INHIBITOR PREVENTS CORROSION, SCALE IN CHINESE WATERFLOOD

Wang Yong, Wang Jian-hua Shengli
Design Institute
Dongying, China

An imidazoline derivative-based series inhibitor has prevented both corrosion and scale formation in produced-water treatment and water-injection equipment in China National Petroleum Co.'s (CNPC) Shengli oil field.

Development of the inhibitor started in 1986, and after successful field trials the chemical is now being extensively applied.

To increase oil recovery, water injection is widely used in China's onshore oil fields. Oil production in the Shengli oil field, for example, requires injection of about 4 bbl of water/1 bbl of oil produced.

The large volumes of produced formation water contain many substances that can cause serious corrosion and scale. Also, the makeup water from other sources, subsurface or surface, complicates water handling.

CORROSION AND SCALE

Substances that contribute to corrosion and scale in produced waters include the following:

High salt concentrations such as seawater containing calcium (Ca) and magnesium (Mg) ions at thousands of ppm. High salt concentrations tend to accelerate corrosion.
Dissolved gases such as carbon dioxide (CO2), hydrogen sulfide (H2S), and oxygen (O2). Gas mixtures are often more aggressive corrosive agents than a single gas.
Anions such as chloride (C1), sulfate (SO4), bicarbonate (CHO3), and sulfide (S 2-).
Bacteria, of which the most common are sulfate-reducing bacteria (SRB).
Suspended solids in high concentration.

OXYGEN

The solubility of oxygen in water is a function of pressure, temperature, and chloride concentration. The solubility of oxygen in brine is much less than in freshwater, but produced water can still cause severe corrosion of carbon steel even at the concentration of

In general, there are four corrosion processes created by dissolved oxygen:

Electron-lost ferric ions enter solution.
Free electrons move from anode to cathode.
Dissolved oxygen of polarizer absorbs electrons, and cathodic reaction product (OH) enters solution.
Fe(OH)2 deposits are formed by a combination of anodal and cathodic products.

Anodal reaction:

Fe - 2e - Fe++

Cathodic reaction:

O2 + 2H2O + 4e - 4OH

Besides the cathodic reaction, dissolved oxygen can also oxidize Fe(OH)2 into Fe(OH)3:

Fe(OH)2 + 1/4O2 + 1/2H2O

Fe(OH)3

Generally, the actual corrosion product is a very complicated mix.

CARBON DIOXIDE

Carbon dioxide is an ionizable gas which forms weak carbonic acid when dissolved in water. It contributes greatly to corrosion in water treatment and injection systems.

The solubility Of CO2 in water is related to pressure, temperature, and water components. Solubility increases as pressure increases, and it decreases as temperature increases.

When free CO2 is present in water, the water is acid reactive:

CO2 + H2O - H+ +

HCO3-

The increase of H ions i water produces depolarization corrosion of hydrogen:

Anodal reaction:

Fe - Fe + 2e

Cathodic reaction:

2H+ + 2e - H2

The metal product produced by CO2 corrosion is soluble. Higher water temperature increases carbonic acid ionization and this accelerates corrosion.

H2S AND SRB

Of the several types of bacteria responsible for serious corrosion in China's oil fields, the most troublesome are sulfate-reducing bacteria (SRB).

Large amounts of SRB reduce inorganic sulfate (SO4 2-) to sulfide (S2-) and produce hydrogen sulfide (H2S). Some H2S is present in most produced waters.

The ionization formula of H2S containing water is:

H2S = H+ + HS

HS = H+ + S2-

The mechanism for H2S corrosion is hydrogen depolarization:

Anodal reaction:

Fe - Fe 2 + 2e

Cathodic reaction:

2H + 2e - H2

The combined effect of the anode-reacted product (Fe) and 52 - in water produces iron sulfide, which is an insoluble black precipitate.

Fe++ + S-- = FeS

TEMPERATURE

In produced water, high salt concentration can increase the conductivity of water, accelerating the electrochemical reaction and corrosion. Because higher temperatures can accelerate all chemical reactions, the corrosion rate increases as temperature increases.

Also, higher temperatures increase oxygen diffusion in water and bicarbonate decomposition to release more CO2, again accelerating corrosion.

SCALE

Scale is often one of the major problems for oil field operators. In general, scales are dissolved salts in abnormal solubility states. The most common are calcium carbonate, magnesium carbonate, calcium sulfate, barium sulfate, strontium sulfate, and magnesium salt.

Formation of scales depends on the degree of supersaturation and the process of salt crystal growth. Dissolved salts in water are in the ion state when the concentration is lower than the ionic solubility product.

Once the concentration reaches supersaturation, these supersaturated salts precipitate crystals to become crystallized scale under the catalytic action of rough equipment surfaces and foreign substances.

If produced waters contain high concentrations of carbonate and chloride, carbonate scale can form. Scale starts forming as long as produced water undergoes temperature and pressure changes that break the equilibrium of dissolved substances. Generally, scales from produced water are mixtures because of the depositions of organic substances such as residual oil, bacteria and organic residues, silt, sand, and clay.

INHIBITION

On the basis of the electrochemical theory, any electrochemical corrosion process consists of anodal and cathodic reaction processes where the depolarizer accepts electrons. Injection of corrosion inhibitors can prevent the anodal or cathodic reaction process.

