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When rock is subjected to internal hydraulic pressure and external mechanical loading, the fluid flow properties will be altered by newly induced fractures. In turn, the fluid flow driven by pore pressure gradient can influence the fracturing behaviour. To better capture the complex hydraulic fractures in rock materials, a 'pinch-off' breaking test is numerically conducted to illustrate the tensile failure of a rock specimen within a uniform pore pressure field. A double-notched specimen, with water pressure in one notch while keeping another one open to the atmosphere, is numerically extended to investigate how the water flow direction or the pore pressure gradient will influence the fracturing behaviour. The simulation results indicate that both pore pressure magnitude and the orientation and distribution of pore pressure gradient surrounding the fracture tip can affect fracturing process and macroscopic strength behaviour of rock materials. [ABSTRACT FROM AUTHOR]

Deep un-mineable coal deposits are viable reservoirs for permanent and safe storage of carbon dioxide (CO2) due to their ability to adsorb large amounts of CO2 in the microporous coal structure. A reduced amount of CO2 released into the atmosphere contributes in turn to the mitigation of climate change. However, there are a number of geomechanical risks associated with the commercial-scale storage of CO2, such as potential fault or fracture reactivation, microseismic events, cap rock integrity or ground surface uplift. The present study assesses potential site-specific hydromechanical impacts for a coal deposit of the Upper Silesian Coal Basin by means of numerical simulations. For that purpose, a near-field model is developed to simulate the injection and migration of CO2, as well as the coal-CO2 interactions in the vicinity of horizontal wells along with the corresponding changes in permeability and stresses. The resulting effective stress changes are then integrated as boundary condition into a far-field numerical model to study the geomechanical response at site-scale. An extensive scenario analysis is carried out, consisting of 52 simulation runs, whereby the impacts of injection pressures, well arrangement within two target coal seams as well as the effect of different geological uncertainties (e.g., regional stress regime and rock properties) is examined for operational and post-operational scenarios. The injection-induced vertical displacements amount in maximum to 3.59 cm and 1.07 cm directly above the coal seam and at the ground surface, respectively. The results further demonstrate that neither fault slip nor dilation, as a potential consequence of slip, are to be expected during the investigated scenarios. Nevertheless, even if fault integrity is not compromised, dilation tendencies indicate that faults may be hydraulically conductive and could represent local pathways for upward fluid migration. Therefore, the site-specific stress regime has to be determined as accurately as possible by in-situ stress measurements, and also fault properties need to be accounted for an extensive risk assessment. The present study obtained a quantitative understanding of the geomechanical processes taking place at the operational and post-operational states, supporting the assessment and mitigation of environmental risks associated with CO 2 storage in coal seams. [ABSTRACT FROM AUTHOR]

The surface properties of expanded polytetrafluoroethylene (ePTFE) can he modified by 172 nm excimer lamp irradiation in ammonia (NH3) or acetylene (C2H2) atmosphere. This surface modification is either related to the substitution of fluorine by nitrogen- and/or hydrogen-containing groups or to the deposition of material from the gas phase. Additional oxygen-containing functionalities are incorporated by reactions with the residual gas and/or oxidation in air after treatment. The changes in surface properties are discussed on the basis of water contact-angle measurements, X-ray photoelectron spectroscopy (XPS) and IR spectroscopy on dense PTFE samples. Surface modification yields a reduction of the tear strength and break-down elongation in tensile tests. Field-emission scanning electron microscopy (FE-SEM) revealed that the tensile failure occurred at the contact of fibrils and nodes in the material. With 193 nm ArF excimer-laser irradiation in NH3 atmosphere, both the modification efficiency and material degradation are considerably lower. [ABSTRACT FROM AUTHOR]

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