Method for determining corrosion limit safety surface of downhole corrosion factors and method for selecting materials for three-dimensional chart of downhole pipe string

Abstract

The invention "A method for determining the corrosion limit safety surface of downhole corrosion factors and a method for selecting materials for the three-dimensional chart of downhole pipe strings" belongs to the field of material selection and corrosion protection of oil and gas well pipes. By conducting indoor corrosion experiments under the combined action of multiple main corrosion factors in the actual environment of the well area where materials are to be selected, and according to the corrosion rate control index of the well area where materials are to be selected, using curve, fitting, and surface fitting methods, a three-dimensional material selection chart containing three factors is made, which can intuitively reflect the corrosion safety area and dangerous area of materials. Finally, the downhole corrosion factor values of the well where materials are to be selected are put into the three-dimensional material selection chart, and the tubing materials that meet the requirements are selected according to the positions of the corrosion factor values relative to the safety areas and dangerous areas of various materials in the three-dimensional material selection chart. The invention can select safe tubing materials for ultra-deep wells with coexisting multiple corrosion factors and complex corrosion environments, simplifies the material selection process, and improves the material selection efficiency.

Description

Technical Field

[0001] The present invention belongs to the field of material selection and corrosion protection for oil and gas well pipes, and particularly relates to a method for determining the corrosion limit safety surface of downhole corrosion factors and a method for selecting materials for a three-dimensional chart of downhole pipe strings. Background Art

[0002] The petroleum and natural gas industry standard SY / T6857.1-2012 "Petroleum and Natural Gas Industry - Recommended Practices for the Selection of Carbon Steel and Low Alloy Steel Tubing and Casing in H2S-Containing Oil and Gas Field Environments - Part 1" gives a tubing material selection process table. However, this material selection process is far from sufficient for tubing material selection under the complex corrosion conditions of ultra-deep wells, only based on three environmental parameters such as H2S partial pressure, CO2 partial pressure, and temperature. For tubing material selection in ultra-deep wells, more corrosion factors should be considered, and selections and judgments should be made based on indoor research tests, on-site engineering experience, etc.

[0003] Patent 201510161649.6 discloses a new type of carbon dioxide corrosion protection material selection chart for oil and casing pipes, mainly used to guide how to economically and reasonably select corrosion protection materials for oil and casing pipes in carbon dioxide-containing oil and gas fields in reservoirs. The chart includes carbon steel, 13Cr stainless steel, and low Cr steel, and corrosion protection materials that meet the conditions can be selected according to the distribution ranges of the temperature and carbon dioxide partial pressure values of the oil and gas wells. However, the factors considered in material selection using this chart only include temperature and carbon dioxide partial pressure values, without considering more complex corrosion factors in ultra-deep wells, and the material selection range is also limited.

[0004] Patent 201710528724.7 discloses a method for determining the material selection boundary of gas injection wells. The present invention first measures the corrosion rate values of different materials under different working conditions, then obtains the relationship curve between the corrosion rate of the material and the working condition parameters, then fits the corrosion rate of the material, and finally calculates the corrosion safety factor and draws the corresponding material selection chart. However, the material selection chart drawn by this method is a two-dimensional chart that only includes a single corrosion factor, without considering the influence of the interaction between multiple corrosion factors on the corrosion rate of the material, and when selecting materials, it is necessary to weigh among multiple single-factor material selection charts, increasing the difficulty of material selection and reducing the material selection efficiency.

[0005] In addition, the following deficiencies also exist in the existing methods for selecting materials for downhole tubing in ultra-deep wells: the downhole corrosion working condition environment is complex, and after encountering the actual environment, experimental material selection is carried out. The experiment takes a long time and it is difficult to meet the requirements of rapid production construction; repeated material selection research is carried out for different environments, resulting in duplicate research and wasting resources; material selection does not fully consider various complex corrosion factors in ultra-deep wells; the material selection range is relatively limited; the interaction between multiple corrosion factors is rarely considered during material selection; and there are problems of high material selection difficulty and low efficiency. Summary of the Invention

[0006] In order to overcome the above-mentioned deficiencies existing in the prior art, the present invention can carry out indoor corrosion experiments under the combined action of multiple factors for the complex corrosion environment of ultra-deep wells, make a three-dimensional material selection chart, and according to the measured corrosion environment of the well to be selected for materials, quickly select a tubing material that meets the corrosion safety requirements and is economical for ultra-deep wells by using the three-dimensional material selection chart.

[0007] The technical content of the present invention is as follows:

[0008] A method for determining the corrosion limit safety surface of downhole corrosion factors for downhole string material selection, characterized by comprising:

[0009] (1) Determine the ultimate safety corrosion rate Vd of the downhole string material of the target well;

[0010] (2) Determine several downhole corrosion factors of the target well;

[0011] (3) Take the downhole corrosion factors X, Y, and Z among several downhole corrosion factors as a group; take the downhole corrosion factor X as a variable, and the downhole corrosion factors Y and Z take fixed values respectively, and conduct a corrosion experiment on the downhole string material as the material to be selected;

[0012] (4) Through the corrosion experiment, measure the corrosion rate values Q1-Qn of the material to be selected when the downhole corrosion factor X takes different values X1-Xn, and obtain the formula for the change law of the corrosion rate value of the material to be selected with the downhole corrosion factor X as a variable: Q x = f(Xn); calculate the X corresponding to the ultimate safety corrosion rate Vd when Qx takes the value e which is the ultimate safety value X of the downhole corrosion factor X e1 ;

[0013] In steps (3) and (4), taking the downhole corrosion factor X as a variable and the downhole corrosion factors Y and Z taking fixed values respectively and conducting a corrosion experiment means: among the downhole corrosion factors X, Y, and Z of the determined target well, assume that the value X' of the downhole corrosion factor X ∈ {X1, X2, X3,..., Xn};

[0014] the value Y' of the downhole corrosion factor Y ∈ {Y1, Y2, Y3,..., Yn};

[0015] The value Z` of the downhole corrosion factor Z belongs to {Z1, Z2, Z3, ……, Zn}. Before conducting the corrosion experiment, first fix the values of factor Y and factor Z. For example, the value Y` of factor Y is Y1, and the value Z` of factor Z is Z1. On this premise, taking factor X as a variable, test the corrosion rate value Qx of the candidate material when factor X takes different values, that is, when X` = X1, X2, X3, ……, or Xn, the measured corrosion rate values Qx of the candidate material may be Qx1, Qx2, Qx3, ……, or Qxn. According to the two sets of one-to-one corresponding values of X` = X1, X2, X3, ……, or Xn and Qx = Qx1, Qx2, Qx3, ……, or Qxn, using the value of X` selected from X1, X2, X3, ……, or Xn as the abscissa and the standard scale of the corrosion rate value as the ordinate, and then mark the values of Qx1, Qx2, Qx3, ……, or Qxn representing the Qx value on the coordinate graph to obtain the curve of Qx with X` as the variable, as Figure 1 shown, the formula corresponding to this curve is Qx = f(Xn).

