How to Take 3D Stress Effects into Account in a Basin Simulator Using Non-Linear Poro-Elastic Assumption
Marie-Christine Cacas, Arnaud Arbeaumont, Jean-Luc Rudkiewicz, Isabelle Faille, and Martin Guiton
IFP, 1&4 Av. De Bois-Préau 92852 Rueil-Malmaison Cedex France
Basin simulation challenges include, among others, the prediction of overpressure prior to drilling as well as the prediction of potential reservoir quality in tectonically complex areas. In these two cases, an accurate modelling of porosity and permeability evolution due to 3D nature of the stress tensor at depth becomes crucial.
Presently, most basin simulations assume that the rock behaves as a non-linear poro-elastic medium and that strain is purely 1D and vertical, following the Terzaghi theory.
This hypothesis has a severe impact on the results of the simulations. The first one is that porosity and permeability evolution through time will not take into account some "structure effects" associated to rock material heterogeneity. The second one is that we cannot account for the effect of non-vertical tectonic loading which might enhance the effect of sediment loading on compaction, therefore reducing permeability and increasing overpressure risk.
"Structure effects" result from the rigidity and cohesion of the different geologic bodies, combined to their mechanical property heterogeneity such as compressibility or density. Overhanging salt diapirs for instance, create local density reduction which impacts the vertical stress evaluation. If only 1D vertical stress and strain is considered, vertical stress and permeability profile will show an abrupt and unrealistic change at the edge of the overhang, which propagates downwards to all layers vertically below.
In this presentation, the possible impact of this structure effect is examined on a basin example. To this purpose, a basin simulator, which computes porosity and pressure field thanks to classical Porosity vs. Effective stress law and Darcy law, was fed with results from a geo-mechanical simulator. Geo-mechanical calculations estimate the incremental contribution of the rock skeleton to the strain, in 3D. The resulting 3D strain is then transferred to the basin simulator which updates porosity, permeability and pressure field. The results demonstrate that computed porosity may be significantly different of that obtained with the classical oedometric assumption. These changes are particularly localized against, below, or in the vicinity of geologic bodies characterized by singular density or compressibility, such as salt or overpressured bodies.
The same approach can also model the effect of tectonic loading. To do so, the geo-mechanical simulation can be performed with prescribed displacement at basin lateral boundary. Again, the resulting incremental strain can be injected in the basin simulator in place of the usual estimate calculated from the Terzaghi approach. Application to a test basin shows that incremental porosity reduction during compression events should not be neglected compared to modelling with the usual oedometric assumption.
This study proposes a simple way to turn our common 1D approach of the compaction in basin simulators into a 3D approach, based on the introduction of incremental 3D geomechanical calculations coupled with the basin simulator. We must keep in mind that the strong underlying assumption is that the medium is characterized by isotropic non-linear poro-elastic behaviour. Future perspectives remain in the development of fully coupled hydro-mechanical simulators involving more realistic constitutive laws for the rock.
AAPG Search and Discovery Article #90091©2009 AAPG Hedberg Research Conference, May 3-7, 2009 - Napa, California, U.S.A.