Page 215 - Hydrocarbon

202 The Driving Force for Production

consolidated reservoir rocks and very low porosity systems)

dV ¼½c o V o þ c g V o þ c w V w ŠdP
where the subscripts refer to oil, gas and water. The term dV represents the
underground withdrawal of fluids from the reservoir, which may be a combination
of oil, water and gas. The exact compressibilities of the fluids depend on the
temperature and pressure of the reservoir, but the following ranges indicate the
relative compressibilities

c o ¼ 10 10 6 to 20 10 6 psi 1
c g ¼ 500 10 6 to 1500 10 6 psi 1
c w ¼ 3 10 6 to 5 10 6 psi 1

Gas has a much higher compressibility than oil or water, and therefore expands
by a relatively large amount for a given pressure drop. As underground fluids are
withdrawn (i.e. production occurs), any free gas present expands readily to replace
the voidage, with only a small drop in reservoir pressure. If only oil and water
were present in the reservoir system, a much greater reduction in reservoir pressure
would be experienced for the same amount of production.
The expansion of the reservoir fluids, which is a function of their volume and
compressibility, act as a source of drive energy which can act to support primary
production from the reservoir. Primary production means using the natural energy
stored in the reservoir as a drive mechanism for production. Secondary recovery would
imply adding some energy to the reservoir by injecting fluids such as water or gas, to
help to support the reservoir pressure as production takes place.
Figure 9.1 shows how the expansion of fluids occurs in the reservoir to replace
the volume of fluids produced to the surface during production.
The relationship between the underground volumes (measured in reservoir
barrels) and the volumes at surface conditions is discussed in Section 6.2, Chapter 6.
The relationships were denoted by

Typical range
Oil formation volume factor B o (rb/stb) 1.1–2.0
Gas formation volume factor B g (rb/scf) 0.002–0.0005
Water formation volume factor B w (rb/stb) 1.0–1.1

One additional contribution to drive energy is by pore compaction, introduced in
Section 6.2, Chapter 6. As the pore fluid pressure reduces due to production the grain
to grain stress increases, which leads to the rock grains crushing closer together,
thereby reducing the remaining pore volume, and effectively adding to the drive
energy. The effect is usually small (less than 3% of the energy contributed by fluid
expansion), but can lead to reservoir compaction and surface subsidence in cases
where the pore fluid pressure is dropped considerably and the rock grains are loosely
consolidated.