Chemically, inhibitors are classified as organic and inorganic.

Based on long-term study and oil field performance, organic inhibitors perform much better in produced waters than inorganic. Organic inhibitors are usually composed of polar groups formed by paired-electrons containing oxygen, nitrogen, sulfur, and phosphorus atoms and non-polar groups formed by carbon and hydrogen atoms.

To shield the metal from corrosive agents, the polar groups change the electric double-layer structure and increase the activation energy in the metal ionization process by adsorbing on metal surfaces.

As previously stated, formation of scales depends on both the supersaturation degree and the salt-crystal growth process. The most common scale is carbonate, especially calcium carbonate that forms in a certain lattice.

Based on the solubility-product principle and the crystallization process, inhibitors can increase salt solubility and destroy the normal crystal growth to produce a lattice distortion that inhibits scale formation.

The Series SL-2 inhibitor, an imidazoline-containing phosphate or thiophosphate derivative, contains phosphate that can distort the lattice by destroying the normal crystal growth and the nitrogenous polar groups that can adsorb on metal surfaces. Therefore, SL-2 simultaneously inhibits corrosion and scale.

The phosphate or the thiophosphate imidazoline derivative inhibitor was developed by condensation and addition and esterification reactions of some components. The inhibitor has good compatibility with commonly used bactericides, clarifiers, and other water-treatment agents and does not emulsify the oil-water mixture.

For SL-2, a China National Invention Patent (Application No. 87104569, 5-4, in 1987) was granted in December 1991. The China Patent Office says that it found no other dual-function inhibitors in the world, at that time.

FIELD TRIALS

Field trials indicate that 10-20 ppm of SL-2 can reduce the average corrosion rate of 0.58 mm/year with a pitting rate of 2.25 mm/year to 0.025 mm/year without visible pitting. The scale inhibition efficiency was about 95%.

The inhibitor was tested in Chunliang and Lingpan produced water treatment (PWT) plants, Shengli Oil field. After a 1-year trial in Chunliang, the average corrosion rate of 0.762 mm/year with 2.43 mm/year of pitting was decreased to under 0.04 mm/year without pitting. Scale formation was inhibited completely.

In Lingpan, a 2-year test indicated that the corrosion rate of 0.4 mm/year with 2.0 mm/year pitting rate was reduced to 0.006 mm/year without pitting. The serious scale problem also was eliminated.

APPLICATIONS

Field-wide application of SL-2 started in the second half of 1989. About 17 produced-water treatment plants are now being protected. These plants in the Shengli field are owned by the Shengcai, Hekou, Gudao, Gudong, Xianhe, Binnan, and Dongxin subsidiaries.

As an example, serious corrosion and scale occurred in the Tuoliu plant system, which has a treating capacity of 125,000 bw/d. The average corrosion rate was up to 0.6 mm/year and was accompanied by heavy pitting and a large amount of hard scale adhering to the metal surfaces.

In March through July 1990, with the plant treating about 125,000 b/d of produced water, 800 lb of SL-2, 18 ppm, and bactericide were batch injected once a week.

After the treatment, the corrosion rate decreased to less than 0.0042 mm/year, without pitting or scale formation. The result was considerably lower than the 0.075 midyear specified by CNPC.

In another example, the Lijing produced-water treatment plant, because of serious corrosion and scale, was abandoned and rebuilt in the second half of 1987. In the rebuilt plant, a 10-ppm concentration of SL-2 controlled corrosion to less than 0.008 mm/year, without pitting and scale formation on the smooth surfaces of coupons.

Bacteria monitoring showed that SRB and THB (total heterotrophic bacteria) contents were 10 million colonies/ml and 1 million colonies/ml, respectively. With continued injection of SL-2 since July 1987, no corrosion failure has occurred. The pump cavity and the pipes have remained clean after 2 1/2 years of operation.

The Tuowu produced-water treating plant is a third example. After starting operations in April 1990, no bactericide was injected and serious corrosion and scale formation occurred.

Once SL-2, 15 ppm, injection began, the corrosion was controlled to less than 0.018 mm/year and scale formation was prevented. Bacteria co tents were 10 million colonies/ml SRB and 10,000 colonies/ml THB. Oxygen concentration was 1.5 ppm.

Since the last quarter of 1989, an evaluation of water quality stabilizers (corrosion and scale inhibitors, bactericides) was made on 11 produced-water treatment plants.

The 200,000 bw/d Gudong No. 1 plant experienced corrosion with a different inhibitor. Replacement of the inhibitor with SL-2 at 15 ppm reduced the corrosion to under 0.006 mm/year, without any plant failures due to corrosion. Because of the effectiveness of corrosion and scale control, the inhibitor concentration was reduced to 10 ppm.

ECONOMICS

Because of its success, demand of SL-2 increased in the Shengli Oil field from about 1.8 million lb in 1990 to 2.9 million lb in 1991. The saving from not needing a separate scale inhibitor was about 1.6 million-2.11 million RMB yuan (about US$280,000-370,000) in 1990 and 2.6 million-3.43 million RMB yuan (about US$450,000-600,000) in 1991.

The corrosion and scale inhibition efficiency of SL-2 has been 50% higher than other field-used inhibitors. With SL-2 injection, corrosion-caused shutdowns for maintenance have been avoided and plant operating life has increased.