Page 275 - Geology and Geochemistry of Oil and Gas

238 MATHEMATICAL MODELING IN PETROLEUM GEOLOGY

where r is ion amount (in mg-eq.); and a 1 , a 2 , b 1 , and b 2 are empirical coefficients
determined from data of formation water composition.
For example, formation water of the Pliocene productive formations of the Abs-
heron Archipelago oilfields in the South Caspian Basin are characterized by em-
pirical coefficients shown in the following table:

Field a 1 a 2 b 1 b 2
Darvin Bank 2.40 20 2 10
Pirallaghi Adasi 1.72 26 2 3
Chalov Adasi 1.72 25 2 2
Palchygh Pilpilasi 1.60 22 2 0
Neft Dashlary 1.92 18 2 5

Based on the data on formation waters in the onshore oilfields of Azerbaijan
(Zhabrev and Khatskevich, 1950, in: Samedov and Buryakovsky, 1966, p. 136), these
coefficients were as follows: a 1 ¼ 1:96, a 2 ¼ 6, b 1 ¼ 2, and b 2 ¼ 0.
Data obtained by Eremenko and Bezhayev (1956, in: Samedov and Buryakovsky,
1966, p. 137) from oilfields of Daghestan area (Southeastern Pre-Caucasus oil and
gas province) indicate that these coefficients are different in different oilfields and
stratigraphic sequences. For the Tertiary and Cretaceous formations, the following
values were obtained: a 1 ¼ 1:8, a 2 ¼ 11:5, b 1 ¼ 2, b 2 ¼ 0:5 and a 1 ¼ 2, a 2 ¼1–3,
b 1 ¼ 2, and b 2 ¼ 15, respectively.

+
Type of formation water can be determined by rNa /rCl ratio: water is alkaline
+

+

if rNa /rCl 41 and is hard if rNa /rCl o1.
Considering the system of Eqs. 11.45, one can obtain the following equation:

h . X i
þ X
rNa =rCl ¼ a 1 =b 1 r b 2 r a 2 (11.46)
For the Neft Dashlary Oilfield in the Absheron Archipelago (Azerbaijan),
Eq. 11.46 can be presented as follows:
h X . X i

þ
rNa =rCl ¼ 0:96 r 5 r 18 (11.47)

þ
Transition from the alkaline to hard water occurs at rNa ¼ rCl . Water salinity

þ
at this transition point can be determined from Eq. 11.46 at rNa ¼ rCl :
X
r ¼ ða 1 b 2 b 1 a 2 Þ=ða 1 b 1 Þ ¼ m=n (11.48)
where m ¼ ða 1 b 2 b 1 a 2 Þ and n ¼ ða 1 b 1 Þ.
For example, in the stratigraphic section of Neft Dashlary Oilfield this transition
occurs at a water salinity of P r ¼ 330 mg-eq:=100 g of brine. The transition
point may be found as the abscissa of intersection of two straight lines described by
Eqs. 11.45.
Class of formation water is determined using characteristics proposed by Charles
+
+
Palmer in 1911 (see Sulin, 1948) based on the correlation between (Na +K ) and
2

–
(Cl +SO 4 +NO 3 ) ions (in % mg-eq.). The ion-equivalent form (mg-eq.) converted