[0016] (5) Change the values of other downhole corrosion factors and fix them, and conduct the corrosion experiment again with the downhole corrosion factor X as the variable. Repeat step (4) to obtain the ultimate safety value X of the downhole corrosion factor X e2 ; The other downhole corrosion factors refer to the downhole corrosion factors other than the downhole corrosion factor used as the variable. For example, if the downhole corrosion factor X is used as the variable, then the downhole corrosion factors Y and Z are the other downhole corrosion factors; if the downhole corrosion factor Y is used as the variable, then the downhole corrosion factors X and Z are the other downhole corrosion factors; if the downhole corrosion factor Z is used as the variable, then the downhole corrosion factors X and Y are the other downhole corrosion factors.

[0017] (6) Each time the values of other downhole corrosion factors are changed and fixed, conduct the corrosion experiment again with the downhole corrosion factor X as the variable. Repeat step (4) to obtain the ultimate safety value X of the downhole corrosion factor X e3 , ……, X en ;

[0018] (7) Take the downhole corrosion factor Y as the variable, and the downhole corrosion factors X and Z take fixed values respectively, and conduct the corrosion experiment on the candidate material; repeat steps (4)-(6) to obtain the ultimate safety value Y of the downhole corrosion factor Y e1 , Y e2 , Y e3 , ……, Y en ;

[0019] (8) Take the downhole corrosion factor Z as the variable, and the downhole corrosion factors X and Y take fixed values respectively, and conduct the corrosion experiment on the candidate material; repeat steps (4)-(6) to obtain the ultimate safety value Z of the downhole corrosion factor Z e1,Z e2 ,Z e3 ,……,Z en ;

[0020] (9) According to the ultimate safety value X of the downhole corrosion factor X e1 ,X e2 ,X e3 ,……,X en , the ultimate safety value Y of the downhole corrosion factor Y e1 ,Y e2 , Y e3 ,……,Y en , the ultimate safety value Z of the downhole corrosion factor Z e1 ,Z e2 ,Z e3 ,……,Z en A set of corrosion ultimate safety surfaces of the downhole corrosion factors X, Y, and Z of the to-be-selected material is obtained by fitting.

[0021] A set of corrosion ultimate safety surfaces of the downhole corrosion factors X, Y, and Z of the to-be-selected material is a three-dimensional surface formed on a three-dimensional coordinate diagram with the ultimate safety value X of the downhole corrosion factor X e1 ,X e2 ,X e3 ,……,X en as the X coordinate axis, the ultimate safety value Y of the downhole corrosion factor Y e1 ,Y e2 ,Y e3 ,……,Y en as the Y coordinate axis, and the ultimate safety value Z of the downhole corrosion factor Z e1 ,Z e2 ,Z e3 ,……,Z en as the Z coordinate axis, as shown in Figures 6-7 shown.

[0022] The "target well" in this article refers to an oil well that will apply the to-be-selected material as the downhole string.

[0023] In step (1), determine the ultimate safety corrosion rate V of the downhole string material in the well area of the to-be-selected material d It refers to determining the ultimate safety corrosion rate of the downhole string material in the well area of the to-be-selected material according to the method described in the standard document "NACE TM0177-2005". The "ultimate safety corrosion rate" in this article is the corrosion rate value in the range of > 0.25 mm / y determined by the corrosion rate measurement method described in the standard document "NACE TM0177-2005" for the downhole string material (Table 2 on page 15 of "NACE TM0177-2005").

[0024] Step (2) further includes: grouping the downhole corrosion factors of several target wells;

[0025] Preferably, the downhole corrosion factors with great interaction are grouped into one group.

[0026] The downhole corrosion factors are selected from: temperature, water cut, CO2 partial pressure, H2S partial pressure, Cl - concentration, elemental sulfur content.

[0027] Those skilled in the art can add other important corrosion factors according to the well conditions, for example, salinity, pressure, oxygen content, bacteria, etc.

[0028] Preferably, temperature, water cut, and CO2 partial pressure are grouped into one group;

[0029] H2S partial pressure, Cl - concentration, and sulfur element content are grouped into one group. Cl - Concentration refers to chloride ion concentration.

[0030] For each group of downhole corrosion factors of each candidate material, a corrosion limit safety surface can be determined;

[0031] Preferably, the candidate materials are selected from 15Cr, P110S, S13Cr, 25Cr, titanium alloy, and 028 alloy.

[0032] The corrosion experiment in step (3) includes treating the candidate material specimens for more than 6 h under the simulated downhole string working environment, then cleaning, drying, and weighing;

[0033] Preferably, the calculation formula for the corrosion rate V of the candidate material specimen is as follows:

[0034]

[0035] Wherein, V is the corrosion rate of the candidate material specimen, with the dimension of mm / a; W1 is the weight of the candidate material specimen before the test, in g; W2 is the weight of the candidate material specimen after treatment, with the dimension of g; 87600 is a calculation constant; A is the surface area of the candidate material specimen, with the dimension of cm 2 ; T is the treatment time, with the dimension of h; D is the density of the specimen material, with the dimension of g / cm 3 ; The dimensionless value Qn of the corrosion rate of the candidate material is the corrosion rate V;

[0036] The simulated downhole string working environment means: putting the prepared oilfield water sample into a high-temperature and high-pressure reactor; the downhole corrosion factors simulated in the reactor include: temperature of 30 - 210 °C, pressure of 20 - 200 Mpa, CO2 partial pressure of 0.1 - 12 Mpa, H2S partial pressure of 0.001 - 2 Mpa, and the downhole corrosion factors in the prepared oilfield water sample include: water cut of 5% - 100%, Cl- with a concentration of 1.0×10 4 mg / L to 15×10 4 mg / L and an elemental sulfur content of 10 - 200 ppm.

