Analysis of the causes of corrosion of underground pipe columns in an oil field in Bohai Sea.

 Introduction:
polymer drive as the main way of oil production three times in oil fields, because of the significant effect of oil increase, the technology is widely used in domestic on-land oil fields. Bohai oil field as early as 2003, carried out a single-well polymer drive experiment, and achieved good development results. So far, Bohai oil field has carried out large-scale polymer oil drive field test and other oil fields such as the 36-1 oil field under the jurisdiction of Liaodong

. Water injection well pipe corrosion, soil accumulation in the well and polymer scale card and other problems are increasingly prominent, resulting in a decline in injection production, seriously affecting the normal production and economic benefits of oil fields.
present, the problem of underground pipe column corrosion in Bohai oil field is very prominent, which has a great impact on oil field exploitation, according to the different corrosion factors, the causes of underground pipe column corrosion are also different. In order to carry out anti-corrosion measures in a targeted way, the first problem is to identify the causes of corrosion, so this paper provides a theoretical basis for the subsequent corrosion prevention by analyzing the inlet fluid, chloride ion corrosion prediction, SRB content analysis, natural gas composition analysis, extracting liquid corrosion evaluation, corrosion product analysis, etc.
analysis of liquid corrosion
during the repair of wells in Bohai Oilfield in 2013-2017, a total of 14 wells were found to be perforated by the corrosion of the benefit wells, of which the separator corrosion broke 7 times. For example, the oil field A13 well in September 2015 pipe change column operation also found serious corrosion of the pipeline, the electric pump separator has a diameter of 1 cm corrosion perforation, in January 2018, the pipeline corrosion perforation was again found.
1 A13 well Y joint above the pump head above the double male short section perforation
Figure 2 A17 well 116th, 117th trout connection (depth 1136.41m)
A17 well Wells found that the underground oil pipe has corrosion perforation phenomenon, the perforation of the pipeline is mainly large and small, sharp edges, corrosion products slightly attached, showing a pitting appearance, the thickness of the pipe wall around the hole from the outward inward gradually thinning, which shows that the perforation is formed by gradually corrosion from the outside inward. The hole is horseshoe-shaped, with sharp edges, and it can be assumed that there is constant media flushing during corrosion. A13 well outside the wall of a large number of corrosion pits, local thinning of more than 60%, no corrosion pits, corrosion uniform.
well fluid corrosion analysis
cation corrosion: water containing potassium, sodium and calcium ions corrodes N80 steel at an average corrosion rate of 0.087 mm/a, 0.081 mm/a, 0.11 mm/a, respectively. At the same ion concentration, the most corrosive is calcium ions.
anion corrosion: N80 steel in chlorine-containing ionized water, the corrosion rate is 0.09 mm/a, while in water containing the same concentration of hydrocarbon ions, the corrosion rate is 0.04 mm/a, chlorine ion corrosion is greater than the corrosion of hydrocarbon ions. Water quality analysis shows that the water type is calcium chloride type, chlorine ions between 4,284 to 4,339 mg/L, the column may be serious chlorine corrosion.
prediction of chloride ion corrosion
the use of Labor index method to predict chlorine ion corrosion. When the LI value is greater than 0.5, it is easy to occur cl-based electrochemical corrosion.
of liquid corrosion
found that a number of well oil pipe corrosion, it is initially suspected that the well extraction liquid may have some corrosion of the pipe column. The corrosion rate of the water extracted from the A17 well by measuring the N80 pendant was carried out, and the corrosion of the water extracted from the well was studied.
the experiment refers to the "SY/T 5405-1996 acidification corrosion inhibitor performance test method and evaluation index" in the "3. normal pressure static corrosion rate, corrosion rate measurement method and evaluation index", the N80 hanging piece was placed in the A17 well extraction water, placed 72 h in the 65 degrees C thermostat oven, by determining the quality of the aging before and after hanging piece, the study of water corrosion.
Figure 3 Before and after the cleaning of the hanging pad
analysis results show that the average corrosion rate of the well-extracted water on the N80 trailer is 0.09675 mm/a, referring to the SY/T-5329-2012 debris rock reservoir water quality index and analysis method≤ From Figure 3, it can also be clearly seen that rust spots already exist on the surface of the corroded gaug, indicating that the corrosion rate of the water extracted from the well is high, and that there is a relationship between the corrosion of the well's pipe column and the high corrosion rate of the well's water itself. Some of the pitting found in this evaluation may have come from chlorine ion corrosion.
Water-like SRB bacterial content analysis
If the well liquid contains sulphate reducing bacteria SRB, when the new environment generated by mining is conducive to bacterial growth, these species will multiply to produce hydrogen sulfide, and then corrode the column, so the water-like sulphate reducing bacteria SRB content needs to be tested. Water-like sulphate reduction bacteria SRB content detection reference SY/T 5329-2012 debris reservoir water quality indicators and analysis methods, continuous monitoring of water-like sulphate reducing bacteria SRB content, thermostat culture temperature of 59 degrees C, culture time of 7 d, the results show that: no SRB bacteria were observed.
