Dynamic Development of Hydrofracture
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Pure and Applied Geophysics
Dynamic Development of Hydrofracture IRFAN GHANI,1 DANIEL KOEHN,2 RENAUD TOUSSAINT,3,4 and CEES WILLEM PASSCHIER1 Abstract—Many natural examples of complex joint and vein networks in layered sedimentary rocks are hydrofractures that form by a combination of pore fluid overpressure and tectonic stresses. In this paper, a two-dimensional hybrid hydro-mechanical formulation is proposed to model the dynamic development of natural hydrofractures. The numerical scheme combines a discrete element model (DEM) framework that represents a porous solid medium with a supplementary Darcy based pore-pressure diffusion as continuum description for the fluid. This combination yields a porosity controlled coupling between an evolving fracture network and the associated hydraulic field. The model is tested on some basic cases of hydro-driven fracturing commonly found in nature, e.g., fracturing due to local fluid overpressure in rocks subjected to hydrostatic and nonhydrostatic tectonic loadings. In our models we find that seepage forces created by hydraulic pressure gradients together with poroelastic feedback upon discrete fracturing play a significant role in subsurface rock deformation. These forces manipulate the growth and geometry of hydrofractures in addition to tectonic stresses and the mechanical properties of the porous rocks. Our results show characteristic failure patterns that reflect different tectonic and lithological conditions and are qualitatively consistent with existing analogue and numerical studies as well as field observations. The applied scheme is numerically efficient, can be applied at various scales and is computational cost effective with the least involvement of sophisticated mathematical computation of hydrodynamic flow between the solid grains. Key words: DEM, hydrofracture, fluid–solid dynamics, pore pressure gradient, pattern formation.
1
Tektonophysik, Institut fu¨r Geoswissenschaften, Johannes Gutenberg-Universita¨t Mainz, Mainz, Germany. E-mail: [email protected] 2 School of Geographical and Earth Sciences, University of Glasgow, Glasgow, UK. 3 Institut de Physique du Globe de Strasbourg, UMR 7516, Universite´ de Strasbourg/EOST, CNRS, 5 rue Rene´ Descartes, 67084 Strasbourg Cedex, France. 4 Centre for Advanced Study at the Norwegian Academy of Science and Letters, Drammensveien 78, 0271 Oslo, Norway.
1. Introduction Brittle deformation of rocks in association with overpressured fluid plays an important role in the geophysical, geochemical and structural mechanics of the Earth’s crust in a wide variety of geological settings (FYFE et al., 1978). A number of fluid expansion mechanisms, e.g., burial compaction, clay dehydration, organic matter decomposition and aquathermal expansion as well as impermeable rock units which behave as barrier to subterraneous fluid flow render the pore fluid overpressure, which if in excess of the least principal stress (Pf [ r3 ) may lead to load parallel or load oblique tension fracturing (Fig. 1) in depths of the Earth’s crust. The
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