Prediction method for long-term corrosion rate of on-site oil pipe

Abstract

The invention discloses a method for predicting the long-term corrosion rate of an on-site oil pipe. The prediction method comprises the following steps that S1, the initial inner diameter, the initial wall thickness and the initial outer diameter of an oil pipe before the oil pipe goes into a well are obtained; s2, an oil pipe is put into a shaft for production; and S3, the oil pipe is taken out, section cutting is carried out at different depth positions of the oil pipe, the depth, the inner diameter and the outer diameter of the corrosion pit at each section are measured, and then the long-term corrosion rate of the oil pipe is obtained. According to the method, the long-term corrosion rate and pitting rate of the oil pipe at different depth positions can be accurately calculated, and the corrosion prediction accuracy is improved.

Description

Technical Field

[0001] The invention relates to a method for predicting the long-term corrosion rate of an on-site oil pipe, and belongs to the field of oil and gas production wellbore integrity. Background Art

[0002] In the process of oil and gas production, the damage of oil pipes due to corrosion has become a major problem that plagues the oil and gas production industry. Oil pipes are severely corroded in harsh environments (such as high temperature, high pressure, high mineralization, high water content, rich in CO2 or H2S, etc.), which seriously affects the oil and gas production process. Although anti-corrosion technology (such as research on corrosion-resistant pipes, coating of oil casing surfaces, cathodic protection, injection of corrosion inhibitors, etc.) can control and slow down the corrosion of oil pipes, it is still impossible to completely avoid it. Therefore, it is necessary to evaluate the corrosion performance of oil pipes in service conditions.

[0003] CO2 often exists in oil and gas as an associated gas of petroleum and natural gas. In addition, existing technologies often use CO2 as an oil displacement agent to increase the recovery rate of oil and gas. However, when evaluating the corrosion of oil casing in a CO2 environment, existing technologies mostly use hanging pieces to directly test the corrosion resistance of the oil pipe material. In actual applications, the test results of this test method are far from the corrosion results of the oil pipe under actual working conditions. The indoor experimental test environment cannot fully simulate the actual working conditions downhole, and the test time in the indoor high-pressure reactor is limited, which cannot simulate the long-term corrosion of the actual downhole oil pipe.

[0004] In the prior art, the judgment and early warning of oil pipe corrosion are often performed based on classical theoretical formulas or laboratory simulation data, which often only analyzes a single factor and fails to fully consider the complexity of the downhole environment. Therefore, the prior art has significant limitations in accurately judging the degree of oil pipe corrosion and cannot provide reliable corrosion prediction data, thereby affecting the safety and production efficiency of oil and gas wells. The present invention is specially proposed. Summary of the invention

[0005] The purpose of the present invention is to provide a method for predicting the long-term corrosion rate of oil pipes on site. The present invention can accurately calculate the long-term corrosion rate and pitting rate of oil pipes at different depths, improve the accuracy of corrosion prediction, and provide reliable data for corrosion assessment of downhole environments, thereby helping to optimize the maintenance and management strategies of oil and gas wells.

[0006] The method for predicting the long-term corrosion rate of an oil pipe provided by the present invention comprises the following steps: measuring and recording the initial dimensions of the oil pipe, including the inner diameter, wall thickness and outer diameter, before the oil pipe is lowered into the well, and recording the time when the oil pipe is lowered into the wellbore and taken out, so as to determine the service time of the oil pipe; after the oil well has been producing for a period of time, the oil pipe is taken out, and the corrosion pit depth, inner diameter and outer diameter are measured at different depths, and the maximum corrosion pit depths of the outer wall and the inner wall are calculated and compared; and the long-term corrosion rate of the inner wall, the long-term corrosion rate of the outer wall and the pitting rate of the oil pipe are calculated through these data and the service time.

[0007] Specifically, the method for predicting the long-term corrosion rate of an oil pipe provided by the present invention comprises the following steps: S1, obtaining the initial inner diameter, initial wall thickness and initial outer diameter of the oil pipe before it is lowered into the well; S2, lowering the oil pipe into the wellbore for production; S3, taking out the oil pipe, cutting the sections at different depths of the oil pipe, measuring the depth, inner diameter and outer diameter of the corrosion pit at each section, and then obtaining the long-term corrosion rate of the oil pipe.

