• Experimental part
• detection method and principle of organic chlorine in crude oil

The standard GB/T18612-2011 determination of organic chlorine content in crude oil is adopted for the detection of organic chlorine in crude oil. Most of the organic chlorine in the crude oil is concentrated in the light and medium fractions, while the chlorine in the heavy fraction is inorganic chlorine. Therefore, the total amount of organic chlorine in the naphtha fraction before 320℃can be approximately regarded as the total amount of organic chlorine in the crude oil.

• physical model experiment of chemical assistant or measure fluid and crude oil in Bamianhe Oilfield

The background organic chlorine content of Bamianhe crude oil and the content of organic chlorine in crude oil after adding water injection corrosion inhibitor, water injection bactericide and acidification solution were determined to determine whether the additives or solution had an impact on the organic chlorine of crude oil under the simulated conditions. The experimental steps are as follows:

(1) The background organic chlorine of typical crude oil in Bamianhe Oilfield was determined;

(2) The crude oil is dehydrated by electricity to get enough dehydrated crude oil;

(3) Prepare 2000ml injection water (for injection aid) or tap water of acid station (for acidification measure liquid) for each aid or measure liquid according to the specified concentration, and then mix it with 2000ml dehydrated crude oil of corresponding block to form a 1:1 mixing system, in 80℃water bath (for water injection aid) or in 70℃and 10MPa cotest high temperature and high pressure reactor (for acidification measure liquid) The total heating time is about 24h;

(4) After the reaction, the oil layer was separated and dehydrated again, then the organic chlorine in the crude oil was determined;

(5) For the background organic chlorine content of crude oil and the organic chlorine content after the reaction between crude oil and auxiliary agent or measure solution, the influence of auxiliary agent or measure solution on the organic chlorine content of crude oil is analyzed.

2. Results and discussion

2.1 effect of water injection additives and injected water on organic chlorine in crude oil of Bamianhe Oilfield

The effect of water injection additives on the organic chlorine content of crude oil in Bamianhe Oilfield through physical model reaction is shown in Table 1. It can be seen from the table that: compared with the results of the reaction of crude oil and blank water sample, the mass concentration of the organic chlorine in the crude oil after the reaction of crude oil and water injection assistant does not change much, both of which are within ± 0.3mg/l of the blank value. Even if the amount of corrosion inhibitor or bactericide is increased to 10 times of the on-site amount (the mass fraction is higher than 0.01%), the increment of organic chlorine in the crude oil after the reaction with crude oil is very small. Considering the systematic errors of experimental methods and measuring instruments, it can be concluded that the addition of water injection additives has little effect on the organic chlorine content of crude oil in Bamianhe Oilfield, but has little relation with its own chemical structure. Since the organic chlorine content of the water injection additives used in the experiment is up to the standard (mass fraction < 0.05%, Sinopec standard), it can be inferred that the organic chlorine content of the additives can be up to the standard and the chemical additives with the mass fraction not higher than 0.01% will not affect the organic chlorine content of the raw oil in Bamianhe Oil field.

On the other hand, this experiment also shows that the organochlorine content of Bamianhe crude oil is relatively sensitive to injected water. The mass concentration of background organic chlorine in M1 well crude oil is 1.002mg/l, which increases to 1.954mg/l after model reaction with injected water of combined station; the mass concentration of background organic chlorine in J5 well crude oil is 1.597mg/l, which increases to 2.793mg/L after model reaction with injected water of nanblock station. After inspection, the mass concentrations of chloride ions in the injected water of the combined station and nanblock station are 13450.18mg/L and 19741.65mg/l, respectively, which are much higher than the concentration of the water injection additives, and should have a greater impact on the organic chlorine in crude oil.

In order to analyze the content of water-soluble organic chlorine in three kinds of injected water, toluene (analytical alcohol) was used to extract the injected water, and then the content of organic chlorine in toluene before and after extraction was determined. The results showed that the content of organic chlorine in toluene decreased after extraction compared with that before extraction, which may be caused by the transfer of sulfur-containing impurities in toluene (affecting the detection of organic chlorine in the instrument) to the injected water, and also showed that the injected water basically did not contain organic chlorine. The impact of the injected water on the organic chlorine in crude oil should come from the rich inorganic chlorine ions.

The average sulfur content of crude oil transported from Bamianhe Oil field is about 1.660%, which is in the same order of magnitude as that of chloride ion injected into water. A small amount of sulfur in petroleum is in the form of elemental sulfur and hydrogen sulfide, and most of it is in the form of organic sulfide, such as mercaptan, thiophene, cyclic sulfide, disulfide, thiophene and their homologues. Under certain conditions, these substances will react with inorganic chloride ions to produce organic chloride. In this reversible substitution reaction, a small amount of reaction (1-10mg / L) should be able to change the content of organic chlorine in crude oil.

