Abstract
Fractures and faults are critical elements affecting the geomechanical integrity of CO2 storage sites. In particular, the slip of fractures and faults may affect reservoir integrity and increase potential for breach, may be monitored via the resulting seismicity. This paper presents an experimental study on shale samples from Draupne and Rurikfjellet formations from the North Sea and Svalbard, Norway, using a laboratory test procedure simulating the slip of fractures and faults under realistic stress conditions for North Sea CO2 storage sites. The motivation of the study is to investigate whether the slip along the fractures within these shales may cause detectable seismic events, based on a slip stability criterion. Using a direct shear apparatus, frictional properties of the fractures were measured during shearing, as a function of the shear velocity and applied stress normal to the fracture. We calculated the friction coefficient of the fractures during the different stages of the shear tests and analysed its dependency on shear velocity. Information on velocity-dependent friction coefficient and its evolution with increasing slip were then used to assess whether slip was stable (velocity-strengthening) or unstable (velocity-weakening). Results showed that friction coefficient for both Draupne and Rurikfjellet shales increased when the shear velocity was increased from 10 to 50 µm/s, indicating a velocity-strengthening behaviour. Such a behaviour implies that slip on fractures and faults within these formations may be less prone to producing detectable seismicity during a slip event. These results will have implications for the type of techniques to be used for monitoring reservoir and caprock integrity, for instance, for CO2 storage sites.
Highlights
Carbon capture and storage in the subsurface is considered by the Intergovernmental Panel on Climate Change (IPCC) as one of the major mitigation methods to climate change [1]
This study focuses on the potential of seismicity for Draupne and Rurikfjellet shales using laboratory shear tests
The vertical load was measured with a load sensor installed inside the vertical load actuator just above the upper shear box, and vertical displacement was measured by four linear variable displacement transducers (LVDTs) recording the change in distance between the top and bottom box
Summary
Carbon capture and storage in the subsurface is considered by the Intergovernmental Panel on Climate Change (IPCC) as one of the major mitigation methods to climate change [1]. Storage reservoirs should ideally have porosity and permeability sufficient to accommodate large volumes of CO2 injected at relatively high rates. For site selection and location of CO2 injection wells, zones of high risk such as proximity to faults and major fractures should be avoided. Large-scale CO2 storage will require the utilization of reservoirs that are faulted (e.g., the Smeaheia fault block offshore Norway). To maintain seal integrity and ensure that injected CO2 remains within the defined storage reservoir, the operation of a storage site must be conducted in a manner that reservoir pressure does not exceed the fracture pressure, to avoid failure of the reservoir or seal units. It is usually difficult to Energies 2020, 13, 6275; doi:10.3390/en13236275 www.mdpi.com/journal/energies
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