Page 49 - Primer on Enhanced Oil Recovery

40 Primer on Enhanced Oil Recovery

More phenomena makes liquids flow through the porous media a very intricate
process. Firstly, it is the presence of so named fines. The fines are particles of smal-
ler size than the main mineral grains and are not cemented into the structure. The
fines can have different mineralogical origin than the dominant formation rock or
can be produced as the main rock gets fractured or gets less consolidated. The fines
also can be produced from some rock types due to chemical changes during oil
extraction.
An example of fines is shown on Fig. 5.2. In this case the fines have other the
main rock origin (they can be a clay, for instance). The fines easily move with the
liquid flow and eventually block some pores interconnections (throats). The faster
is the flow, the faster the blockage can be established. Presence of especially clay
fines due to their plate-shape, ability to swell and complex influences on liquid rhe-
ology represent one of the highest challenge during oil extraction.
The pores in an oil reservoir contain reservoir fluids hydrocarbons (oil) and
some connate water. At this point we need to introduce a term of saturation.
Saturation (S) is defined as a fraction of pores volume occupied by the reservoir
liquids. Usually saturation divided into saturation of water, nominated as S w , and
saturation of oil S o . During oil extraction both of those saturations change. The
effect of saturation on reservoir behavior will be discussed a bit later.
The minerals in formation can be water-wet or oil-wet. In means that in many
cases the wetting phase will coat an entire grain with thin layer as shown in
Fig. 5.3.
Type of wetting affect dynamics of liquid flow during oil extraction and the res-
ervoir final saturations. In many cases not only rock type defines the wettability in
the dynamic situation but also the past saturation. Establishes layers of water on the
grain surface within surrounding of oil only can be well developed if the pores in
the past were water saturated. Small pores tend to be predominantly occupied by
the wetting phase.
Saturation and capillary pressure behaves very special ways when the liquids
drained from the pores. A schematic example is presented on the Fig. 5.4. In this

Figure 5.2 Schematic of fines in a grain structure.