1019
Macro- and micro-FE modelling of wellbore damage due to drilling and coring
processes
Modélisation par les éléments finis aux échelles micro et macro de l’endommagement dû au forage
et au carrotage
Khoa H.D.V., Grande L., Jostad H.P.
Norwegian Geotechnical Institute, Oslo, Norway
ABSTRACT: This paper presents the application of the finite element method to evaluate tensile fracturing at different scales in the
formation during drilling and coring operations. The first part focuses on evaluating the formation damage process at the macro scale.
Both full 3D and axisymmetric macro FE-model are established in order to identify the potential damage mechanisms, to study the
size of damage zone as well as to indentify the critical stress changes causing failure cracks at one specific location close to the
wellbore tip. In the second part of the paper, the potential mechanisms of damage are investigated in detail at the micro scale (i.e.
grain scale) by using a complex 2D micro FE-model reproducing a realistic grain structure taken from the Scanning Electron
Microscope (SEM). The stress changes calculated from the macro FE-model are applied at the boundaries of the micro FE-model to
simulate the effects from drilling and coring operations. The calculated results show that the 2D micro FE-model is closer to explain
the formation damage observed around the wellbore and more pertinent to investigate the physical processes of damage, while it is
almost impossible with the macro FE-model.
RÉSUMÉ : Cet article présente l'application de la méthode des éléments finis pour évaluer une rupture en traction à différentes
échelles dans une formation rocheuse au cours des opérations de forage et de carottage. La première partie se concentre sur
l'évaluation du processus de détérioration de la formation à l'échelle macro. Les deux cas de modélisation 3D et de symétrie axiale par
les éléments finis sont mis en place afin d'identifier les mécanismes d’endommagements potentiels et d'étudier la taille de la zone
endommagée ainsi que pour identifier les changements de contraintes critiques causant une fissuration. Dans la deuxième partie de
l'étude, les mécanismes potentiels d’endommagement sont étudiés en détail à l’échelle micro (
i.e.
échelle de grain) en utilisant un
modèle 2D complexe qui reproduit une structure de grain réaliste issue d’observations par microscope électronique à balayage
(MEB). Les changements de contraintes calculés à partir de la modélisation macro sont appliqués aux frontières du modèle Eléments
Finis à l’échelle micro pour simuler les effets de forage et de carottage. Les résultats calculés montrent que le modèle 2D micro est
plus apte à expliquer la détérioration de la formation observée dans le puits de forage et plus pertinent pour étudier les processus
physiques d’endommagement qui sont presque impossibles à aborder avec le modèle macro.
KEYWORDS: Multiscale modelling, wellbore damage, finite element
1 INTRODUCTION
Drilling and coring operations disturb and generate stress
changes in the rock surrounding the wellbore. These induced
stresses, which are quite different in magnitude and sometimes
orientation, as compared to the initial conditions, can cause a
number of events such as wellbore stability and fracture
initiation.
In general, well damage is governed by the in-situ stresses,
pore pressure and rock strength. In addition to these dominant
parameters, during the drilling and coring operations, wellbore
stability may directly or indirectly be affected by the three
following effects:
•
Stresses from drill bit (shear stresses from torque, lateral
stresses due to stress release due to drilling vibration and
weight of bit);
•
Stresses released due to drilling: difference between
mud pressure and the in-situ stresses and reduction by mud-
fluid flowing into the formation;
•
Stresses released due to temperature reduction due to
cooling of the formation rock near the wellbore by the colder
mud-fluid.
Recent developments in geophysical logging i.e sonic
scanner logging tool together with the Diapole Shear Radial
Profiling algorithm have given new insight into the evaluation
of 3D field of stresses and material properties around the
wellbore (Sayers et. al, 2009). The information from such logs
along the well are crucial with respect to predicting potential
geomechanical challenges during drilling and coring operations.
It was found that one possible mechanism of wellbore damage
is tensile failure of the formation during effective stress
unloading caused by radial stress release, pore pressure increase
by mud-fluid flowing into the formation close to the wellbore
tip as well as temperation reduction.
The paper focuses on applying the finite element method to
evaluate tensile fracturing at different scales (multiscale) in the
