Page 347 - Numerical Analysis and Modelling in Geomechanics

328 RESERVOIR COMPACTION, SUBSIDENCE AND WELL DAMAGE
reservoirs and water aquifers, the dimensions of which are on an even smaller or
local scale, that of the wellbore. In particular, compaction-induced shearing
causes severe localized deformation of a casing over small lengths, often less
than 10 feet (3 metres). The interaction of soft rock with the casing, and
moreover the stiffness and strength of the rock, governs the geometry of the
damaged, deformed casing. If effective and lasting damage mitigation and failure
prevention measures are to be designed and implemented, then an understanding
of the mechanisms of shear-induced casing damage is required.
Compaction induces bending and shearing deformations around the edges of
the area of surface subsidence, or the “subsidence bowl”. Under such
shearing, thin weak-rock layers and geologic unconformities fail or undergo
significant shear deformations. Shear-induced, relative motions between layers,
called “slip”, cause localized shear deformation. Such shearing can cause
deformation and failure of wells penetrating the layers. It is not feasible to
include the details of a wellbore in field-scale models in order to determine a
relation between compaction and wellbore deformation. Instead, it is necessary
to develop a correlation between shear-induced slip computed from large-scale
models to either observations of casing shear damage or the results of wellbore-
scale modeling. This procedure was followed in the study of the South Belridge
field discussed later in this chapter. 37

Casing damage

Casing damage is herein defined as a change in the dimension of the casing
which may affect the usability or serviceability of the casing, but for which the
casing may be repaired to render it useable once again. Casing failure is defined
as a change in casing dimension, which renders the casing irreparable or
unusable, or separates the casing into one or more pieces, which is referred to as
a catastrophic failure. Damage and failure of the casing due to shear deformation
is focused upon in this chapter. Damage to the casing due to reservoir
compaction has been the focus of several investigators. Casing deformation in
the Ekofisk field and other fields of the North Sea has resulted in considerable
Similarly, reservoir compaction has caused significant
operational difficulty. 18– 23
difficulty and expense in the oilfields of southern California, most prominently in
the Wilmington field, 24, 28,40 the Belridge field, 28,32,37–42 and in the Lost Hills
field. 34,35 Rock-casing interactions leading to damage and failure have also
occurred in Montana and North Dakota 43 and in the Gulf of Mexico 43,44
Deforming salt has been studied as a mechanism for casing collapsed and failure. 45
In addition, salt and permafrost, a mixture of ice and rock, will thaw due to
heating of the wellbore during production, which can result in high compressive
strain and casing deformation. 46,47 Perforation holes in the casing can also lead to
casing deformation due to compaction or flow of weak, plastically deforming
rock. 48,49 Finally, numerous general studies of casing damage due to casing-