Structural and mechanical characterisation of natural hydrofractures in 3D (HyFract3D)

• Applicant: Professor Christophe Pascal, Ph.D.
• Subject Area: Palaeontology
• Term: from 2014 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 257439590

Hydrofractures are mostly tensile fractures created by relatively high pore fluid pressures. Natural examples of hydrofractures are mineral veins, joints and dykes promoted by pore fluid or magma overpressures. Hydrofractures control fluid and heat transfer in the crust that, in turn, influence a variety of geological factors and processes. These structures provide preferential pathways for crustal fluids and, often, conduct economic fluids and host mineral deposits. Hydrofractures can also be artificial and made in the course of various industrial operations, including stimulation of oil or geothermal reservoirs. Hydrofractures should ideally present a penny shape in 3D. This ideal shape is not expected in nature, because propagation of 3D cracks occurs in highly heterogeneous media and both stress and structural anisotropies tend to divert and/or confine crack paths. However, the current knowledge on hydrofracture mechanics in real geological media is mainly based on 2D field data and theoretical studies. Complete 3D exposure of natural hydrofractures is indeed seldom, unless access to underground excavations is granted by industry. Until now, no attempt has been made to quantify the mechanics of hydrofracture systems in three dimensions, using state of the art finite element modelling calibrated by structural mapping on 3D field exposures and paleostress reconstructions. In this project, we propose to address the 3D structure and mechanics of hydrofracture systems, with emphasis on the influence of stress and structural anisotropies on their complex propagation paths. To this aim we will study parts of the network of quartz veins exposed in selected outcrops of the Panasqueira Mines, Portugal, as natural examples. The extensive network of underground excavations (i.e. ~600 km) allows for exceptional 3D exposure of the geological structures. We will study in much detail the structural relationships between the veins and the pre-existing structures in order to isolate their mechanical interactions. In addition, we will conduct paleostress analyses using both fault slip and vein data, complemented by fluid inclusion and geothermometry studies. This combination of methods will allow for full quantification of the stress state, including pore fluid pressure, coeval to the formation of the hydrofractures. We will finally use an innovative numerical method (i.e. X-FEM) in order to model the propagation of hydrofractures in real 3D geological media and explore the effects of both stress and structural anisotropies. The models will be calibrated according to our structural observations and paleostress reconstructions.

• DFG Programme: Research Grants
• International Connection: Portugal
• Participating Persons: Dr. Paulo Ferraz; Professor Dr. Fernando Noronha; Professor Dr. Bernhard Stöckhert