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Abstract: Fluids and Stress in Fractured Rocks: A Case Study with Applications to Hydrodynamic Modelling (Lodeve basin, South France)

Thierry Bruel, Roland Guerin, Gerard Massonnat, Jean-Pierre Petit

In order to stock nuclear waste safely in fractured rock or optimize the development and exploitation of hydrocarbons in fractured reservoirs the same fracture parameters have to be investigated through field studies. It is essential to define the relationships between tectonics, 3D fracture networks and fluid flows well as the effect of past stress states on the network evolution and the effect of the present-day stress state on hydrodynamics.

A case study was carried out on a site of the COGEMA Uranium mine with a very high density of wells and galleries in a permian silicoclastic basin. Fracturation on all scales from individual fractures (with their connectivity and hydraulic behaviour) to fracture network was described. A multidisciplinary approach was necessary to understand the importance of fluids and to upscale metric scale observations to the well identified fracture network:

- 3D organization and evolution of the fracture network under the influence of paleostress was obtained from tectonic analysis and response of the reservoir in terms of fracture dilatancy or closure to present day stress was characterized by in situ stress measurements;

- water geochemistry and hydrogeology showed the large scale flow characterized by the presence of main drains and barriers; and

- geophysical prospecting and study of gaseous emanations from the soil helped to localize the subsurface fracture pattern, especially dilatant fractures.

All this data is used to build a 3D model of the fracture pattern - and its flow networks.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France