[0037] A method for selecting materials for a three - dimensional chart of downhole pipe strings, characterized in that, using the method for determining the corrosion limit safety surface of downhole corrosion factors for downhole pipe string material selection as described above, determine the corrosion limit safety surface of each group of downhole corrosion factors for each candidate material, and form a three - dimensional material selection chart for the corresponding group;

[0038] Measure the measured values of each group of downhole corrosion factors of the target well where the corresponding candidate material is about to be applied: x mk , y mk , z mk ;

[0039] On the X, Y, and Z coordinate axes where the three - dimensional material selection chart for the corresponding group is located, determine the Mk point of the downhole corrosion factors of the corresponding group of the target well in the three - dimensional coordinate chart according to the measured values x mk , y mk , z mk Determine the Mk point of the downhole corrosion factors of the corresponding group of the target well in the three - dimensional coordinate chart.

[0040] The inner side of the corrosion limit safety surface is the corrosion safety area, and the outer side is the corrosion danger area;

[0041] If the Mk point falls within the corrosion safety area of the corrosion limit safety surface of the downhole corrosion factors of the corresponding group of the corresponding candidate material, it indicates that the corresponding candidate material can be used for the downhole pipe string of the target well.

[0042] The three - dimensional material selection chart formed by the corrosion limit safety surface of the first group of downhole corrosion factors is Tu1, and the three - dimensional material selection chart formed by the corrosion limit safety surface of the second group of downhole corrosion factors is Tu2;

[0043] Measure the measured values of the first group of downhole corrosion factors of the target well where the corresponding candidate material is about to be applied: x1, y1, z1, and the measured values of the second group of downhole corrosion factors: x2, y2, z2;

[0044] On the X, Y, and Z coordinate axes where the three - dimensional material selection chart Tu1 is located, determine the M1 point of the downhole corrosion factors of the corresponding group of the target well in the three - dimensional coordinate chart according to the measured values x1, y1, z1, and on the X, Y, and Z coordinate axes where the three - dimensional material selection chart Tu2 is located, determine the M2 point of the downhole corrosion factors of the corresponding group of the target well in the three - dimensional coordinate chart according to the measured values x2, y2, z2;

[0045] The candidate material corresponding to the point M1 falling within the corrosion safety area of ​​the corrosion limit safety surface, and the candidate material corresponding to the point M2 falling within the corrosion safety area of ​​the corrosion limit safety surface, the intersection of the two is taken, and the candidate material determined is the material applicable to the downhole tubing of the target well.

[0046] The present invention provides a method for selecting materials of downhole pipe strings using a three-dimensional drawing, which is characterized by comprising the following steps:

[0047] Step 1: According to the service life of the oil pipe or the corrosion rate control index of the well area to be selected, determine the limit safe corrosion rate of the oil pipe material within the design service life of the oil pipe in the well area to be selected;

[0048] Step 2: Determine the main corrosion factors and their value ranges in the ultra-deep wells in the well area to be selected, and divide the main corrosion factors into groups of three. Corrosion factors with large interactions are preferentially divided into the same group;

[0049] Step 3: Conduct indoor corrosion experiments to determine the limit safety values ​​of a group of three main corrosion factors corresponding to each pre-selected oil pipe material. The specific method is as follows:

[0050] S1: Among the three main corrosion factors, two of the main corrosion factor values ​​(respectively recorded as Y factor and Z factor) are set to a fixed value within their variation range, and the value of another main corrosion factor (recorded as X factor) is changed within its variation range. Through corrosion experiments, the corrosion rate values ​​of the pre-selected oil pipe material when the changed main corrosion factor (X factor) takes different values ​​are tested;

[0051] S2: Based on the corrosion rate values ​​of the pre-selected tubing material when the main corrosion factor (X factor) is changed to different values, the fitting method is used to obtain the change law formula of the pre-selected tubing material when the two main corrosion factor values ​​(Y factor and Z factor) are fixed values: x =f(x);

[0052] S4: Let f(x) = v d , calculate the limit safety value of the other main corrosion factor (X factor) of the pre-selected oil pipe material when two of the main corrosion factors (respectively denoted as Y factor and Z factor) take a certain fixed value under the limit safety corrosion rate requirement.

[0053] S5: Two of the main corrosion factors (respectively recorded as Y factor and Z factor) take different fixed values ​​within their value variation range, and using the methods described in S1-S4, the limit safety value of another main corrosion factor (X factor) can be obtained when the two main corrosion factors (respectively recorded as Y factor and Z factor) take different fixed values ​​within their value variation range; when the other main corrosion factor (X factor) takes the limit safety value, the corresponding fixed values ​​of the two main corrosion factors (respectively recorded as Y factor and Z factor) are the limit safety values ​​of the two main corrosion factors (respectively recorded as Y factor and Z factor);

[0054] Step 4: Take each set of limit safety values ​​of a certain set of three main corrosion factors of each material as x, y, z coordinate values, and record them as a coordinate point in a three-dimensional rectangular coordinate system;

[0055] Step 5: Using the fitting method, the coordinate points corresponding to all the limit safety values ​​of a group of three main corrosion factors of each material are fitted into a material corrosion limit safety surface in a three-dimensional rectangular coordinate system, denoted as: U ck =F ck (x, y, z), if a point is taken on one side of the limit safety surface, and the corrosion rate of the material under the conditions of the three main corrosion factors corresponding to the point is higher than the limit safety corrosion rate of the material, then this side of the limit safety surface is recorded as the outer side, and the other side is recorded as the inner side. The area enclosed by the inner side of the limit safety surface and the coordinate plane is the corrosion safety zone of this material, and the area enclosed by the outer side of the limit safety surface and the coordinate plane is the corrosion danger zone of this material. The corrosion limit safety surfaces of all pre-selected materials and the coordinate plane of the three-dimensional rectangular coordinate system constitute a three-dimensional material selection chart for a group of three main corrosion factors of the oil pipe material;

[0056] Step 6: According to the method of step 3-step 5, make a three-dimensional material selection chart of all the main corrosion factors in the other three groups, recorded as Tu k ;

[0057] Step 7: Test the underground corrosion environment of the ultra-deep well to be selected, obtain the measured values ​​of all major corrosion factors, and record these measured values ​​of major corrosion factors in groups of three corresponding to the three-dimensional material selection chart as follows: M k (x mk ,y mk ,z mk );

[0058] Step 8: Select all the materials in the 3D drawing of the tubing material k In the figure, the measured values ​​of the main corrosion factors are respectively plotted against the coordinate points M k (x mk ,y mk , z mk), and according to the coordinate point M k The safe pipe material is selected based on the position relationship of the corrosion limit safety surface of various pre-selected materials in the three-dimensional plate of oil pipe material selection: k If the point is above or inside the corrosion limit safety surface of a certain material, it means that this material can be selected as the oil pipe material for the ultra-deep well. k The point is outside the corrosion limit safety surface of a certain material, indicating that this material cannot be selected as the tubing material for the ultra-deep well to be selected;

[0059] Step 9: Create a 3D map of all selected materials k The selected tubing materials are intersected, and the materials in the intersection are the tubing materials for ultra-deep wells that are finally selected.

