Page 148 - Hydrocarbon Exploration and Production Second Edition

Reservoir Description 135

Location Pressure
well 1
x x
x x gas gradient
x well 2 x
Depth GDT PGOC Depth possible
gas - oil contact
OUT
x x
x x oil gradient
x x
x = RFT pressure
measurement
Figure 6.25 The gradient intercept technique.

each well may be used to predict where the possible gas–oil contact (PGOC) lies. This
method is known as the gradient intercept technique (Figure 6.25).

6.2.8.2. Normal and abnormal pressure regimes
In a normally pressured reservoir, the pressure is transmitted through a continuous
column of water from the surface down to the reservoir. At the datum level at
surface the pressure is 1 atm. The datum level for an offshore location is the mean
sea level (MSL), and for an onshore location, the groundwater level.
In abnormally pressured reservoirs, the continuous pressure–depth relationship is
interrupted by a sealing layer, below which the pressure changes. If the pressure
below the seal is higher than the normal (or hydrostatic) pressure, the reservoir is
termed overpressured. Extrapolation of the fluid gradient in the overpressured
reservoir back to the surface datum would show a pressure greater than 1 atm. The
actual value by which the extrapolated pressure exceeds 1 atm defines the level of
overpressure in the reservoir. Similarly, an underpressured reservoir shows a pressure
less than 1 atm when extrapolated back to the surface datum.
In order to maintain underpressure or overpressure, a pressure seal must be present.
In hydrocarbon reservoirs, there is by definition a seal at the crest of the accumulation,
and the potential for abnormal pressure regimes therefore exists (Figure 6.26).
The most common causes of abnormally pressured reservoirs are

uplift/burial of rock, whereby permeable rock, encapsulated by thick layers of
shale or salt, is either uplifted (causing overpressure) or downthrown (causing
underpressure). The OBP is altered, but the fluid in the pores cannot escape, and
therefore absorb the change in overburden stress
thermal effects, causing the expansion or contraction of water which is unable to
escape from an encapsulated system
rapid burial of sediments consisting of layers of clay and sand, the speed of which
does not allow the fluids to escape from the pore space as the rock compacts – this
leads to overpressures. Most deltaic sequences show this to some degree