Characterizing the physical and mechanical properties of fractured porous rocks and their interactions with the injected fluid at representative geo-reservoir conditions is crucial to assess the feasibility of any geothermal system and to minimize the risks (particularly the induced seismicity) associated with any geothermal exploitation activity.
At the laboratory scale, a novel Geo-Reservoir Experimental Analogue Technology (GREAT) cell has been developed by the collaboration between the University of Göttingen (UoG), the University of Edinburgh (UoE) and Heriot Watt University. This polyaxial apparatus is capable of testing large (200x200 mm) cylindrical samples at conditions equivalent of depths of 3-4 km, with the unique capability of applying a rotatable stress field.
A big limitation of the UoE GREAT cell is the absence of an acoustic emissions (AEs) and ultrasonic monitoring system to capture the nucleation and coalescence of microcracks during hydraulic fracturing experiments. To overcome this, a second poliaxial apparatus, equipped with an array of 20 (but expandable to 32) AEs sensors, has been recently delivered at UoG. In addition, the UoG GREAT cell is capable of simulating deeper conditions (vertical stress representative of depths of 6-7 km) or larger differential stresses (σ1 - σ3 ≈ 200 MPa), larger injection pressure (100 MPa), higher temperature conditions (120°C, compared to 100°C at UoE) on a larger sample (250x250 mm).
Preliminary experiments focus on comparing the results of the two apparatuses and to study the scale effect between the two samples, by recreating the same stress field, as in previous tests at UoE, on the same rock material, without generating any fracture.
The scale effect is also investigated during hydraulic fracturing and subsequent fracture permeability (with horizontal stress rotation) experiments on the same rock but tested in both the UoE and UoG GREAT cells, and in a conventional triaxial cell (sample size 250x100 mm), also available at UoG. The latter is fitted in the same rig, as this and the GREAT cell are interchangeable, but does not allow stress rotation.
The aim is not only to confirm both numerical simulations and previous laboratory tests with the result acquired with the new apparatus but also to show the importance of the unique capability of the rotatable stress field. In particular because direct comparisons could be done between samples of the same rock tested in the same laboratory and rig.
