Page 99 - Geology and Geochemistry of Oil and Gas

72 WATER

Radioactivity of formation water is determined by the presence of dissolved
radium, uranium, radon, and thorium compounds. Their content in oilfield
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formation water is as low as 10 –10 11 .
The capability of water to dissolve gaseous hydrocarbons is reviewed in Chapter 6.
Liquid hydrocarbons under standard conditions (0.1 MPa and 20 1C) are almost
insoluble in water. A film of surface tension is clearly observed between water and
liquid hydrocarbons. This solubility increases as temperature and pressure increase.
This process is especially clear at a temperature of about 150 1C and near-critical
pressure (21.8 MPa). The critical point of the water–oil binary solution is not studied
well. Besides, such a solution can hardly be called ‘‘binary,’’ because the oil itself is a
complex system of interacting components with very diverse critical temperatures
and pressures.
The study of critical temperature of oils is not easy, because their flash points are
often below their critical temperatures. There is a drastic increase in the solubility of
liquid hydrocarbons in water with increasing temperature and upon exceeding the
critical pressure of water.
If oil is not completely dissolved in water, chemical composition of the dissolved
oil may be substantially different from that of the original oil. Light oil is more
soluble in water than the heavy oil, and its critical temperature is lower than that of
the heavy oil. Possibly, the relatively light oils may have a high solubility at lower
temperatures (below 100 1C). The reason for the latter supposition is that the
solubility of hydrocarbons belonging to the same chemical type decreases with
increasing molecular weight. With the same number of carbon atoms in the
molecule, aromatic hydrocarbons are more soluble than the other hydrocarbons.
The writers did not analyze here the colloidal–emulsion solution mechanism of
hydrocarbons in water (although sometimes this type of solubility may be higher
than the true solubility). The reason is that the colloidal–emulsion solution
mechanism appears to play a very limited role in the hydrocarbon transfer in nature.
Beletskaya (1990, p. 40) lists the following reasons for this:
(1) difficulties associated with the micelle movement in the pore space of rocks;
(2) destruction of colloids with temperature increase;
(3) adsorption of surfactants by rocks and the destruction of micelles and
emulsions;
(4) electric barrier — hydrocarbon micelles are negatively charged, are hydrophilic,
and move with difficulty along the mineral surfaces;
(5) the need for the presence of surfactants at reservoir conditions to a degree
sufficient to overcome the barrier of the critical micelle-formation concentration.
Beletskaya (1990) stresses the fact that surfactants lose their properties with
increasing temperature. In the experience of writers, water-in-oil emulsions break at
higher temperatures.
On basis of the aforementioned fact, it is reasonable to suggest the absence, in
many cases, of a clear-cut oil–water contact (OWC) and the presence of a transition
zone. A stable oil–water emulsion can form within a transition zone as a transitional
state between the two immiscible liquids. The thickness of the transition zone
depends on the chemical properties of oil and water and on the subsurface