[0060] A downhole pipe string three-dimensional plate material selection method, characterized in that: three main corrosion factors are: temperature, water content, CO2 partial pressure, indoor corrosion test method can refer to the standard document "NACE TM0177-2005" for implementation, as follows:

[0061] SS1: Preparation of produced water samples from oil fields: According to the formation water components of ultra-deep wells in the well area to be selected, the test solution with a water content of 100% is prepared using reagents that meet the requirements and distilled water. The solutions for tests with different water contents use the solution of crude oil + water.

[0062] SS2: Deoxygenation of test medium: put the prepared water sample into the deoxygenation bottle; connect the nitrogen cylinder, rubber tube and glass tube to form a ventilation system; open the main valve and partial pressure valve of the nitrogen cylinder, and pass nitrogen into the deoxygenation bottle to drive out oxygen for 6 hours;

[0063] SS3: High temperature and high pressure reactor corrosion test, using the following steps: cleaning the reactor, putting the reactor on, checking air tightness, pressurizing and heating, unloading the reactor, and taking the test piece;

[0064] SS4: Removal of corrosion products from the test piece: After the test piece is taken out from the reactor, it is cleaned with tap water and anhydrous ethanol, wrapped with filter paper and placed in a desiccator for 12 hours before being weighed;

[0065] SS5: Calculation of corrosion rate: The corrosion rate is calculated using the weight loss of the specimen before and after the test.

[0066] The calculation formula of corrosion rate is:

[0067]

[0068] X-test piece corrosion rate mm / a; W1-test piece weight before test g; W2-test piece weight after test g;

[0069] 87600-calculation constant; A-surface area of ​​the test piece cm 2; T - test time h; D - density of the test piece material g / cm 3 .

[0070] A method for selecting materials for the three - dimensional chart of downhole pipe string, characterized in that: the main corrosion factors considered in the tubing selection method include: temperature, water cut, CO2 partial pressure, H2S partial pressure, Cl - concentration, elemental sulfur content.

[0071] A method for selecting materials for the three - dimensional chart of downhole pipe string, characterized in that: the material selection range of the tubing selection method includes: 15Cr, P110S, S13Cr, 25Cr, titanium alloy, 028 alloy. The above materials are all common steel or alloy materials in the field and can be obtained commercially.

[0072] The present invention has the following technical effects:

[0073] 1. The present invention can, for the special environment of ultra - deep wells with multiple co - existing corrosion factors and complex corrosion environments, make a chart based on indoor corrosion experiment data, and select tubing materials according to the actual downhole environment on site. It can effectively avoid the situation where the anti - corrosion level of tubing materials is designed too high or too low, resulting in waste of casing use or failure to reach the service life, ensure the safe production of oilfields, and reduce the tubing cost.

[0074] 2. The three - dimensional material selection chart established by the present invention can visually reflect the corrosion safety performance of multiple tubing materials under the combined action of three corrosion factors in one chart. After obtaining the corrosion factor values of the well to be selected, they can be directly input into the chart to quickly determine the optimal material, simplifying the material selection process and improving the material selection efficiency.

[0075] The present invention is applied to the field of material selection and corrosion protection of oil and gas well pipes. It can effectively avoid the situation where the anti - corrosion level of tubing materials is designed too high or too low, resulting in waste of casing use or failure to reach the service life, ensure the safe production of oilfields, and reduce the tubing cost.

[0076] The present invention discloses a method for selecting materials for the three - dimensional chart of downhole pipe string. The present invention conducts indoor corrosion experiments under the combined action of multiple main corrosion factors in the actual environment of the well area to be selected. According to the corrosion rate control index of the well area to be selected, using curve, fitting, and surface fitting methods, a three - dimensional material selection chart containing three factors that can visually reflect the corrosion safety area and dangerous area of materials is made. Finally, the downhole corrosion factor values of the well to be selected are put into the three - dimensional material selection chart, and the tubing materials that meet the requirements are selected according to the positions of the corrosion factor values relative to the safety area and dangerous area of various materials in the three - dimensional material selection chart. The present invention can select safe tubing materials for ultra - deep wells with multiple co - existing corrosion factors and complex corrosion environments, simplify the material selection process, and improve the material selection efficiency. Brief Description of the Drawings

[0077] Figure 1 The corrosion rate curve of Material C1 at different water contents when the CO2 partial pressure is p1 MPa and the temperature is t1 °C obtained from an experimental example of the present invention.

[0078] Figure 2 The curve corresponding to the variation law formula of the corrosion rate value of the to-be-selected Material C1 with the water content as a variable and the wellbore corrosion factors: temperature of 80 °C, 160 °C, and 180 °C when the CO2 partial pressure is p1 MPa obtained from another experimental example of the present invention.

[0079] Figure 3 The distribution diagram on the coordinate axis of the limit safety value of the water content of Material C1 at different temperatures when the CO2 partial pressure is p1 MPa obtained from an experimental example of the present invention.

[0080] Figure 4 The distribution diagram on the coordinate axis of the limit safety value of the water content of Material C1 at different CO2 partial pressures and different temperatures obtained from another experimental example of the present invention.

[0081] Figure 5 The corrosion limit safety surface of a group of wellbore corrosion factors of Material C1: temperature, water content, and CO2 partial pressure obtained from an experimental example of the present invention.

[0082] Figure 6 The three-dimensional material selection chart formed by the corrosion limit safety surfaces of a group of wellbore corrosion factors of four to-be-selected materials: temperature, water content, and CO2 partial pressure obtained from another experimental example of the present invention.

[0083] Figure 7 The three-dimensional material selection chart Tu1 (left figure) formed by the corrosion limit safety surfaces of a group of wellbore corrosion factors of four to-be-selected materials: temperature, water content, and CO2 partial pressure obtained from an experimental example of the present invention, and the three-dimensional material selection chart Tu2 (right figure) formed by the corrosion limit safety surfaces of another group of wellbore corrosion factors of the four to-be-selected materials: H2S partial pressure, Cl - concentration, and elemental sulfur content are used for material selection and the tubing material is preferably selected.

