Page 343 - Numerical Analysis and Modelling in Geomechanics

324 RESERVOIR COMPACTION, SUBSIDENCE AND WELL DAMAGE
rock. Therefore, the underburden is usually assumed to provide a non-
deformable boundary at the bottom of the reservoir. The rock boundaries at the
sides of the reservoir are called flanks.
Ubiquitous to any large, subsurface rock unit, and as indicated in Figure 11.1,
are faults, fractures and interfaces between rocks of differing mechanical
properties, and geologic unconformities. Generally, these various geologic
1,2
features are referred to as discontinuities. Thin layers of weak, plastic shale or
clay, often present as a natural consequence of depositional processes, are
classified herein as discontinuities. Geological unconformities may be the
locations of significant changes in mechanical stiffness, which can be
responsible for focusing shear deformation, and these are also classified as
discontinuities for the purposes of this chapter. Additionally, changes in pore
3
pressure can initiate slip on faults and fractures. Discontinuities have low shear
strength or resistance to shear displacement, so they may be the location of large
shear deformations. Water or oil wells that penetrate actively shearing
discontinuities may be deformed to the extent that they may be considered
damaged, which is the focus of the discussion on well damage and failures in this
chapter.
As a prelude to later discussions, consider a petroleum reservoir that has
undergone significant reservoir compaction as a result of eighteen years of field
operation. Depicted in Figure 11.2 is a cross section from a finite element
analysis of the South Belridge field reservoir compaction and subsidence process,
which is presented later in this chapter. The displacements calculated in the
analysis have been exaggerated by ten times to illustrate the effects of reservoir
compaction. The cross section shown has superimposed on it a color contour plot
of reservoir pore pressures at the end of a simulation of eighteen years of field
operation. The color contours in the center of the cross section represent the total
pressure reduction due to production from wells. As fluids (oil, gas and water)
are produced from the reservoir through wells, pore pressure and pore volume
decrease. As a result, compressive effective stress and compressive vertical strain
both increase, along with a reduction of the thickness of the reservoir. The
vertical compressive strains in the reservoir result in “sagging” of the overburden
layers, resulting in downward displacements at the surface. The decrease in
elevation of the surface is termed surface subsidence. Water may be pumped into
the reservoir for pressure maintenance to sustain productivity, or sometimes
above the reservoir in an attempt to “pump up” the reservoir to counteract
compaction and subsidence. The dark contours in Figure 11.2 are contours of
pressure increase due to water injection. One important feature that can be seen
in Figure 11.2 is that the rock units on the flanks of the reservoir are “dragged”
toward the center of the field, which are the result of the horizontal components
of strain. This effect may exacerbate well damage near the flanks.