[0008] In the prediction method of the present invention, in step S1, the initial inner diameter, the initial wall thickness and the initial outer diameter are average values ​​at three different positions of the oil pipe.

[0009] In the prediction method of the present invention, in step S2, there is no specific requirement for the production time. Indoor experimental tests are generally 3-17 days, which is considered to be short-term corrosion. In actual on-site production, the casing is generally used for several months, several years or even decades.

[0010] In the prediction method of the present invention, in step S3, before cross-section cutting, the oil pipe is cleaned to remove attached sediments and corrosion products to ensure the accuracy of measurement.

[0011] In the prediction method of the present invention, in step S3, the long-term corrosion rate of the inner wall of the oil pipe is obtained according to the following formula: V i =(D i-1 -D i ) / 2t Where V i Indicates the long-term corrosion rate of the inner wall, D i-1 Indicates the inner diameter of the oil pipe, D i represents the initial inner diameter of the oil pipe, and t represents the service time of the oil pipe.

[0012] In the prediction method of the present invention, in step S3, the long-term corrosion rate of the outer wall of the oil pipe is obtained according to the following formula: V o =(D o -D o-1 ) / 2t Where V o Denotes the long-term corrosion rate of the outer wall, D o-1 Indicates the outer diameter of the oil pipe, D o represents the initial outer diameter of the oil pipe, and t represents the service time of the oil pipe.

[0013] In the prediction method of the present invention, in step S3, the pitting rate of the inner wall of the oil pipe is obtained according to the following formula: V ip =D k / t Where V ip Indicates the pitting rate of the inner wall, D k It represents the maximum corrosion pit depth of the inner wall of the oil pipe, and t represents the service time of the oil pipe.

[0014] In the prediction method of the present invention, in step S3, the pitting rate of the outer wall of the oil pipe is obtained according to the following formula: V op =D j / t Where V op Denotes the pitting rate of the outer wall, D j It represents the maximum corrosion pit depth of the outer wall of the oil pipe, and t represents the service time of the oil pipe.

[0015] In the prediction method of the present invention, in step S3, the corrosion pit depth, the inner diameter and the outer diameter are all average values ​​at three different positions of the oil pipe.

[0016] In the prediction method of the present invention, in step S3, "different depths" refers to the depths to which the actual production oil pipe is lowered, such as tens of meters or even thousands of meters; there is no requirement for the value interval, and any depth position is applicable to the present invention.

[0017] The method of the invention can accurately calculate the long-term corrosion rate and pitting rate of the oil pipe at different depths, thereby improving the accuracy of corrosion prediction.

[0018] The method of the present invention has the following beneficial technical effects: Improve accuracy: By actually measuring the corrosion of in-service oil pipes, their long-term corrosion rate and pitting rate can be accurately calculated, overcoming the problem of large gap between simulation experiments and actual working conditions in existing technologies.

[0019] Comprehensive assessment: Measuring the corrosion of the tubing at different depths provides comprehensive corrosion data, which helps to accurately assess the corrosion status of the downhole environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of the oil pipe dimensions before and after corrosion.

[0021] Figure 2 It is a schematic diagram of the overall process of the method of the present invention. DETAILED DESCRIPTION

[0022] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0023] Unless otherwise specified, the materials and reagents used in the following examples are all commercially available. Before the tubing was lowered into the well, the initial dimensions of the tubing were recorded using measuring tools, including: inner diameter of 88.9 mm, wall thickness of 12.7 mm, and outer diameter of 114.3 mm. The above data were measured at three points and the average value was taken. The time the tubing was lowered into the wellbore was recorded for 2 years.

[0024] After 2 years of production (assuming time T = 2 years), the oil pipe is taken out and the inner and outer surfaces of the oil pipe are cleaned using a high-pressure water cleaning device to remove attached deposits and corrosion products.

