Equipment and sample preparation
Find out more about the GREAT Cell technology and equipment capabilities. We have also provided information on sample preparation and synthetic samples.
The GREAT Cell serves as a mixed type “polyaxial cell” capable of creating principal stresses from multiple directions, and even irregular distributions of stress without the need to re-position the sample.
It represents a significant advancement in testing capability with respect to sample size, stress control and monitoring technology.
The GREAT cell is designed to recreate in situ conditions found at depths of 3 to 4 km in geoenergy applications in terms of:
- triaxial polyaxial stresses to 100 MPa
- temperatures up to 100 °C
- fluid pressure up to 40 MPa with flow.
The sample size of 0.2 m diameter facilitates the investigation of fracture networks, with specially-positioned fluid ports selecting specific fluid flow channels/features within the samples.
Rotation of the stress field during experiments enables investigation of the behaviour of fractures and fracture networks under changing loading conditions.
The strain response and temperature response of the samples are monitored through the use of state-of-the-art fibre optic cable, providing thousands of detailed all around measurements. Multiple pressure sensors provide detailed real-time stress and fluid pressure measurements of the loading applied.
The GREAT cell is rated to provide:
- Up to 60MPa axial load and anisotropic radial confining pressure (σ1>σ2>σ3).
- Up to 60MPa fluid pressure.
- Up to 100oC rock temperature.
- Up to 100oC injection fluid temperature.
- Single and multiphase fluid flow including nitrogen, water and brines.
- Flow rates from 0.001ml/min to 20ml/min depending on pump type.
- ISCO Syringe pumps with 100ml capacity
- CP Class dual-headed, positive displacement piston pumps with constant pressure or flow rate control.
- Upstream, downstream and differential pressure measurement and logging.
- Fluid sampling
- Full lab view control and logging
- ~200mm diameter and length samples
200mm diameter and length GREAT cell samples require unique sample preparation technologies to ensure the samples are prepared to the tolerances of ISRM 1972 with regards to square, straight, parallel ends.
These technologies have been developed at the University and include:
- 200mm Hilte diamond-tipped core trimmer, capable of being deployed in the field to take direct outcrop samples.
- Large diameter Clipper saw.
- Rock turning rig to ensure the sample diameter is precisely 193.75.
- End facing grinder rig to create a completely flat end surface with a tolerance of approximately 0.06-0.1 mm across the 196 mm diameter
Clear polyethylene samples are used in the GREAT Cell to visually highlight the outcome of mechanical processes under true triaxial conditions and encase rock samples that are too small to be directly tested in the GREAT Cell such as well cores.
The procedure for creating the synthetic samples is as follows:
- The synthetic samples constructed for GREAT Cell experiments are made from a water-clear polyester casting resin cured with an MEKP catalyst at 1% concentration.
- Each sample is built up in a series of individual pours that are allowed to cure individually to prevent the sample from reaching too high a temperature during the curing process.
- Once the catalyst is mixed into the resin as evenly as possible, this is poured into a specifically designed mould made from High-Density Polyethylene (HDPE) with non-stick properties for ease of removal of the sample once cured.
- The mould is then placed in a vacuum degassing chamber and a vacuum applied to remove any air bubbles in the mixture. The vacuum pump can pull vacuum to < 1 mBar.
- Following degassing the mould is removed from the chamber and allowed to continue curing in the fume cupboard.