[0084] The markings in the figure are listed as follows: Material C1-P110S; Material C2-S13Cr; Material C3-15Cr; Material C4-titanium alloy material. Detailed implementation manners

[0085] The technical solution of the present invention will be further described below in conjunction with the accompanying drawings and through specific implementation manners. The embodiments of the present invention are merely examples for clearly explaining the present invention and are not intended to limit the implementation manners of the present invention.

[0086] The first group of embodiments, the determination method of the corrosion limit safety surface of the present invention

[0087] This group of embodiments provides a method for determining the corrosion limit safety surface of downhole corrosion factors for downhole string material selection. All embodiments in this group have the following common features: The method for determining the corrosion limit safety surface of downhole corrosion factors for downhole string material selection includes:

[0088] (1) Determine the ultimate safety corrosion rate Vd of the downhole string material in the target well;

[0089] (2) Determine several downhole corrosion factors in the target well;

[0090] (3) Take the downhole corrosion factors X, Y, and Z among several downhole corrosion factors as a group; take the downhole corrosion factor X as a variable, and the downhole corrosion factors Y and Z take fixed values respectively, and conduct a corrosion experiment on the downhole string material as the material to be selected;

[0091] (4) Through the corrosion experiment, measure the corrosion rate values Q1 - Qn of the material to be selected when the downhole corrosion factor X takes different values X1 - Xn, and obtain the variation law formula of the corrosion rate value of the material to be selected with the downhole corrosion factor X as a variable: Q x = f(Xn); Calculate the X corresponding to the ultimate safety corrosion rate Vd when Qx takes the value e which is the ultimate safety value X of the downhole corrosion factor X e1 ;

[0092] (5) Change the values of other downhole corrosion factors and fix them, take the downhole corrosion factor X as a variable and conduct a corrosion experiment again, and repeat step (4) to obtain the ultimate safety value X of the downhole corrosion factor X e2 ;

[0093] (6) Each time the values of other downhole corrosion factors are changed and fixed, take the downhole corrosion factor X as a variable and conduct a corrosion experiment again, and repeat step (4) to obtain the ultimate safety value X of the downhole corrosion factor X e3 , ……, X en ;

[0094] (7) Take the downhole corrosion factor Y as a variable, and the downhole corrosion factors X and Z take fixed values respectively, and conduct a corrosion experiment on the material to be selected; repeat steps (4) - (6) to obtain the ultimate safety values Y of the downhole corrosion factor Y e1 , Y e2 , Y e3 , ……, Y en ;

[0095] (8) Take the downhole corrosion factor Z as a variable, and the downhole corrosion factors X and Y take fixed values respectively, and conduct a corrosion experiment on the material to be selected; repeat steps (4) - (6) to obtain the ultimate safety values Z of the downhole corrosion factor Ze1 , Z e2 , Z e3 , ……, Z en ;

[0096] (9) According to the limit safety value X e1 , X e2 , X e3 , ……, X en , the limit safety value Y of the downhole corrosion factor Y e1 , Y e2 , Y e3 , ……, Y en , the limit safety value Z of the downhole corrosion factor Z e1 , Z e2 , Z e3 , ……, Z en A set of corrosion limit safety surfaces of the downhole corrosion factors X, Y, and Z of the to-be-selected material is obtained by fitting.

[0097] In the above steps (3) and (4), taking the downhole corrosion factor X as a variable and taking the downhole corrosion factors Y and Z as fixed values respectively for the corrosion experiment means: among the downhole corrosion factors X, Y, and Z of the determined target well, assuming that the value X` of the downhole corrosion factor X ∈ {X1, X2, X3, ……, Xn};

[0098] the value Y` of the downhole corrosion factor Y ∈ {Y1, Y2, Y3, ……, Yn};

[0099] the value Z` of the downhole corrosion factor Z ∈ {Z1, Z2, Z3, ……, Zn}. Before conducting the corrosion experiment, first take the factors Y and Z as fixed values. For example, the value Y` of the factor Y = Y1, and the value Z` of the factor Z = Z1. On this premise, taking the factor X as a variable, test the corrosion rate value Qx of the to-be-selected material when the factor X takes different values, that is, when X` = X1, X2, X3, ……, or, Xn, the measured corrosion rate values Qx of the to-be-selected material may be Qx1, Qx2, Qx3, ……, or, Qxn. According to the two sets of one-to-one corresponding numerical values of X` = X1, X2, X3, ……, or, Xn and Qx = Qx1, Qx2, Qx3, ……, or, Qxn, taking the X` value selected from X1, X2, X3, ……, or, Xn as the abscissa and the corrosion rate value standard scale as the ordinate, and then marking the values such as Qx1, Qx2, Qx3, ……, or, Qxn representing the Qx value on the coordinate graph, a curve of Qx with X` as the variable is obtained, as Figure 1 shown, and the formula corresponding to this curve is Qx = f(Xn).

[0100] The corrosion limit safety surface of a set of downhole corrosion factors X, Y, and Z of the candidate material is a three-dimensional surface formed on a three-dimensional coordinate graph with the limit safety values Xe1, Xe2, Xe3, ……, Xen of the downhole corrosion factor X as the X coordinate axis, the limit safety values Ye1, Ye2, Ye3, ……, Yen of the downhole corrosion factor Y as the Y coordinate axis, and the limit safety values Ze1, Ze2, Ze3, ……, Zen of the downhole corrosion factor Z as the Z coordinate axis, as shown in Figures 6-7 shown.

[0101] The "target well" in this article refers to an oil well that will apply the candidate material as the downhole string.

[0102] In a specific embodiment, in step (1), the limit safety corrosion rate V of the downhole string material in the well area of the candidate material is determined d which refers to determining the limit safety corrosion rate of the downhole string material in the well area of the candidate material according to the method described in the standard document "NACE TM0177-2005". The "limit safety corrosion rate" in this article is the corrosion rate value in the range of > 0.25 mm / y determined for the downhole string material according to the corrosion rate measurement method described in the standard document "NACE TM0177-2005" (Table 2 on page 15 of "NACE TM0177-2005").

[0103] In a further embodiment, step (2) further includes: grouping the downhole corrosion factors of several target wells;

[0104] Preferably, the downhole corrosion factors with large interactions are grouped into one group.

[0105] In a specific embodiment, the downhole corrosion factors are selected from: temperature, water cut, CO2 partial pressure, H2S partial pressure, Cl - concentration, elemental sulfur content.

