flowing fluid electrical conductivity (FFEC) logging of this borehole identified nine discrete transmissive intervals from a depth of 250 m to the borehole bottom. Based on these results, a new field campaign was conducted using the Step-rate Injection Method for Fracture In-situ Properties (SIMFIP) tool. This instrument uses straddle packers together with a deformable cage that can monitor real-time 3D mechanical deformation and associated pressure variations due to water injection. Three intervals were evaluated: a transmissive zone associated with gently dipping fractures at ~506 m that had been previously identified by FFEC logging, a steeply dipping tight fracture at 515.1 m, and an unfractured interval at 485.2 m. Within the transmissive interval at ~506 m, three distinct fluid entry zones were detected using an in situ conductivity probe: the most prominent feature at ~504.5 m, a second at ~506 m, and a weak entry at ~507.2 m; each of these inflow zones correspond to fractures observed in core and televiewer logs. The 504.5 m flowing fracture was isolated using packers, and then a step rate test, followed by a stepped constant pressure test, were conducted to evaluate the transmissivity of this feature. Similar tests conducted on the high angle sealed fracture at 515.1 m showed almost no hydraulic response until the fracture was reactivated above an injection pressure of ~ 8MPa. Stress measurement tests conducted on the unfractured interval at 485.2 m resulted in no observed pressure decrease (leak-off) for lower pressure step injections, until a hydrofracture was created at a high injection pressure. A series of leak-off tests were conducted to determine the fracture initiation pressure, (~13.09 to 16.19 MPa), the fracture opening pressure (~8.4 MPa), and the fracture shut-in pressure (~8 MPa). Ongoing analysis of the SIMFIP data will provide constraints on the orientation and magnitude of the principal stresses and their influence on permeability of the fracture zones.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.H32B..05G
- Keywords:
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- 1805 Computational hydrology;
- HYDROLOGY;
- 1822 Geomechanics;
- HYDROLOGY;
- 1829 Groundwater hydrology;
- HYDROLOGY;
- 1832 Groundwater transport;
- HYDROLOGY