Page 130 - Geochemistry of Oil Field Waters

118 ANALYSIS OF OILFIELD WATERS

Constituent Concentration( mg/l)
Na+ 13,500
K+ 400
Li+ 10
Ca+ 2,000
Mg+ 1,200
c1- 23,500
Br- - 500
HC03 1,200
S04-2 1,200
Total dissolved solids 43,500

Spent acid

Hydrochloric acid is the oldest and most common solution used in oil-well
acidizing (Halliburton Company, 1970)..Many additives and other acids may
be used in conjunction with HC1, for example HF and HAc. Normally, 15%
HCl is used; however, other strengths are quite common. These acid solu-
tions are pumped into carbonate formations to dissolve and remove a part of
the formation.
After reacting with carbonate rocks or being “spent” on the formation,
the solutions are returned to the surface by various means. Often, they are
mixed with formation water, and an operator may want to know when the
spent acid has been recovered, or if formation water or a mixture of solution
and water is being produced.
When 15% HC1 is completely spent on CaC03 or MgC03, the resulting
solution will contain 90,000 mg/l Ca or Ca equivalent. The normal formation
water contains only about 10,000 mg/l Ca or Ca equivalent. The procedure is
based on these differences.

Reagents and equipment. The necessary reagents and equipment include:
Calcium carbonate, 10-mesh.
NH4 OH, reagent.
Whatman No.31 filter paper.
Plastic funnel.
150-ml beakers.
Graduated cylinder, 25 ml.
1-ml syringe or pipette, preferably plastic.
0.5% Eriochrome Black T indicator.
2N NaOH solution.
CDTA solution (disodium dihydrogen 1,2-cyclohexanediamine-tetraace-
tate): dissolve 100.0 g CDTA in 900 ml water and dilute to 1 liter. 1 ml
equals 9.0 mg Ca.