[0025] Cut sections at different depths of the oil pipe (for example, every 10 meters) to show the corrosion of the inner and outer walls. Assuming that the oil pipe is 1,000 meters deep, a three-coordinate measuring machine is used to measure the maximum corrosion pit depth of 2.521mm and 2.643mm, the inner diameter of 89.254mm and the outer diameter of 110.132mm at the section. The inner and outer diameters of each section are measured at three points and the average value is taken.

[0026] Calculate the long-term corrosion rate V of the inner wall of the oil pipe i and the long-term corrosion rate of the outer wall V o , and the pitting rate V of the inner and outer walls ip and V op The formula is as follows: Long-term corrosion rate of inner wall: V i =(D i-1 -D i ) / 2t=0.089mm / a; Long-term corrosion rate of outer wall: V o =(D o -D o-1 ) / 2t=1.042mm / a; Inner wall pitting rate: V ip =D k / t=1.261mm / a; Outer wall pitting rate: V op =D j / t=1.322mm / a.

[0027] The method of the present invention can significantly improve the accuracy of corrosion rate prediction by directly measuring the long-term corrosion and pitting rates at multiple points of the oil pipe in actual downhole conditions, combining data acquisition of high-precision equipment, averaging of multi-point measurements, elimination of interference factors, and separate calculation of long-term corrosion and pitting rates. The method of the present invention can more realistically reflect the corrosion situation in a complex downhole environment, avoid the limitations of laboratory testing and the deviation of single factor analysis, and provide a more reliable scientific basis for oil pipe anti-corrosion decision-making and maintenance strategies.

Claims

1. A method for predicting the long-term corrosion rate of an oil pipe, comprising the following steps: S1, obtaining the initial inner diameter, initial wall thickness and initial outer diameter of the oil pipe before it is lowered into the well; S2, lowering the oil pipe into the wellbore for production; S3, taking out the oil pipe, cutting the sections at different depths of the oil pipe, measuring the depth, inner diameter and outer diameter of the corrosion pit at each section, and then obtaining the long-term corrosion rate of the oil pipe.

2. The prediction method according to claim 1, characterized in that: In step S1, the initial inner diameter, the initial wall thickness and the initial outer diameter are average values ​​at three different positions of the oil pipe.

3. The prediction method according to claim 1 or 2, characterized in that: In step S3, the oil pipe is cleaned before cross-section cutting.

4. The prediction method according to any one of claims 1 to 3, characterized in that: In step S3, the long-term corrosion rate of the inner wall of the oil pipe is obtained according to the following formula: V i =(D i-1 -D i ) / 2t Where V i Indicates the long-term corrosion rate of the inner wall, D i-1 Indicates the inner diameter of the oil pipe, D i represents the initial inner diameter of the oil pipe, and t represents the service time of the oil pipe.

5. The prediction method according to any one of claims 1 to 4, characterized in that: In step S3, the long-term corrosion rate of the outer wall of the oil pipe is obtained according to the following formula: V o =(D o -D o-1 ) / 2t Where V o Denotes the long-term corrosion rate of the outer wall, D o-1 Indicates the outer diameter of the oil pipe, D o represents the initial outer diameter of the oil pipe, and t represents the service time of the oil pipe.

6. The prediction method according to any one of claims 1 to 5, characterized in that: In step S3, the pitting rate of the inner wall of the oil pipe is obtained according to the following formula: V ip =D k / t Where V ip Indicates the pitting rate of the inner wall, D k It represents the depth of corrosion pits on the inner wall of the oil pipe, and t represents the service time of the oil pipe.

7. The prediction method according to any one of claims 1 to 6, characterized in that: In step S3, the pitting rate of the outer wall of the oil pipe is obtained according to the following formula: V op =D j / t Where V op Denotes the pitting rate of the outer wall, D j It represents the depth of corrosion pits on the outer wall of the oil pipe, and t represents the service time of the oil pipe.

8. The prediction method according to any one of claims 1 to 7, characterized in that: In step S3, the corrosion pit depth, the inner diameter and the outer diameter are all average values ​​at three different positions of the oil pipe.

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