Page 348 - Numerical Analysis and Modelling in Geomechanics

RESERVOIR COMPACTION, SUBSIDENCE AND WELL DAMAGE 329
structure interactions for a variety of rock types, rock deformation mechanisms,
and casing configurations (i.e., casing inclination) have been conducted. 50–54
With respect to compaction-induced shearing damage to casing, which is the
mechanism for casing damage focused upon in this chapter, such damage can
result from shearing of a thin, weak shale layer as depicted in the insert of
Figure 11.2. Such shearing of thin, weak shale layers occurs in the overburden
due to the bending (or sagging) of the overburden as a result of reservoir
compaction. While the overall displacements in the overburden strata appear to
be large in the figure, larger shear displacements are “focused” on the weak shale
layer. That is, the weak layers “absorb” most of the shear deformation. As a
result, the shear deformation of the casing is termed localized. Shearing reaches
maximum magnitudes nearest the flanks of the reservoir. Faults and fractures can
similarly affect casing damage. In fact, earthquakes have been induced by
increases in shear stress in and around compacting reservoirs, resulting in a
completely severed casing. 24
In addition to localized shearing, several other modes of casing damage due to
reservoir compaction are possible, which have been discussed in some detail by
Cernocky and Scholibo. 45 and Dusseault et al. 53 Casing damage caused by
vertical compressive strains includes axial column-type or Euler buckling, if the
well is vertical or nearly vertical. Crushing or collapse of the casing cross section
may occur when the well is inclined (i.e., deviated), the worst case occurring
when the casing is horizontal. Depending on the stiffness of the surrounding rock
and the ratio of the casing outside diameter to its wall thickness, the pipe may
experience cross-sectional, “can-type” buckling near the threaded connections.
This type of compressive failure mode has been documented for compression
caused by the thawing of permafrost, 47 In some cases, under sufficiently high
compressive strains, threads in threaded connections may shear off, resulting in a
“telescoped” connection. Tensile failure modes are also associated with
compaction, but occur most often within the overburden strata. Large tensile
strains may occur in the overburden due to vertical straining as the overburden
rock layers sag downward, much as the roof of an unsupported tunnel. Such
tensile strains can result in the “pull-out” failure of threaded connections. Such a
failure mode may also occur if a connection is placed in a zone of localized
shearing.

Mechanics of porous media
The mechanics of geologic materials has come to be referred to as
geomechanics. Both experimental and computational geomechanics have
subtleties unique from the study of solid, non-porous continua. The materials of
interest are typically rocks or soils, with air and liquids residing in the pores. The
cornerstones of the theory of the mechanics of porous media are a stress
decomposition principle, the balance laws of linear momentum and mass, the
balance of energy, and an appropriate constitutive model. Beginning with the