During the injection of carbon dioxide (CO 2 ) for CO 2 capture and storage (CCS) operations, the near-well (including casing, cement, and rock around it) can undergo several thermal loadings. These loadings can significantly increase or decrease the pore pressure and can thus lead to mechanical failure of the cement sheath and rock formation. When these failures appear in the caprock, they can compromise the integrity of the storage site. The understanding of thermo-mechanical behaviour of a potential caprock shale is, therefore, of great importance for the success of CCS operations. In this paper, experiments were performed on Pierre II shale, under confining and initial pore pressures comparable to field conditions. A 60 °C loading amplitude (between 30 and 90 °C) was applied on the shale material both under undrained and drained conditions. The results, analysed within the framework of anisotropic thermo-poro-elasticity, highlight the anisotropic behaviour of the thermal expansion coefficients, as well as of the Skempton coefficient. The thermal pressurization coefficient was also evaluated and showed a potential pore pressure change as high as 0.11 MPa/°C.

Laboratory tests were conducted in a triaxial load frame with acoustic emission and transmission capability to investigate mechanisms that might be initiating the microseismicity experienced in CO2 injection operations. Although often related to reactivation of mapped faults or local fracturing due to reduced injectivity, the case of the Illinois Basin - Decatur Project is used here to illustrate the need for better understanding of what triggers microseismic events in relatively large permeability, good reservoir candidates. There, microseismicity has occurred in the CO2 storage target formation, the Mt. Simon sandstone, as well as in the underlying Precambrian basement. The microseismicity in the Mt. Simon sandstone occurred ahead of CO2 plume arrival and at relatively low injection pressure conditions, well below the fracturing pressure at the injection well. A hypothesis is suggested for the occurrence of such events in the field, whereby critically stressed planes are activated by the passage of the pressure front at injection start; these faults are small and thus not visible in the seismic survey. In order to test this hypothesis, sandstone plugs were prepared by two different methods to incorporate a fracture plane, which we attempted to reactivate by pore pressure pulses. The reactivation was successful at low pressure for a fracture created in the laboratory at reservoir conditions but was unsuccessful except at a much higher pore pressure in a saw-cut artificial fracture. The results suggest that tortuous, rough stress-induced fractures may be easier to reactivate because of the higher probability that sections are already favorably oriented with respect to critical shear stress at a low pore pressure increase. Saw-cut fractures may close completely under isotropic stress loading and may be difficult to activate unless exactly oriented with respect to critical shear stress at a low pore pressure increase. Acoustic emission accompanying fracture reactivation was also recorded and analyzed. This revealed a different event distribution energy between creating and reactivating the fracture.

Abstract We adapted a scratch apparatus used to evaluate shear strength and stiffness of rocks to use on soft filter cakes, to obtain quantitative information on cake properties for modelling purposes. The modelling will help design better drilling fluids, in terms of their filter cake's resistance to oil production onset. Scratch testing of rock specimens is now a well established method to obtain reliable strength and stiffness measurements as a function of distance along the scratched surface. The apparatus consists of a rigid frame holding a cutter, being pushed at a constant velocity over a rock specimen. A micrometric screw allows the user to choose a precise cutting depth, while a bidirectional load cell monitors the shear and normal forces on the cutter. The shear force can be related to the specific energy of cutting, which in turn correlates with the Unconfined Compressive Strength (UCS) of the rock. New low-resolution load cells have been installed in anticipation of the orders of magnitude lower expected values when scraping filter cakes. Rocks were exposed to different fluids to assess the impact on filter cake quality and inner filter cake properties. The effect of the internal filter cake on the rock was also addressed by scratching the rocks prior to filtration and once again after, scratching through the filter cake, on the exposed rock face. Quantitative and detailed probing of filter cake elastic properties is now possible using the scratch method. Concerns about removing the filter cake all at once (especially when OBM systems are tested, known for their low adhesion) turned out to be unfounded; the tool was capable of scraping out clean, well-defined cake layers, down to a thickness of 0.1 mm.