Page 385 - Numerical Analysis and Modelling in Geomechanics

366 RESERVOIR COMPACTION, SUBSIDENCE AND WELL DAMAGE

Figure 11.18 Relative slip plotted against the year of well life.
Mitigation of well damage
Several casing damage mitigation procedures have been investigated during the
course of this work. A method of damage repair proposed by Dale et al. 42
involved reaming the damaged section of casing to reduce the curvature of the
sheared casing, thus enabling the passage of through-casing tools after the repair.
Under-reaming the hole during drilling has been proposed as a means of
allowing large displacement of a fault before rock can contact the casing and
24
transmit shear. The under-reamed section of the hole was referred to as a “bell
hole.” The utility of this design was investigated in this work by modifying the
three-dimensional wellbore model. Elements were removed from the model just
outside of the casing and above and below the slip surface. The deformed shape
of the casing for a standard well design is compared with that for the bell hole
design simulation in Figure 11.22. The effect of the bell hole is to distribute the
shear deformation over a greater length, thereby reducing the curvature of the
casing in the sheared section. The curvature is what causes workover tools to get
stuck and tubing to become permanently bent. The plots show that increasing the
length of the under-reamed section significantly reduces the casing curvature. The
primary requirement for a bell hole design is an annulus fluid that transmits little
or no shear load to the casing.