[0106] Those skilled in the art can add other important corrosion factors according to the well conditions, for example, salinity, pressure, oxygen content, bacteria, etc.

[0107] Preferably, temperature, water cut, and CO2 partial pressure are grouped into one group;

[0108] H2S partial pressure, Cl - concentration, and sulfur element content are grouped into one group. Cl - concentration refers to chloride ion concentration.

[0109] In some embodiments, the corrosion limit safety surface can be determined for each group of downhole corrosion factors of each candidate material;

[0110] Preferably, the candidate materials are selected from 15Cr, P110S, S13Cr, 25Cr, titanium alloy, and 028 alloy.

[0111] In a specific embodiment, the corrosion experiment in step (3) includes treating the candidate material specimens for more than 6 hours under the simulated downhole string working environment, and then cleaning, drying, and weighing them.

[0112] Preferably, the calculation formula for the corrosion rate V of the candidate material specimens is as follows:

[0113]

[0114] Wherein, V is the corrosion rate of the candidate material specimens, with the dimension of mm / a; W1 is the weight of the candidate material specimens before the test, in g; W2 is the weight of the candidate material specimens after treatment, with the dimension of g; 87600 is a calculation constant; A is the surface area of the candidate material specimens, with the dimension of cm 2 ; T is the treatment time, with the dimension of h; D is the density of the specimen material, with the dimension of g / cm 3 ; The dimensionless value Qn of the corrosion rate of the candidate material is the dimensionless value of the corrosion rate V.

[0115] In some other embodiments, the simulated downhole string working environment means: putting the prepared oilfield water sample into a high-temperature and high-pressure reactor; the downhole corrosion factors simulated in the reactor include: temperature of 30 - 210 °C, pressure of 20 - 200 Mpa, CO2 partial pressure of 0.1 - 12 Mpa, H2S partial pressure of 0.001 - 2 Mpa, and the downhole corrosion factors in the prepared oilfield water sample include: water cut of 5% - 100%, Cl - concentration of 1.0×10 4 mg / L to 15×10 4 mg / L, and elemental sulfur content of 10 - 200 ppm.

[0116] In a specific embodiment, the other downhole corrosion factors in steps (5) and (6) refer to the downhole corrosion factors other than the downhole corrosion factors taken as variables. In some embodiments, taking the downhole corrosion factor X as a variable, then the downhole corrosion factors Y and Z are the other downhole corrosion factors; in some other embodiments, taking the downhole corrosion factor Y as a variable, then the downhole corrosion factors X and Z are the other downhole corrosion factors; in some embodiments, taking the downhole corrosion factor Z as a variable, then the downhole corrosion factors X and Y are the other downhole corrosion factors.

[0117] Group 2 embodiments, a method for selecting materials for a three-dimensional chart of downhole strings of the present invention

[0118] This group of embodiments provides a method for selecting materials for the three-dimensional chart of downhole strings. All embodiments in this group have the following common features: The method for selecting materials for the three-dimensional chart of downhole strings uses the method for determining the corrosion limit safety surface of each group of downhole corrosion factors for material selection in downhole strings in any one of the first group of embodiments to determine the corrosion limit safety surface of each group of downhole corrosion factors for each candidate material, forming a three-dimensional material selection chart for the corresponding group;

[0119] Measure the measured values of each group of downhole corrosion factors of the target well where the corresponding candidate material will be applied: x mk ,y mk ,z mk ;

[0120] On the X, Y, and Z coordinate axes where the three-dimensional material selection chart for the corresponding group is located, determine the Mk point of the downhole corrosion factors for the corresponding group of the target well in the three-dimensional coordinate chart according to the measured values x mk ,y mk ,z mk If the Mk point falls within the corrosion safety area of the corrosion limit safety surface of the downhole corrosion factors for the corresponding group of the corresponding candidate material, it indicates that the corresponding candidate material can be used for the downhole string of the target well.

[0121] In some embodiments, the inner side of the corrosion limit safety surface is the corrosion safety area, and the outer side is the corrosion danger area;

[0122] If the Mk point falls within the corrosion safety area of the corrosion limit safety surface of the downhole corrosion factors for the corresponding group of the corresponding candidate material, it indicates that the corresponding candidate material can be used for the downhole string of the target well.

[0123] In some other embodiments, the three-dimensional material selection chart formed by the corrosion limit safety surface of the first group of downhole corrosion factors is Tu1, and the three-dimensional material selection chart formed by the corrosion limit safety surface of the second group of downhole corrosion factors is Tu2;

[0124] Measure the measured values of the first group of downhole corrosion factors of the target well where the corresponding candidate material will be applied: x1, y1, z1, and the measured values of the second group of downhole corrosion factors: x2, y2, z2;

[0125] On the X, Y, and Z coordinate axes where the three-dimensional material selection chart Tu1 is located, determine the M1 point of the downhole corrosion factors for the corresponding group of the target well in the three-dimensional coordinate chart according to the measured values x1, y1, z1, and on the X, Y, and Z coordinate axes where the three-dimensional material selection chart Tu2 is located, determine the M2 point of the downhole corrosion factors for the corresponding group of the target well in the three-dimensional coordinate chart according to the measured values x2, y2, z2;

[0126] For the candidate materials corresponding to the M1 point falling within the corrosion safety area of the corrosion limit safety surface, and the candidate materials corresponding to the M2 point falling within the corrosion safety area of the corrosion limit safety surface, take the intersection of the two to determine the candidate materials that are applicable to the downhole string of the target well.

[0127] One of the most specific embodiments of this group provides a method for selecting materials for the three-dimensional chart of downhole strings, which is characterized by the following steps:

[0128] Step 1: According to the service life of the tubing or the corrosion rate control index of the well area to be selected, determine the ultimate safe corrosion rate of the tubing material within the designed service life of the tubing in the well area to be selected, such as Figure 1 V in d . The determination of the ultimate safe corrosion rate can refer to the standard document "NACE TM0177-2005".

