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Chapter 4

WATER

4.1. PHYSICAL AND CHEMICAL PROPERTIES OF WATERS

In the environment of the Earth’s upper crust, water plays a vital role. Even
subaerial deposits became enriched in water content during diagenesis and
epigenesis. Water reaches a maximum density at +4 1C. Its critical temperature is
374 1C and its critical pressure is 21.8 MPa. Water density declines as its composition
changes from hard to alkaline. Water viscosity is 1.789 cp at 0 1C and 0.282 cp at
100 1C. Water viscosity decreases with increasing temperature and increases with
increasing density and content of calcium and magnesium. Water can reach its
critical pressure at a depth of about 2000 m, which affects some features of the
hydrocarbon habitat in the Earth’s crust (for instance, hydrocarbon dissolution).
Water is a stable chemical compound. Noticeable decomposition of water occurs at a
temperature of greater than 2,000 1C.
At great depth (about 100 km), when pressure is nearly 1,000 MPa and
temperature is 600–900 1C, electrons are totally or in part torn-off from the atoms;
the atoms are close together and are tightly packed. In such an environment, water
as a chemical compound, H 2 O, cannot exist. Water dissociation into H + and OH
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ions under standard conditions is low — a mere 10 . The amount of free hydrogen
ions in water substantially affects its properties as solvent and catalyst for various
chemical reactions. The pH is the negative logarithm of H + concentration. Alkalis
and alkaline earth’s metals increase the pH. At pH ¼ 7, the medium is neutral; when
pH is greater than 7, it is alkaline, and when pH is lower than 7, it is acidic.
The pH of the ground water ranges from 5.6 to 8. The pH is strongly affected by
CO 2 content in water and much less affected by H 2 S, HCl, and some other volatile
components. An increase in CO 2 content in the water lowers pH. Consequently, at
depth, the stability domain for various minerals shifts.
The interfacial tension between the water and oil is highest for hard water
(22.671–5 N/cm) and lower, for the alkaline water (4.471–5 N/cm). The greater the
time of contact between the oil and water, the lower is the interfacial tension. In case
of bacterial infestation, a clear interface between the oil and water may be absent.
Water compressibility is very low. Bulk volume elasticity coefficient for the oilfield
formation water lies between 3 10 4 and 5 10 4 MPa. Dissolved gases in the
water and increasing temperature drastically raise the bulk volume elasticity
coefficient. In petroleum engineering, the formation volume factor (FVF) is
calculated by dividing the water volume at reservoir conditions by its volume under
standard conditions (0.1 MPa, 20 1C).
Heat conductivity of water depends on the concentration of dissolved salts and
temperature and can be determined by logging (see Khilyuk et al., 2000).