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Sonja Leonie Philipp, Geoscience Centre, University of Goettingen (Germany)
Belinda Larsen, University of Bergen (Norway)
Agust Gudmundsson, Royal Holloway, University of London (United Kingdom)
Silke Meier, Geoscience Centre, University of Goettingen (Germany)
Dorothea Reyer, Geoscience Centre, University of Goettingen (Germany)
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Hydrofractures, i.e., fractures formed by internal fluid overpressure, include dykes, mineral veins, many joints and man-made hydraulic fractures. Theoretical models of hydrofractures in homogeneous and isotropic rocks show that any hydrofracture with a significant fluid overpressure (fluid pressure exceeding the fracture-normal stress) develops such high tensile stresses at its tips that it should continue its propagation upwards and eventually reach the earth surface. Rocks, however, are normally heterogeneous and anisotropic, in particular, most sedimentary rocks and volcanic rocks and many metamorphic rocks are layered. Changes in grain size, mineral content, fracture frequencies, or facies often coincide with changes in mechanical properties, particularly the Young's moduli.
Here we compare detailed field studies of natural hydrofractures with numerical models (finite-element and boundary-element methods) of the stress fields affecting hydrofracture propagation in mechanically layered carbonate rocks. The field results presented here are (1) from the Lower Jurassic Blue Lias Formation in the Bristol Channel Basin, South Wales and Southwest England, (2) from the Middle Triassic Muschelkalk Formation in the Kraichgau area, Southwest Germany, and (3) from the Lower Cretaceous San Giovanni di Rotondo Formation on the Gargano Peninsula, Italy. In all these outcrops different types of limestone (different Young's moduli; but generally stiff, high Young's modulus) and marl or shale (soft, low Young's modulus) alternate on a centimetre to decimetre scale. Measurements of thousands of joints and mineral veins show that most hydrofractures become arrested at layer contacts, particularly at contacts between layers with contrasting mechanical properties. The field results also indicate that the mechanical layering affects hydrofractures differently: for some hydrofractures the lithological layering coincides with the mechanical layering. Other hydrofractures propagate through many layers, but often change their attitudes (strike and dip) and apertures from one layer to the next. The results of the numerical models show that stresses commonly concentrate in stiff layers. Also, at the contacts between soft and stiff layers, the directions of the principal stresses may rotate. Which layers become stress barriers to fracture propagation depends on the external loading conditions: When a layered rock is subject to horizontal tension, the stiff layers are likely to take up most of the tensile stresses. The stiff layers then may become highly stressed and develop more fractures, whereas soft layers tend to act as stress barriers. By contrast, when such a layered rock is subject to horizontal compression, compressive stresses concentrate in the stiff layers, which may then act as barriers to hydrofracture propagation. To understand hydrofracture propagation in layered rocks we need to combine numerical modelling results and quantitative field studies.
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