SHALE SYMPOSIUM
Finite Element simulation of wellbore breakout and spalling in shale
H. Xu1, C. Arson1, A. Pouya2, A.-P. Bois3
haoxu@gatech.edu, chloe.arson@ce.gatech.edu, ahmad.pouya@enpc.fr, apbois@curistec.com
1. School of Civil and Environmental Engineering, Georgia Institute of Technology, GA, USA
2. Laboratoire Navier – CERMES, IFSTTAR-Ecole des Ponts Paris Tech, France
3. CurisTec, Lisieu, Franc
Abstract
The orientation, shape and size of wellbore breakouts can be used to measure in situ stresses. It is well
known that breakouts grow in the direction of minimum compression and that plastic deformation around
wellbores tends to arrest breakouts (Zoback et al., 1985). In jointed rocks the breakout shape and
orientation is strongly conditioned by the geometry and physical properties of joints. Also in brittle rocks,
the rock can be fractured in the damage zone around the borehole. Fractures play a determinant role in
spalling and block detachment during the stages of fluid injection and withdrawal that follow drilling. The
objective of this work is to model shale wellbore spalling during hydrocarbon extraction. The rock mass
is modeled as a jointed continuum with POROFIS Finite Element program (Pouya, in prep.). The problem
is treated in plane strain. Simulations are performed in three stages: excavation of a circular hole, fluid
injection and fluid withdrawal. The first loading path simulated is purely mechanical: excavation followed
by wellbore pressurization and pressure relaxation. The second loading path simulates the injection and
withdrawal of a single-phase fluid in a porous rock (in which both the matrix and the joints are porous).
Joint elements are assigned a damage model coupled to a failure criterion: this allows simulating fracture
propagation in shale, and subsequent block detachment and spalling.
In the first section of the paper, parametric studies are presented to relate breakout size to wellbore
diameter and joint spacing. First, Finite Elements modeling shale blocks are assumed to be elastic: it is
verified that breakouts grow in the direction of least compression. Second, Drucker-Prager plasticity
model is assigned to the Finite Elements to study the interaction between the accumulation of plastic
deformation around the well and the growth of breakouts by spalling. Third, sensitivity analyses are
conducted to examine the variations of pressure in the fractures during fluid injection or withdrawal, and
the consequent stability of the near-wellbore zone. In the second section of the paper, a real shale
geological site is modeled. The model is verified against in situ measures such as caliper data and wave
velocities. The sensitivity of the model to mesh refinement and joint spacing is discussed in the last part
of the paper. The numerical approach proposed in this work will be used to recommend wellbore
operation modes so as to avoid excessive spalling and clogging.
Keywords: shale, wellbore, spalling, breakout, Finite Element Method, joint element, injection,
withdrawal
References
Pouya, A. (in preparation). Formlation and development of a Finite Element Method for POROus rocks
with FISsures (POROFIS).
Zoback, M. D., Moos, D., Mastin, L., & Anderson, R. N. (1985). Well bore breakouts and in situ stress.
Journal of Geophysical Research: Solid Earth (1978–2012), 90(B7), 5523-5530.