Caprock Characterization

Caprock Characterization

Caprock formations play a key role for the safe geological storage of carbon dioxide (CO2). Shales are considered among the best candidates due their low permeability, high water retention capacity, and self-sealing properties. The assessment of the integrity and sealing capacity of caprock materials during CO2 injection and storage is a fundamental aspect to design injection pressures and flow rates. The Chair “Gaz naturel” Petrosvibri is currently investigating these aspects. The determination of capillary-entry pressure of water saturated geomaterials is the most important parameter to assess the sealing capacity and the sequestration of CO2 during the injection phase (short term) where the CO2 overpressure compared with respect to the reservoir might trigger the penetration in the caprock. In the long-term, the sealing efficiency of shale caprocks can be significantly affected by chemical effects. The reaction of CO2 with brine acidifies the pore fluid (pH about 4), which can in turn generate dissolution of carbonate minerals. This aspect can generate deformation problem in the reservoir formation, as well as increasing the permeability in the caprock formation. Long-term experiments are performed at the Chair to investigate these issues using also special fluids to reproduce acid conditions generates by the CO2 dissolved in the pore fluid.

Laboratory testing currently carried out at the Chair aim at analysing the hydro-mechanical behaviour of the Opalinus Clay as shaly caprock representative material when put in contact with CO2 or CO2-rich brine. The Chair deploys advanced testing equipment specifically developed to perform experimental investigation in the framework of CO2 storage. A special system to inject CO2 in gas, liquid, and supercritical conditions up to a pressure of 26 MPa has been developed; the injection can be performed with flow rate in the range 0.001ml/min – 100ml/min. This system can be connected to two main testing set-ups described in the following.

High-pressure oedomtric cell

The cell is designed to test specimens with diameter of 35 mm and height of 12 mm. An axial stress up to 100 MPa can be applied. Fluid pressure can be controlled at both upstream and downstream sides up to pressure of 16 MPa with two different pumps. Pressure transducers are also installed in the drainage lines to monitor fluid pressure evolution when undrained conditions are applied. The behaviour of the tested specimens is monitored with three external LVDTs with a resolution 0.2 microns.

This testing set-up allows the analysis of the mechanical response during stress variations and fluid injection. CO2 injection tests are usually performed to evaluate the sealing capacity of the tested materials, as well as the transport properties such as the permeability to both water and CO2 .

The high-pressure oedometric cell has been designed and manufactured at the Laboratory for Soil Mechanics of the EPFL. Its mechanical testing capabilities for shales have been also widely demonstrated in various works carried out in the context of nuclear waste disposal.

High-pressure Triaxial Cell

The cell is designed to test specimens with diameter of 50 mm and height of 100 mm. An axial load up to 1 MN can be applied, while the confining stress can be controlled up to 70 MPa. Fluids can be injected at both upstream and downstream sides up to a pressure of 16 MPa with two different pumps; flow rates are controlled in the range 0.001 ml/min – 100 ml/min. The deformations of the tested specimens are monitored with two axial LVDTs, and one radial LVDT mounted on a clamp system; all the LVDTs have a resolution of 0.2 microns and are mounted inside the vessel in direct contact with the specimen.

This device aims at determining mechanical properties of geomaterial having put it in contact with CO2. The elastic behaviour can be analysed by performing unloading/reloading stress cycles, as well as strength properties by loading the tested specimens up to their failure.

Other Testing

The Chaire Gaz Naturel is also versed in Standard Geotechnical Lab Testing.