Frictional Properties and Seismogenic Potential of Caprock Shales

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.

Experimental evaluation of predicted undrained pore pressure generation as function of stress path and orientation in the Draupne shale

“…The mineralogy and mechanical properties of the Draupne core material from this well have been characterized by several authors in recent studies (e.g. Bohloli et al, 2020;Koochak Zadeh et al, 2017;Skurtveit et al, 2015;Smith, 2019;Soldal et al, 2021a). The clay content of the Draupne shale is approximately 50 % and the total organic content is between 6 and 8 wt.%.…”

Laboratory Evaluation of Mechanical Properties of Draupne Shale Relevant for CO2 Seal Integrity

“…The Draupne shale is a major caprock of the North Sea and important in the assessment of potential CO 2 storage sites in the area [24]. Several recent publications have touched upon specific mechanical aspects of Draupne derived from testing on the same core material as in the current study (e.g., [25][26][27][28][29]). The first mention of the core was by Skurtveit et al [28] in 2015.…”

Continuous assessment of landslides by measuring their basal temperature

“…Moreover, Blasio et al (2017) presented a study on the effect of the frictional melting of a crystalline gouge of a landslide due to the increase in temperature when the sliding mass accelerates. On the other end, several authors have applied velocity stepping on experimental tests on clays and shales for shearing zones (e.g., Cappa et al 2019;Bohloli et al 2020), finding that the material's strength exhibits velocity hardening (i.e., stable slip on seismic faults). Additional studies of velocity stepping in salt rock for fault gouges show a dependency in the rate hardening and rate weakening depending on the range of velocity that is applied, as well as the particle size of the mineralogy (Rattez and Veveakis 2020) and its orientation (Niemeijer et al 2010).…”