[0129] Step 2: Determine the main corrosion factors and the range of change of their factor values in the ultra-deep well downhole in the well area to be selected. Divide the main corrosion factors into groups of three, and the corrosion factors with large interaction are preferably assigned to the same group. Since the main downhole corrosion factors in different well areas are different, the main corrosion factors need to be determined based on on-site experience or experimental analysis and evaluation. For example, it can be determined according to the methods described in documents such as "Factors Influencing the Hydrogen Cracking Sensitivity of Pipeline Steels", CORROSION / 76, paper no.144 and "Analysis of Corrosion Factors of J55 and N80 Casing Pipes in the Downhole of Yanchi Oilfield, Ningxia". Group the determined main corrosion factors according to the on-site experience or experimental analysis and evaluation data according to the degree of influence of the corrosion factors. According to the book "Orthogonal Test Method", use orthogonal experimental design to evaluate the degree of influence of each corrosion factor on the corrosion situation or the correlation between each corrosion factor. The corrosion factors with similar degrees of influence or large correlations are grouped together.

[0130] The main corrosion factors for ultra-deep well downhole include: temperature, water cut, CO2 partial pressure, H2S partial pressure, Cl - concentration, elemental sulfur content. Divide them into two groups. The first group: temperature, water cut, CO2 partial pressure; the second group: H2S partial pressure, Cl - concentration, elemental sulfur content. CH Figure 1The figure shows the corrosion rate of material C1 (P110S) at different water contents when the temperature and CO2 partial pressure in the first group of three main corrosion factors (temperature, water content, CO2 partial pressure) are fixed. According to the actual well conditions, several fixed values ​​can be taken within the CO2 partial pressure range. For example, in general, the CO2 partial pressure range of the actual well conditions is 0.1-12Mpa. When the CO2 partial pressure range is 0.1-3Mpa, the CO2 partial pressure can be taken as 0.1Mpa, 0.5Mpa, 1Mpa, 2Mpa, 3Mpa and other fixed values; when the CO2 partial pressure range is 3-12Mpa, the CO2 partial pressure can be taken as 4Mpa, 6Mpa, 8Mpa, 10Mpa, 12Mpa and other fixed values.

[0131] Step 3: Conduct indoor corrosion experiments to determine the limit safety values ​​of a group of three main corrosion factors corresponding to each pre-selected oil pipe material. The specific method is as follows:

[0132] S1: Among the three main corrosion factors, two of the main corrosion factor values ​​(respectively denoted as Y factor and Z factor) are set to a fixed value within their variation range, and the value of another main corrosion factor (represented as X factor) is changed within its variation range. Through corrosion experiments (the specific operation of the corrosion experiment can refer to the standard document "NACE TM0177-2005"), the corrosion rate values ​​of the pre-selected oil pipe material when the changed main corrosion factor (X factor) takes different values ​​are tested;

[0133] like Figure 1 As shown in the figure, the temperature and CO2 partial pressure of the three main corrosion factors: temperature, moisture content, and CO2 partial pressure are taken as constant values, the moisture content is changed, and the corrosion rate corresponding to different moisture contents is measured through corrosion experiments. The moisture content and corrosion rate are recorded as X-coordinate values ​​and Y-coordinate values ​​in the rectangular coordinate system as coordinate points, as shown by coordinate point Qk in the figure.

[0134] S2: Based on the corrosion rate values ​​of the pre-selected tubing material when the main corrosion factor (X factor) is changed to different values, the fitting method is used to obtain the change law formula of the pre-selected tubing material when the two main corrosion factor values ​​(Y factor and Z factor) are fixed values: x =f(x);

[0135] like Figure 1 As shown, the curve Ls is the moisture content-corrosion rate law curve obtained by fitting the coordinate point Qk, and the corresponding formula is: x =f(Xn).

[0136] S4: Let f(x) = v d, calculate the limit safety value of another main corrosion factor (X factor) when two of the main corrosion factors (denoted as Y factor and Z factor respectively) take certain fixed values under the requirement of the limit safety corrosion rate (i.e., the "limit safety corrosion rate" in Step 1) for the preselected tubing material.

[0137] As Figure 1 shown, the straight line Ld intersects the curve Ls at the point (P d , V d ), and P d is the limit safety value of the water cut when the tubing material C1 takes certain fixed values of temperature and CO2 partial pressure. The straight line Ld is a straight line parallel to the horizontal axis determined according to the position of the limit safety corrosion rate Vd on the vertical axis.

[0138] S5: Take different fixed values for two of the main corrosion factors (denoted as Y factor and Z factor respectively) within their value change ranges. By using the methods described in S1 - S4, the limit safety values of another main corrosion factor (X factor) can be obtained when two of the main corrosion factors (denoted as Y factor and Z factor respectively) take different fixed values within their value change ranges; when another main corrosion factor (X factor) takes the limit safety value, the fixed values taken by the corresponding two of the main corrosion factors (denoted as Y factor and Z factor respectively) are the limit safety values of these two main corrosion factors (denoted as Y factor and Z factor respectively).

[0139] Figure 2 is the variation law of the corrosion rate of P110S with the water cut at different temperatures measured by the indoor corrosion experiment. As Figure 2 shown, that is, when the CO2 partial pressure is taken as a fixed value and the temperatures are taken as 80, 160, and 180 °C respectively, the corrosion rate values of P110S at different water cuts are obtained, and these corrosion rate values are fitted into curves, which intersect with the straight line Ld respectively, so as to obtain the limit safety values of the water cut at a fixed CO2 partial pressure and three temperatures. Figure 3 represents the limit safety values of the water cut at a fixed CO2 partial pressure and multiple temperatures.

[0140] Similarly, by changing the value of the CO2 partial pressure and using the same method above, the limit safety values of the water cut at different CO2 partial pressures and different temperatures can be obtained, as Figure 4 shown.

[0141] Step 4: Take the limit safety values of each group of three main corrosion factors of each material as the x, y, and z coordinate values respectively, and record them as a coordinate point in a three - dimensional rectangular coordinate system.

[0142] Figure 4The coordinate points in are the coordinate points corresponding to the limit safety values ​​of the three main corrosion factors of CO2 partial pressure, temperature and moisture content of material C1P110S under different CO2 partial pressure, temperature and moisture content.

[0143] Step 5: Using the fitting method, the coordinate points corresponding to all the limit safety values ​​of a group of three main corrosion factors of each material are fitted into a material corrosion limit safety surface in a three-dimensional rectangular coordinate system, denoted as: U ck =F ck (x, y, z), if a point is taken on one side of the limit safety surface, and the corrosion rate of the material under the conditions of the three main corrosion factors corresponding to the point is higher than the limit safety corrosion rate of the material, then this side of the limit safety surface is recorded as the outer side, and the other side is recorded as the inner side. The area enclosed by the inner side of the limit safety surface and the coordinate plane is the corrosion safety zone of this material, and the area enclosed by the outer side of the limit safety surface and the coordinate plane is the corrosion danger zone of this material. The corrosion limit safety surfaces of all pre-selected materials and the coordinate plane of the three-dimensional rectangular coordinate system constitute a three-dimensional material selection chart for a group of three main corrosion factors of the oil pipe material;

[0144] Figure 5 The surface in the figure is the corrosion limit safety surface of material P110S obtained by fitting the coordinate points corresponding to the limit safety values ​​of the three main corrosion factors: CO2 partial pressure, temperature, and water content. Area I in the coordinate system is the corrosion safety area of ​​material P110S; Area II is the corrosion danger area of ​​material P110S.

