reservoir geomecanics

38 Reservoir geomechanics
production from previously drilled wells in what was mapped as the same reservoir.
Thus, this reservoir appears to be compartmentalized at a smaller scale than that mapped
seismically, presumably by relatively small, sub-seismic sealing faults that subdivide
the sand into small compartments.
Figure 2.10 illustrates compartmentalization in a Miocene sand (the Pelican sand)
in Southern Louisiana (Chan and Zoback 2007). A structure contour map indicating
the presence of faults that compartmentalize the reservoir is superimposed on an air
photo of the region. As shown in the inset, the pressure in the wells penetrating this
sand in fault blocks I, II and III were initially at a pressure of ∼60 MPa. By 1980
fault blocks I and III had depleted along parallel, but independent depletion paths to
∼5MPa (all pressures are relative to a common datum at 14,600 ft). Wells B and C are
clearly part of the same fluid compartment despite being separated by a fault. Note that
in ∼1975, the pressure in the fault blocks I and III differed by about 10 MPa. However,
the pressure difference between fault blocks I and III and fault block II after five years
of production is quite dramatic. Even though the first two fault blocks were signicantly
depleted when production started in fault block II in the early 1980s, the pressure was
still about 60 MPa. In other words, the pressure in wells E and F was about 55 MPa
higher than that in wells B and C in the same sand. The fault separating these two
groups of wells is clearly a sealing fault whereas the fault between wells B and C is
not.
An important operational note is that drilling through severely depleted sands (such
as illustrated in Figure 2.10a) to reach deeper reservoirs, can often be problematic
(Addis, Cauley et al. 2001). Because of the reduction of stress with depletion
described in Chapter 3, amud weight sufficient to exceed pore pressure at greater
depth (and required to prevent flow into the well from the formation) might inadvertently
hydraulically fracture the depleted reservoir (Chapter 6) causing lost circulation.
This is addressed in Chapter 12 both in terms of such drilling problems but also from
the perspective of the opportunity reservoir depletion offers for refracturing a given
formation.
It is worth briefly discussing how pore pressure can appear to increase with depth
at gradients greater than hydrostatic. In Figure 2.2, atdepths greater than ∼11,000 ft,
pore pressure increases with depth at approximately the same rate as the overburden
stress increases with depth. This would suggest that a series of compliant, isolated
reservoirs is being encountered in which the reservoir pressure is supporting the full
overburden stress. However, an extremely high pressure gradient is seen between 9000 ft
and 11,000 ft (much greater than the overburden stress gradient). One should keep in
mind that data sets that appear to show pressure gradients in excess of hydrostatic
are compilations of data from multiple wells which penetrate different reservoirs at
different depths, even though a hydrostatic pressure gradient is observed within each
individual reservoir (assuming that water is in the pores).