Figure 4 Water-like sulfate reducing bacteria SRB content detection
natural gas composition analysis
take the well with anger, using the composition of natural gas analysis gas chromatography GB/T 13610-2003 detection gas composition. The results of natural gas analysis show that the above wells may have carbon dioxide corrosion. Combined with the bottom pressure, the calculation of A13 well and A17 well CO2 pressure is greater than 0.07 MPa, refer to the QHS 14015-2012 offshore oil and gas well oil and gas well oil pipe and casing corrosion design guide, the use of N80 ordinary carbon steel pipe production, there is a risk of corrosion.
analysis
outer wall corrosion sample analysis
the outer wall is a block corrosion product, using the German ZEISS Zeiss EVO MA15 scanning electroscope for scale sample morphology and element content analysis. The pattern of scale samples is determined by scanning the electroscope (SEM) and the samples of different forms are classified (Figure 5).
are often used in conjunction with X-ray energy spectrometers (EDS) to analyze the types and content of component elements in the micro-regions of the material. EDS analysis shows that the outer wall corrosion products are FeCO3, Fe2O3 and Fe3O4, carbon dioxide corrosion products and iron oxides. Some corrosion samples contain chlorine, indicating the presence of Cl-local corrosion.
Figure 5 corrosion scale scanning electroscope map
inner wall corrosion sample analysis
inner wall corrosion is more uniform, corrosion products are powdered fine particles, XRD element analysis results show that the inner wall corrosion products are mainly composed of C, O, Fe, Si, Cl and other elements, Cl element content of 0.4%, indicating that regardless of whether the column perforated, the pipe wall inside and outside Cl - in the process of local corrosion plays an important role. FeCO3 (card number 29-0696) and Fe2 (OH) 2CO3 (card number 33-0650) are present according to the search PDF card library.
results show that the corrosion products of the inner and outer walls of the tube column are different, and the corrosion products are different in shape, possibly because of the different corrosion paths of the two. Electrochemical corrosion occurs internally, so for uniform corrosion, gap corrosion occurs in the outer wall, so the corrosion pit and perforated site is obvious.
analysis of the causes of corrosion of the underground pipe column
through the above experiments and the analysis of the composition of corrosion products, it is concluded that the causes of corrosion of the well pipe column in the oil field are as follows:
❶ electrochemical corrosion. Electrochemical corrosion refers to electrochemical reactions in metal tubes and external environmental media. During the reaction, there are separate cathode and anode regions, and electrons flow from the cathode region to the anode region. The metal surface is dosed with many impurities, and when it comes into contact with salt water (electrolyte solution), there must be many tiny cathodes and anodes present at the same time, and the metal surface forms many tiny primary batteries, called microcables. The salt content of different parts of the wellhead is different, so that the electrode level of each section of the wellhead is different, and the underground oil casing can be regarded as a macro-battery corrosion. In electrochemical corrosion, Cl-influence is greater, followed by the influence of Ca2 plus and Mg2 plus content. Therefore, reference to water quality analysis, chlorine, high mineralization is one of the reasons for
❷ corrosion perforation; Single well natural gas, scanning electroscope SEM, energy spectrum analysis EDS and X-ray polycrystalline diffraction XRD analysis, the corrosion products are mainly carbon dioxide corrosion products and iron oxides, iron oxides may be produced after exposure in the air. Therefore, pipeline corrosion is mainly caused
❸ by carbon dioxide corrosion; At present, it is found that most of the failure trouting is a well for injection benefit wells, the pH of the well fluid is 7.5 to 7.8, is weak alkaline, when there is free state oxygen in the injection, the metal in the pipeline and the outer wall is prone to oxygen absorption corrosion. At the same time, the penetration of polymers accelerates the stripping of corrosion products, reducing the thinning of pipe walls.
Through the above analysis, the following conclusions are concluded:
❶ no SRB bacteria, no hydrogen sulfide gas, corrosion factors can eliminate bacteria and hydrogen sulfide corrosion;
❷ layer water chloride, calcium ion content is high, is an important factor in corrosion
❸
❹ Underground pipe column corrosion products are mainly carbon dioxide corrosion products and iron oxides, corrosion causes are mainly carbon dioxide corrosion,
❺ recommends the suspension of injection wells corresponding to injection benefit wells, and detection of casing gas, polymer oxygen content, analysis of oxygen corrosion sources, as well as changes in carbon dioxide content, and improve the corrosion resistance level of the pipeline, the use of corrosion-resistant materials, reduce pipeline corrosion.
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