Page 138 - Hydrocarbon Exploration and Production Second Edition

Reservoir Description 125

defined as 601F (298 K) and 1 atm (14.7 psia or 101.3kPa), but may vary from
location to location, and between gas sales contracts.
In gas reservoir engineering, the gas expansion factor, E, is commonly used.
However, in oil reservoir engineering it is often more convenient to refer to the gas
formation volume factor, B g , which is the reciprocal of E, and is expressed in units of
rb/scf (using field units):
1
B g ðrb=scf Þ¼
5:615E
The reason for this will become apparent in Chapter 9.

6.2.4.2. Gas density and viscosity
Density is the most commonly measured property of a gas, and is obtained
experimentally by measuring the specific gravity of the gas (density of the gas
relative to air ¼ 1). As pressure increases, so does gas density, but the relationship is
non-linear since the dimensionless gas compressibility (z-factor) also varies with
pressure. The gas density (r g ) can be calculated at any pressure and temperature
using the real gas law:
MP
r ¼
g
zRT
where M is the molecular weight of the gas (lb/mol or kg/kmol).
Gas density at reservoir conditions is useful in calculating the pressure gradient of
the gas when constructing pressure–depth relationships (see Section 6.2.8).
When fluid flow in the reservoir is considered, it is necessary to estimate the
viscosity of the fluid, since viscosity represents an internal resistance force to flow
given a pressure drop across the fluid. Unlike liquids, when the temperature and
pressure of a gas is increased, the viscosity increases as the molecules move closer
together and collide more frequently.
2
Viscosity is measured in poise. If a force of 1 dyn, acting on 1 cm , maintains a
velocity of 1 cm/s over a distance of 1 cm, then the fluid viscosity is 1 P. For practical
purposes, the centipoise (cP) is commonly used. The typical range of gas viscosity in
the reservoir is 0.01–0.05 cP. By comparison, a typical water viscosity is 0.5–1.0 cP.
Lower viscosities imply higher velocity for a given pressure drop, meaning that gas
in the reservoir moves fast relative to oil and water, and is said to have a high
mobility. This is further discussed in Chapter 9.
Measurement of gas viscosity at reservoir pressure and temperature is a complex
procedure, and correlations are often used as an approximation.

6.2.4.3. Surface properties of hydrocarbon gases
Wobbe index. The Wobbe index (WI) is a measurement of the quality of a gas
and is defined as
Gross calorific value of the gas Energy density
WI ¼ 0:5 or 0:5
ðSpecific gravity of the gasÞ ðRelative density of the gasÞ