Page 40 - Primer on Enhanced Oil Recovery

Basic physical and chemical concepts 31

vapor will start actively dissolving in the liquid. The pressure when all vapor will
dissolve is a dew point.
When a chemical compound is in a liquid or solid state the attraction of mole-
cules to each other can be higher than the other molecular forces (repulsion forces
plus thermal motion). When two chemically different matters brought in contact the
molecules at the boarder layer start to interact. The fluid tends to minimize the sur-
face area due to the molecules mutual attraction and will form a droplet. The liquid
resists any surface area alteration, in the scientific terms some energy needs to be
supplied to increase surface area. As the result we can say that there is a force
which is named surface tension. If we want to increase the surface area we will
need to work against this surface tension. For instance, for water in air at room tem-
perature (say 20 C) the surface tension is at around 74 mN/m. If we talk about the

area than we will need to talk about the surface energy.
Mixing or blending of different compounds is extremely important for the hydro-
carbon extraction. Without going into much of a detail we will state that gasses do
mix without too much difficulties. Gasses and liquids, liquids and liquids, liquids
and solids will mix only in certain combinations and only in the defined conditions.
If two phases (let’s say two liquids) can be mixed at all ratios and create only
one phase then these two phases (liquids) are referred to as miscible. In this case there
is no interface in the volume which will separate two phases. It looks easy but it is not.

For instance, water and alcohols at temperatures below 0 C will separate water will
freeze and alcohol will remain a liquid until it freezes too at even lower temperature.
If at certain temperature and pressure two liquids (or gas and liquid) mix immedi-
ately as they brought into a contact (e.g. they produce a single phase) then this named
First-Contact Miscibility (FCM). In this case two phases already mix at the lowest
possible pressure. Miscibility increases with pressure and at high enough pressure
almost everything makes a single phase. At the same time high pressure is expensive
to produce, implement and sustain. From this point of view it is important for
the given reservoir conditions to know the minimum miscibility pressure. At the mul-
tiple contact miscibility (MCM) two phases first exchange components. By exchange
process the phases obtain undistinguishable properties and create a single phase. It is
widely accepted that the pressure for MCM is lower than for FCM. Volatile compo-
nents in oil play an important role in defining the oil properties. As the result, misci-
bility of live oil (oil with volatile components) and dead oil (without volatile
components) do mix differently with other fluids. The general rule is that the solubil-
ity increases with the pressure and decreases with temperature. Interplay between
influences of oil properties, pressure and temperature on miscibility is very complex
and needs laboratory data for proper assessment from all points of view.
If two liquids do not mix (do not create a continuous single phase system) it is
still possible to create a solution containing both phases. It is generally known as
emulsion (colloidal system is more encompassing term). Emulsion preparation,
properties and behavior are very complex subjects and we will just touch on them
here and provide very simplified description as an introduction.
An emulsion is a dispersive system when one phase is spread in another phase in
form of droplets. Simple emulsions are not stable and over time will separate into