Predicting deformation properties of argillaceous sediments for geomechanical analyses
|Location||International Geological Congress,oslo 2008|
|Author||Grande, Lars; Cuisiat, Fabrice|
|Holding Date||23 September 2008|
Deformation properties of argillaceous sediments are commonly needed in offshore engineering, whether for characterizing the surface of the sea bed for foundation design of offshore structures, or at greater depths, for problem related to e.g. borehole stability, subsidence prediction and cap rock integrity of petroleum reservoirs. However, except for geotechnical sampling of the sea bed and sparse characterization of the formation directly overlying the reservoir, geomechanical tests on overburden material are limited. Indirect methods relying for instance on correlations with acoustic velocities measured in wells are used to provide profiles of mechanical properties with depth in theses formations.
In this paper, we have reviewed geomechanical properties of clay / shales collected from nearly 50 laboratory experiments performed on core samples from petroleum reservoir on the Norwegian continental shelf. The experiments consist of triaxial test (isotropically consolidated, sheared under drained or undrained conditions) carried out to infer the stiffness and strength properties of argillaceous formations. In addition measurements of P and S wave velocities were performed at various stages of loading in the experiments.
After a close examination of the collected data (quality check, grouping of tests according to stress level, orientation with respect to lamination...), trend lines between various parameters such as porosity, acoustic velocities and static mechanical properties are drawn. The trend lines are compared to other correlations presented in the literature.
The trend lines are applied to predict the vertical variation of shear moduli of argillaceous formations overlying a gas reservoir. Such vertical parameters are required to estimate sea bed subsidence due to reservoir compaction associated with gas pressure reduction during production. The consequence of parameter uncertainty and consequence for prediction of sea bed subsidence is shown.