Page 160 - Fundamentals of Gas Shale Reservoirs

140 PORE PRESSuRE PREdIcTIOn FOR ShAlE FORmATIOnS uSInG WEll lOG dATA

Pore uids Ef cient communication Rock grains Pressure/stress (psi)
of pore uids

Top of overpressure
Depth (m)

FIGURE 7.1 Example of a situation where pore fluids communicate
efficiently and develop normal pore pressure regime in sedimentary
basins. Effective stress Normal pressure
Overburden stress Overpressure

discovered that the overburden stress S is supported by pore FIGURE 7.2 Real example of pore pressure and vertical stresses
pressure P and the effective stress σ. An example of an as functions of depth.
overpressure situation is when the pore fluids are trapped
within the pore space due to a lack of communications (low pressure increases abnormally near the base of the over-
permeability barriers) between the sediments that are being pressure zone and returns to the normal trend below the
compacted and the overlying sediments. This process is overpressured section. The vertical effective stress σ may
referred to as compaction disequilibrium or under‐compaction. increase or decrease depending on the overpressure‐
generating mechanisms. In this particular example, the
S P (7.3) vertical effective stress decreases due to fluid expansion
mechanisms which will be discussed in detail in
It can be observed from Equation 7.3 that if overburden stress Section 7.2.2.
S at a certain point increases and water is allowed to escape,
the effective stress σ increases and pore pressure P remains
constant at hydrostatic pressure. however, if water is not 7.2 OVERPRESSURE-GENERATING
allowed to escape as overburden stress S increases, both pore MECHANISMS
pressure P and the effective stress σ increase.
Overpressure is also generated by the increase in volume An accurate prediction of pore pressure involves proper
resulting from the expansion of the pore fluids such as understanding of overpressure‐generating mechanisms as
hydrocarbon generation, heating, and expulsion/expansion different origins of overpressure have different signatures on
of intergranular water during clay diagenesis. In fluid expan- petrophysical properties of the formations. The theories of
sion processes, overpressure develops as the rock matrix overpressure‐generating mechanisms were well described
restricts the escape of the pore fluids as the latter increase by several authors such as Watts (1948), draou and Osisanya
in volume. (2000), Shunhua et al. (2006), and Butler (2011). The two
A typical method of plotting pore pressure/stresses main generating mechanisms of overpressure in sedimentary
versus depth is illustrated in Figure 7.2. As illustrated in rocks can be classified as (1) loading mechanisms, for
this real example, pore pressure P increases hydrostati- example, under‐compaction (compaction disequilibrium)
cally up to the top of the overpressure zone. Then the pore and lateral tectonics compression, and (2) unloading