Page 146 - Hydrocarbon Exploration and Production Second Edition

Reservoir Description 133

As pressure increases, so does water density, though the compressibility is small
1
(typically 2–4 10 6 psi ). Small amounts of gas (typically CO 2 ) are dissolved in
water. As temperature increases, the density reduces due to expansion, and the
opposing effects of temperature and pressure tend to offset each other. Correlations
are available in the chartbooks available from logging companies.
The formation volume factor for water (B w , reservoir volume per stock tank
volume) is close to unity (typically between 1.00 and 1.07 rb/stb) depending on
amount of dissolved gas and reservoir conditions, and is greater than unity due to the
thermal contraction and evolution of gas from reservoir to stock tank conditions.

6.2.7.2. Formation water viscosity
This parameter is important in the prediction of aquifer response to pressure drops
in the reservoir. As for liquids in general, water viscosity reduces with increasing
temperature. Water viscosity is in the order of 0.5–1.0 cP, and is usually lower than
that of oil.
The fluid properties of formation water may be looked up on correlation charts,
as may most of the properties of oil and gas so far discussed. Many of these
correlations are also available as computer programmes. It is always worth checking
the range of applicability of the correlations, which are often based on empirical
measurements and are grouped into fluid types (e.g. California light gases).

6.2.8. Pressure–depth relationships
The relationship between reservoir fluid pressure and depth may be used to define
the interface between fluids (e.g. gas–oil or oil–water interface) or to confirm the
observations made directly by wireline logs. This is helpful in determining the
volumes of fluids in place, and in distinguishing between areas of a field which are in
different pressure regimes or contain different fluid contacts. If different pressure
regimes are encountered within a field, this is indicative of areas which are isolated
from each other either by sealing faults or by lack of reservoir continuity. In either
case, the development of the field will have to reflect this lack of communication,
often calling for dedicated wells in each separate fault block. This is important to
understand during development planning, as later realisation is likely to lead to a
sub-optimal development (either loss of recovery or increase in cost).
Normal pressure regimes follow a hydrostatic fluid gradient from surface, and are
approximately linear. Abnormal pressure regimes include overpressured and underpressured
fluid pressures, and represent a discontinuity in the normal pressure gradient. Drilling
through abnormal pressure regimes requires special care, as discussed in Section 4.7,
Chapter 4.

6.2.8.1. Fluid pressure
Assuming a normal pressure regime, at a given depth below ground level, a certain
pressure must exist which just balances the overburden pressure (OBP) due to the
weight of rock (which forms a matrix) and fluid (which fills the matrix) overlying