Electrochemical impedance spectroscopy as a mean of measuring film thickness of calcium sulfate precipitate on an XC70 carbon steel

In this paper we present the use of electrochemical impedance spectroscopy measurement as a tool to measure the thickness of calcium sulphate film formed on an XC70 carbon steel. The solution used is a mixture of sodium sulphate and calcium chloride solution at different concentrations. The film thickness of calcium sulphate is estimated, after an immersion time of 4 days. The results indicate reciprocal relationship between measured capacitance and thickness of calcium sulphate film.


INTRODUCTION
Usually at the first stage of production only a small fraction of crude oil contained in the reservoir (about 30 %) can be recovered, in this case oil is spontaneously extracted under the only effect of the pressure in the reservoir. As the extraction of crude oil continued, the internal pressure of the reservoir decreases towards atmospheric pressure. This decrease in reservoir pressure affect the yield of the recovery of crude oil from the reservoir [1].
For many years, research has been conducted to improve the ultimate recovery of reserves of crude oil, one of the most commonly used method is called "Secondary" or "Enhanced recovery" and is consist of injection of albian water into the formation, but this injection can cause deposition of calcium sulphate which can provoke blockage of the reservoir rock, due to the incompatibility between the injected albian water, extracted from albian aquifer with high concentration of sulphate ions and formation waters, with high concentrations of calcium ions [2][3].
This present work is a contribution to estimate the quantity of calcium sulphate precipitation on an XC70 carbon steel using electrochemical impedance spectroscopy (EIS) measurements.

EXPERIMENTAL
EIS measurements were performed with PGP301 Potentiostat/Galvanostat in a frequency range between 100 kHz and 10 mHz, using a conventional three-electrode cell with a platinum wire as the auxiliary electrode and an Hg/Hg 2 Cl 2 (saturated with KCl) as reference. Working electrode was an XC70 carbon steel with a surface of 7.07 mm 2 . Before use, it was carefully polished to a mirror finish with 1.0, 0.3, and 0.05 µm alumina slurries, successively. The working electrode was then immerged in a solution of a mixture of sodium sulphate/calcium chloride at concentration of 79.5/5, 37.95/2.25, 18.85/1.25 g/l. The results are represented graphically using complex plane Nyquist plots. The diagrams of Nyquist presented in Figure 1 can be interpreted by a simple equivalent circuit Figure 2, where Rs is the solution resistance, and C dl and R ct are the double layer capacitance and the resistance of charge transfer respectively.

RESULTS AND DISCUSSION
International Letters of Chemistry, Physics and Astronomy Vol. 32 Impedance data obtained from the equivalent circuit of figure 2 are presented in Table 1.
Table1. Data extracted from Nyquist diagrams of figure 1.

1. Capacitance determination
The value of the imaginary impedance of the equivalent electrical circuit presented in Figure 2, is given by the following equation  Figure 3. The capacitance was obtained by extrapolation of the linear fitting of the obtained curve to the intersection with . Data obtained from curves of Figure 3 are regrouped in Table 2. As it can be seen experimental capacitance increase when solution concentration increase.

2. Thickness measurement
The thickness of calcium sulphate precipitate on XC70 carbon steel is calculated from the well-known relation that linking thickness precipitate to its capacitance, equation 4.
where is the dielectric constant of calcium sulphate (5.6) [9], is the primitive of vacuum (8.85.10-12 F·m -1 ) and A is the exposed area of the working electrode (0.707 cm 2 ). Results are shown in Table 3.  Figure 4 show the independence of calcium sulphate film thickness on the solution concentration expressed as mol/l. Thickness of calcium sulphate precipitate increases when both concentration of sodium sulphate (Figure 4 (a)) and calcium chloride (Figure 4 (b)) increases.

CONCLUSION
The thickness of calcium sulphate film formed on an XC70 carbon steel was estimated from capacitance determination using electrochemical impedance spectroscopy measurements in a mixture of sodium sulphate and calcium chloride solution. It has been found that film thickness increase when solution concentration increase.