Page 171 - Geochemistry of Oil Field Waters

158 INORGANIC CONSTITUENTS AND PHYSICAL PROPERTIES

Phosphorus

Phosphorus occurs in the earth’s crust almost exclusively as the
ion, and a large percentage of it is contained in the apatite group of minerals,
which primarily are related to igneous rocks. The crust of the earth contains
about 0.12 wt.% of phosphorus. It is a member of the V A group of elements
with oxidation states ranging from -3 to +5. In contrast to nitrogen,
phosphoric acid and phosphates are not oxidizing agents.
The phosphorus species present in most natural waters probably is the
phosphate anion, and it usually is reported as an equivalent amount of the
orthophosphate ion (PO4 )-3, the final dissociation product of phosphoric
acid, H3 PO4. This dissociation occurs in four steps, giving four possible
phosphate forms: H3 PO4, H2 PO4-, HP04-2, and P04-3. In the alkalinity
titration, any HP04-2 is converted to H2P04- and appears as bicarbonate.
Shales, sandstones, and carbonates contain about 700, 170, and 400 ppm,
respectively, of phosphorus. Sea water contains about 0.07 mg/l. A detailed
study of the content of phosphorus in subsurface brines has not been made,
but of the few that have been analyzed, most have contained less than 1
mg/l.

Arsenic

Arsenic is a member of the V A group of elements and probably occurs in
nature mainly in the form of arsenides and sulfarsenides; it rarely occurs in
its elemental form. It is comparatively rare, and the earth’s crust contains
about 0.0005 wt.% of it (Fleischer, 1962). In an acidic environment, the
oxidized ion, AsO~-~, mobile, and mineral arsenates tend to be solu-
is
bilized. The arsenates usually are formed in oxidation zones in contact with
atmosphere and free oxygen, and arsenic will precipitate with ferric iron
hydroxide. Glauconitic sediments have been found which contain up to 70
ppm of arsenic (Goldschmidt, 1958).
Subsurface oilfield brines may contain arsenic as HAs02 - or H2 As04,
depending upon the Eh and pH. A low Eh may favor the HAs02- form.
Shales, sandstones, and carbonates contain about 13, 1, and 1 ppm,
respectively, of arsenic (Mason, 1966). Sea water contains about 0.003 mg/l
and subsurface oilfield brines contain from 0 to 10 mg/l. Compounds con-
taining arsenic sometimes are used in corrosion inhibitors; therefore, infor-
mation concerning well treatments should be obtained before assuming that
any arsenic found occurs naturally.

Oxygen

Oxygen is the most abundant element in the earth’s crust, which contains
about 49 wt.% of it (Fleischer, 1962). It is capable of existing in many types
of combinations, and even though it is highly active, it occurs extensively in