Page 366 - Numerical Analysis and Modelling in Geomechanics

RESERVOIR COMPACTION, SUBSIDENCE AND WELL DAMAGE 347
geomechanical analysis is that the initial geostatic stress must be included in the
model. Rock behavior depends on the confining stress and at any depth in the
subsurface the geostatic stress state acts as the confining stress. Changes in the
effective stress, due to the pore pressure changes, result in local changes in the
stress field from the initial geostatic stress.
The stress state at any depth is generally characterized by the principal
stresses: one vertical and two principal stresses oriented horizontal to the surface
(i.e., in the plane perpendicular to the vertical). The vertical stress at any depth is
taken to be the pressure due to the weight of the material above that point. The
horizontal principal stress components may not be equal and their magnitude
depends upon the geologic processes, are difficult to determine with even
reasonable accuracy, but can be estimated from hydraulic fracturing tests or
borehole ellipticity measurements. 91,92 Horizontal principal stresses are usually
expressed as fractions of the vertical component, so that if the vertical component
is σ , then:
v
(11.44)

where usually σ >σ >σ . The factors, K and K are referred to as horizontal
oH
oh
v
H
h
stress factors.
Modeling casing damage and failure under shearing
The mechanisms of compaction-induced casing damage may be categorized into
two general forms: localized shear damage and gross casing deformation.
Localized shear damage occurs due to displacements, frequently called slip,
along rock discontinuities (i.e., faults and fractures), or due to plastic
deformation of weak layers of soft rock as a result of yielding. Gross
deformations of the casing are due generally to large, compaction-induced
vertical strains. Modeling these modes of casing damage is discussed below.
Localized shear damage of the casing occurs due to the relative displacement,
or slip, at discontinuities. Typically, casing shear damage is manifested as the
relative offset of the casing centerline, thus producing a “kink” in the casing
string. The length over which the offset, or kink, can occur varies, depending on
the width of the discontinuity and the strength and stiffness of the rock layers on
either side of the discontinuity, but this length has been observed to be between six
inches to a few feet (0.152 to 3 metres). If the offset is sufficiently large, well
maintenance can be made difficult due to the inability to pass tools though the
kinked section of the casing. In the most dramatic cases involving large slip
displacements on faults, which produce detectable micro-quakes and even large
earthquakes, complete and catastrophic failure of the casing (i.e., separation of
the casing into two pieces) can occur.