Page 139 - Hydrocarbon Exploration and Production Second Edition

126 Reservoir Fluids

3
3
Measured in MJ/m or Btu/ft , the WI has an advantage over the calorific value of
a gas (the heating value per unit of weight, e.g. Btu/lb), which varies with the
density of the gas. The WI is commonly specified in gas contracts as a guarantee of
product quality. A customer usually requires a product whose WI lies within a
narrow range, since a burner will need adjustment to a different fuel:air ratio if the
fuel quality varies significantly. A sudden increase in heating value of the feed can
cause a flame-out.

6.2.4.4. Hydrate formation
Under certain conditions of temperature and pressure, and in the presence of free
water, hydrocarbon gases can form hydrates, which are a solid formed by the
combination of water molecules and the methane, ethane, propane or butane.
Hydrates look like compacted snow, and can form blockages in pipelines and other
vessels. Process engineers use correlation techniques and process simulation to
predict the possibility of hydrate formation, and prevent its formation by either
drying the gas or adding a chemical (such as tri-ethylene glycol [TEG]), or a
combination of both. This is further discussed in Section 11.1, Chapter 11.

6.2.5. Properties of oils
This section will firstly consider the properties of oils in the reservoir
(compressibility, viscosity and density), and secondly, the relationship of subsurface
to surface volume of oil during the production process (formation volume factor and
GOR).

6.2.5.1. Compressibility of oil
Pressure depletion in the reservoir can normally be assumed to be isothermal, such
that the isothermal compressibility is defined as the fractional change in volume per
unit change in pressure, or
1 dV 1 1
c ¼ ðpsi Þ or ðbar Þ
V dP
The value of the compressibility of oil is a function of the amount of dissolved
6 1
gas, but is in the order of 10 10 psi . By comparison, typical water and gas
6 1 6 1
compressibilities are 4 10 psi and 500 10 psi , respectively. Above the
bubble point in an oil reservoir, the compressibility of the oil is a major determinant
of how the pressure declines for a given change in volume (brought about by a
withdrawal of reservoir fluid during production).
Reservoirs containing low-compressibility oil, having small amounts of dissolved
gas, will suffer from large pressure drops after only limited production. If the
expansion of oil is the only method of supporting the reservoir pressure, then
abandonment conditions (when the reservoir pressure is no longer sufficient to
produce economic quantities of oil to the surface) will be reached after production
of probably less than 5% of the oil initially in place. Oil compressibility can be read
from correlations.