Page 15 - Fundamentals of Enhanced Oil and Gas Recovery

An Introduction to Enhanced Oil Recovery 3

1.3 POROSITY

The grains and particles forming rocks make irregular shapes in the internal
structure of rock. This leads to the creation of void space in rocks, called pore space.
The porosity is the fraction of pore volume over the bulk volume occupied by fluid.
In other words, porosity is the ability of rock to storage and hold fluid.
Mathematically porosity is defined as follows:

pore volume
[5
bulk volume
where [ is porosity.
During sedimentation some pores are isolated from the interconnected pore net-
work. The oil trapped in isolated pores is inaccessible and remains in rock during the
flowing period. Thus an effective porosity is defined as the fraction of interconnected
pore volumes over the total bulk volume.
Porosity is determined by laboratory measurements and well logs like sonic, neu-
tron, and density logs. The laboratory measurements are done by both liquid and gas
phases separately. Methods of calculation are based on basic physical laws including
buoyancy law or Boyle’s law.

1.4 SATURATION

All pores in porous media are filled with different fluids. The portion of each
fluid volume over the total pore volume is called saturation. Therefore the sum of all
saturation is 100%.
The fluids existing in reservoirs are under equilibrium condition. These fluids are
distributed along reservoirs according to present forces including gravity, capillary, and
viscous forces. Gas is placed on the top, oil is in the middle, and water is on the bot-
tom of the reservoir. The water trapped in reservoirs after oil migration is called con-
nate water. The portion of water remaining in rock that cannot be reduced by oil
flooding is called irreducible water saturation.
For the oil phase there are some similar expressions. The saturation of oil in which
the oil becomes moveable is called critical oil saturation. Another term is residual oil
saturation, which is the oil saturation after wet-phase flooding. The residual oil satura-
tion is more than critical oil saturation and can be reduced by side works such as
EOR.