Page 47 - Primer on Enhanced Oil Recovery

38 Primer on Enhanced Oil Recovery

Figure 5.1 Porous rock with oil.

then porosity will be at the maximum level at around 26%. Other grain shapes are
widely presented and the maximum porosity can be higher. In general, as grains
have in many cases very irregular shape and various formation origins, the porosity
can vary in very broad spectrum from 10% to 40% for oil containing geological
formations.
In order for oil to move (to enable an oil extraction), the pores should be con-
nected. Otherwise the oil would not be able to flow through the rock towards the
production well. On the first place we are interested in the volume of connected
pores so named open porosity. It might happened that not all pores contain oil.
In this case one can talk about effective porosity the volume of connected oil
containing pores.
Saturation is defined as a percentage or a fraction of a liquid in the whole pore
volume. Essentially it shows how much of a pore volume is occupied by a liquid.
Related to water it then shown as k r and to oil as k o . The same as porosity, the satu-
ration can be total (related to the total pores volume) and can be effective. The lat-
ter is used much more often.
The ability for reservoir liquids to flow is most important for oil recovery. The
liquids flow through the chains of connected pores. The flow in two dimension
can be easily visualized as a river flow in a map though the series of valleys. In
general, this analogy shows that as the whole the liquid flows between two dis-
tant points but hardly on the straight line. The flow twists and turns, sometimes
makes almost full loops and in some areas flow towards the flow source. The
same, but in three dimensions, happens with the flow of liquid through the porous
media.
The basic equation which guards a liquid flow (as a single phase) though the
porous media has been constructed by French engineer Darcy in 1856. The original
equation was produced for water permeation through sand. After all Darcy was a
water engineer. The equation is a significant simplification and has many limita-
tions. Nevertheless, it has some applications and helps to understand the basic