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Natural underground fractures in rocks provide fluids with preferential flow pathways. The resulting global permeability is exploited for energy production (geothermal energy and oil and gas recovery). However, these fractures represent a threat from the point of view of geological storage site integrity. It is therefore essential to anticipate their impact on rock properties and predict potential fracture extensions and reorientations, under the effect of in situ fluid pressure and mechanical stress existing in the rock. It is to this end that a completely coupled hydro-mechanical numerical model has been developed.

This new model(1) is based on the extended finite element method (X-FEM), making it possible to overcome problems relating to the generation of the calculation mesh in the presence of fractures and its modification consequent to their evolutiona. Fluid flow in fractures is governed by fracture opening and by the fluid exchanges taking place with the surrounding porous medium. The mechanical behavior of the fracture is described by a cohesive zone model.
 

Science 33
Influence of mechanical boundary conditions and stresses on fluid flow pathways for two connected fractures(c) (fluid flow from the lower right corner).


Called HM-XFEM, the hydro-mechanical model has been validated by comparison with analytical solutions and tested on synthetic casesb, in order to verify its capacity to predict fracture behavior, incorporating the effects of mechanical stress (figure). Examples of potential future enhancements to the model are the inclusion of multiphase flow, diffusion of chemical species and mechanical anisotropy of the porous rock.


a - Because the mesh does not need to conform to fracture geometry.
b - Not from real cases.
c - Representation of fracture opening and fluid pressure [Pa].

 


(1) M. Faivre, B. Paul, F. Golfier, R. Giot, P. Massin, D. Colombo, Engineering Fracture Mechanics  2016
    DOI : 10.1016/j.engfracmech.2016.03.029 

 


Scientific contact: daniele.colombo@ifpen.fr

> ISSUE 33 OF SCIENCE@IFPEN