Page 342 - Numerical Analysis and Modelling in Geomechanics

RESERVOIR COMPACTION, SUBSIDENCE AND WELL DAMAGE 323
Belridge field analysis was performed in an effort to reduce or to mitigate the
high rate of well failures associated with the severe compaction problem.

Petroleum industry compaction and subsidence problems

Reservoir compaction and surface subsidence

Compaction is generally taken to mean the increase in density of soil or rock due
to a reduction in porosity. With reference to geological processes, compaction is
a result of the increase in overburden above a layer of sediment due to deposition
on a geological time scale (i.e., lithifaction); in civil engineering compaction is
usually the result of the application of a mechanical force at the surface
(densification through vibratory compactors, rollers, or an additional layer of
soil); and in petroleum engineering compaction is usually associated with the
decrease in pore pressure and pore volume during production. Consolidation is
more strictly related to the transient expulsion of fluid (usually water) from the
pores of a soil. With restriction to reservoir rock behavior, pore compaction, or
simply compaction, is defined in this chapter to mean the change in volume of a
sample of rock due to a change in pore volume. Since the focus of the work
covered in this chapter is on compaction of a hydrocarbon reservoir, compaction
means the reduction in the volume of reservoir rock due to a decrease in
reservoir pressure as a result of production.
Some nomenclature associated with oil and gas reservoir compaction and
surface subsidence is reviewed in this section. Figure 11.1 is a cross section of a
reservoir. The depicted cross section is the east to west cross section of the South
Belridge field, an analysis of which is presented later in this chapter. In this case,
the reservoir is comprised of layers of diatomite rock, which is at a depth of from
500 to 1000 feet (152 to 305 metres) below the surface. The dome-shaped
diatomite reservoir is about 1000 feet (305 metres) at its thickest and about 5000
feet (1524 metres) wide. The rock above the reservoir is referred to as the
overburden. The volume and weight of the overburden rock remains constant
during field development. It may be noted that in the specific case of the South
Belridge field, oil and gas has also been produced from the Tulare sands within
the overburden, but the effect of this production has been neglected in the
analysis of the Belridge diatomite reservoir compaction. The weight of the
overburden constitutes the vertical load applied to the top of the reservoir, which
is in part responsible for driving the compaction of the reservoir rock. As fluid is
withdrawn from the reservoir and pore pressure decreases, the portion of the
overburden stress supported by the pore fluid is transferred to the rock
surrounding the pores, a notion that is captured within the concept of effective
stress, to be addressed in a later section. The rock directly underneath the
reservoir is referred to as the underburden. In the case of soft-rock, compactive
reservoirs, the underburden rock is usually stiffer and stronger than the reservoir