There are about 20% - 30% thioether sulfur in crude oil sulfur, which is the highest content of crude oil sulfur except thiophene sulfur with low reactivity. In general sulfur-containing crude oil, the sulfur ether in diesel oil fraction and vacuum fraction is mainly cyclic sulfur ether. In the presence of inorganic chloride ions, organic sulfur molecules can produce SN1 nucleophilic substitution reaction (taking cyclic sulfur ether as an example) to generate trace organic chlorine substances. Therefore, high sulfur crude oil may generate more organic chlorine substances with inorganic chloride ions rich in injected water.

2.2 effect of acidizing solution on organic chlorine in crude oil of Bamianhe Oilfield

In this part, through the analysis of acid, corrosion inhibitor, drainage aid, iron ion stabilizer and other additives in the acidizing solution, they react with the crude oil of Bamianhe Oilfield under the physical model condition alone or jointly, to find out whether the organic chlorine in the crude oil exceeds the standard, and analyze its change rule.

The experimental results show that:

(1) The reagent blank used in the experiment, i.e. the iron ion stabilizer, the acidizing inhibitor (to prevent the corrosion of the crude oil dehydration instrument) and the demulsifier used to separate the oil layer after the physical model reaction, only have a slight impact on the organic chlorine in the crude oil.

(2) The organic chlorine increment of crude oil will increase with the increase of HCI concentration. Fresh HCI (10% HCI) will increase the organic chlorine increment sharply; residual HCI (1% HCI) will increase slightly, but still have a greater impact on the organic chlorine in crude oil; more residual HCI (0.1%) will have a small impact on the organic chlorine in crude oil.

(3) Compared with hydrochloric acid, fluoboric acid can reduce the increment of organic chlorine in crude oil. Fresh fluoboric acid (8% fluoboric acid) still has a greater impact on the organic chlorine of crude oil, but it is much smaller than fresh hydrochloric acid; as for residual fluoboric acid (0.8%, 0.08%), the impact on the organic chlorine of crude oil is smaller.

(4) The results of the physical model test of the soil acid and hydrochloric acid are similar to those of the fresh HCI, while the results of the fluoroboric acid are similar to those of the fresh fluoroboric acid. Through comparison, it can be found that fluoboric acid has lower chlorine content than conventional acid. Because industrial fluoboric acid also contains about 0.2% inorganic chlorine, the effect of fluoboric acid solution on the content of organic chlorine in crude oil is weaker than that of HCI, but it still has a certain improvement.

Experiments have shown that the organic chlorine increment of crude oil does not exceed 1.5mg/l when 16% sodium chloride solution (the chloride ion concentration provided is similar to that provided by 10% hydrochloric acid) reacts with Bamianhe crude oil in a physical model. Therefore, the fundamental reason for the increase of organic chlorine in crude oil caused by acidizing solution is its strong acidity. When organic sulfur molecules are in contact with a large number of hydrogen ions (for example, cyclic sulfide), the SN1 nucleophilic substituent is easy to occur. After contacting with a large number of acids, cyclic sulfide is easy to open (similar to the acid catalyzed ring opening of epoxy ether). Then inorganic chloride ions will quickly combine with carbon positive ions to generate organic chlorine molecules, and then generate organic chlorine salts due to the nucleophilic nature of sulfur atoms. As a result, the concentration of organic chlorine in crude oil after acid treatment is increased. In other words, strong acids accelerate the reaction of organic sulfur molecules with inorganic chloride ions to produce organic chlorine.

The above experimental results are consistent with the conclusion of an expert et al. That is to say, the high organic chloride content of crude oil after acidizing simulation experiment is likely to come from hydrochloric acid.

In addition, the pH value of the backflow fluid from Bamianhe acidizing well is about 3-5, which is equivalent to the mass fraction of chlorine in residual HCI of 4% ~ 0.04%; there has been a high content of inorganic chlorine in the formation water underground (similar to the injected water), and the content of hydrogen ion in residual acid is hundreds to tens of thousands of times higher than that of neutral hydrogen ion, so the residual acid after acidizing still has a significant impact on the content of organic chlorine in crude oil, This has also been proved by experimental results.

3 conclusions and suggestions

In this paper, through the physical model experiments of water injection additives, injection water, acidizing solution and crude oil of Bamianhe Oilfield, the problem of over standard organic chlorine in crude oil exported from the oilfield and the upstream source of organic chlorine corrosion of refining equipment are studied. The main conclusions and suggestions are as follows:

(1) Excluding the influence of low concentration oilfield chemicals on the organic chlorine in crude oil, oilfield enterprises can normally use oilfield chemicals with low concentration (less than 0.01% in mass) that meet their own organic chlorine standards.

(2) Because of the high content of inorganic chloride ions in injected water, new organochlorine can be produced by micro substitution reaction with organic sulfur molecules in sour crude oil.

(3) In the common acidizing solution of Bamianhe Oilfield, the effect of hydrochloric acid and earth acid acidizing solution on organic chlorine in crude oil is much greater than that of fluoboric acid acidizing solution, because the existence of HCI greatly accelerates the substitution reaction of organic chlorine produced by organic sulfur.

(4) In order to avoid the problem of organic chlorine corrosion in the downstream refining equipment and the problem of crude oil export in the oil field, it is suggested that oil field enterprises gradually replace part of hydrochloric acid and earth acid with fluoborate acid in acidification measures.