World Stress Map Conference - International Lithosphere Program ...

3 rd World Stress Map Conference, 15.-17. October 2008 in Potsdam
Wednesday, October 15 th
Oral Session II: Stress modelling and interpretation at reservoir scale
Simulating the Stress Field around Salt Structures: an
Integrated Approach
Wouter van der Zee
Geomechanics International Inc., Den Haag, The Netherlands
wvanderzee@geomi.com
Martin Brudy, Cem Ozan
Geomechanics International Inc., Houston, USA
Marc Holland
Geomechanics International Inc. Mainz, Germany
Jack Taylor, Obie Djordjevic
Devon Energy Corporation, Houston, USA
Wellbore stability problems while drilling close to salt structures are common. While in general
for wellbore stability analyses the assumption is used that the vertical stress is a principal stress
and equal in its magnitude to the overburden load, this is not a valid assumption anymore drilling
close to a salt body. Because salt structures create, due to their shape and rheological behavior, a
perturbation of the stress field with strong spatial variation of the principal stress magnitudes and
orientations. To provide realistic stress input data for wellbore stability predictions the stress fields
around salt structures are simulated using realistic 3D geometries. This paper presents results of
3D finite-element simulations of stress fields around several salt structures ranging from a classic
salt dome to allochthonous salt sheets. In the finite element models the salt is simulated as a
creeping material which means that it can’t withstand any differential stress, and the adjacent
clastic sediments are simulated as porous-elastic materials. The initial unperturbed stress situation
is derived using well log data, pressure tests, and wellbore failures. 3D seismic data are used to
derive interval velocities, based on which density, overburden and pore pressure can be estimated.
The presentation outlines the workflow used to create the finite-element model from structural
information. We discuss the meshing algorithm developed by the authors which allows for high
density meshing in the area of interest. The various formations are populated with mechanical
properties using either log-derived or seismic data. In the final modeling step the initial stress state
is applied to the model to calculate the detailed stress field around the salt dome. The resulting
fully three-dimensional stress tensor (which describes the stress orientations and magnitudes) is
extracted from the finite element model and exported to a 3D visualization system. The
visualization system allows us to extract the stress tensor along arbitrary well trajectories. To
validate the model output we compare the stress outputs with available stress measurements and
indicators such as (extended) leak-off tests or losses during drilling. After this calibration step we
successfully used the input for wellbore stability predictions. The workflow described in this
presentation provides an efficient way to create realistic 3D finite-element simulations from
complicated structural data. It is optimized for the best resolution around the area of interest (well
trajectory) while limiting the size of the numerical problem to an order that can be handled in
reasonable times. Most importantly, it is set up to reflect the variability of material properties both
in horizontal and vertical direction in a geologically realistic way. Thus, this workflow allows for
a detailed simulation of the stress field around salt bodies that is new to the hydrocarbon industry
and helps to significantly reduce the risk for wellbore failures of increasingly costly wells drilled
to exploit e.g. sub-salt plays in the Golf of Mexico.
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