[0145] Figure 6 In the figure, the corrosion limit safety surfaces of four materials, C1, C2, C3 and C4, based on the three main corrosion factors of CO2 partial pressure, temperature and water content are shown.

[0146] Step 6: According to the method of step 3-step 5, make a three-dimensional material selection chart of all the main corrosion factors in the other three groups, recorded as Tu k ;

[0147] For ultra-deep wells, the method of steps 3 to 5 needs to be used to make the second group of main corrosion factors: H2S partial pressure, Cl - Three-dimensional chart of material selection for concentration and elemental sulfur content. Figure 7 Tu2 in the figure represents the partial pressure of H2S and Cl - Three-dimensional chart of material selection for concentration and elemental sulfur content. One set of corrosion factors corresponds to one three-dimensional chart.

[0148] Step 7: Test the underground corrosion environment of the ultra-deep well to be selected, obtain the measured values ​​of all major corrosion factors, and record these measured values ​​of major corrosion factors in groups of three corresponding to the three-dimensional material selection chart as follows: M k (xmk ,y mk ,z mk ); The purpose of this step is to test the measured values ​​of various downhole corrosion factors in the target well where the candidate material is about to be used. The measured values ​​have nothing to do with the candidate material itself, but only with the downhole environment of the target well.

[0149] Step 8: Select all the materials in the 3D drawing of the tubing material k In the figure, the measured values ​​of the main corrosion factors are respectively plotted against the coordinate points M k (x mk ,y mk , z mk ), and according to the coordinate point M k The safe pipe material is selected based on the position relationship of the corrosion limit safety surface of various pre-selected materials in the three-dimensional plate of oil pipe material selection: k If the point is above or inside the corrosion limit safety surface of a certain material, it means that this material can be selected as the oil pipe material for the ultra-deep well. k The point is outside the corrosion limit safety surface of a certain material, indicating that this material cannot be selected as the tubing material for the ultra-deep well to be selected;

[0150] Figure 7 In the figure, the position of M1 in the three-dimensional material selection chart based on CO2 partial pressure, temperature and water content in Tu1 shows that the optional materials for this ultra-deep well are C2, C3 and C4; the position of M2 in the three-dimensional material selection chart based on H2S partial pressure, Cl - From the position of the concentration and elemental sulfur content in the three-dimensional material selection chart, it can be seen that the optional materials for this ultra-deep well to be selected are C3 and C4.

[0151] Step 9: Create a 3D map of all selected materials k The selected tubing materials are intersected, and the materials in the intersection are the tubing materials for ultra-deep wells that are finally selected.

[0152] Depend on Figure 7 In the figure, the intersection of the two sets of three-dimensional material selection charts is obtained, and it is concluded that the optional materials for the ultra-deep well to be selected are C3 and C4.

[0153] For the same well area, n three-dimensional plates can be made according to the number of groups n of the main corrosion factors. The candidate material needs to meet all three-dimensional plates at the same time, that is, the Mk value must fall into the safe zone in each three-dimensional plate to be considered as meeting the requirements.

[0154] A method for selecting materials based on a three-dimensional plate for downhole pipe strings, characterized in that: three main corrosion factors are: temperature, water content, and CO2 partial pressure; indoor corrosion test methods are:

[0155] SS1: Preparation of oilfield produced water sample: According to the formation water components of ultra-deep wells in the well area to be selected, a test solution with a water cut of 100% is prepared using qualified reagents and distilled water. For solutions with different water cuts, the scheme of crude oil + water is adopted.

[0156] SS2: Deoxygenation of test medium: Fill the prepared water sample into a deoxygenation bottle; Connect the nitrogen gas cylinder, rubber hose, and glass tube to form a ventilation system; Open the main valve and pressure reducing valve of the nitrogen gas cylinder, and introduce nitrogen gas into the deoxygenation bottle to drive out oxygen for 6 hours.

[0157] SS3: Corrosion experiment of high-temperature and high-pressure autoclave, using the following steps: Clean the autoclave body, install the upper autoclave, check the airtightness, pressurize and heat up, unload the autoclave, and take out the test piece.

[0158] SS4: Removal of corrosion products from test pieces: After the test pieces are taken out from the autoclave, they are washed with tap water and anhydrous ethanol, wrapped with filter paper, placed in a desiccator, and weighed after 12 hours.

[0159] SS5: Calculation of corrosion rate: Calculate the corrosion rate using the weight loss of the test piece before and after the test.

[0160] Calculation formula for corrosion rate:

[0161]

[0162] X - Corrosion rate of test piece, mm / a; W1 - Weight of test piece before test, g; W2 - Weight of test piece after test, g; 87600 - Calculation constant; A - Surface area of test piece, cm 2 ; T - Test time, h; D - Density of test piece material, g / cm 3 .

[0163] A method for selecting materials for the three-dimensional chart of downhole strings, characterized in that: The main corrosion factors considered in the tubing selection method include: temperature, water cut, CO2 partial pressure, H2S partial pressure, Cl - concentration.

[0164] A method for selecting materials for the three-dimensional chart of downhole strings, characterized in that: The tubing selection range of the tubing selection method includes: P110S, S13Cr, 15Cr, 25Cr, titanium alloy, 028 alloy.

Claims

1. A method for determining the corrosion limit safety surface of downhole corrosion factors for downhole string material selection, characterized in that, Including: (1) Determine the ultimate safe corrosion rate Vd of the downhole string material of the target well; (2) Determine several downhole corrosion factors of the target wells, group the downhole corrosion factors of several target wells, and group the downhole corrosion factors with great interaction into one group; the downhole corrosion factors are selected from: temperature, water cut, CO2 partial pressure, H2S partial pressure, Cl - concentration, elemental sulfur content; Temperature, water cut, and CO2 partial pressure are grouped together; H2S partial pressure, Cl - concentration, and sulfur element content are grouped together; (3) Take several downhole corrosion factors X, Y, and Z as a group; take downhole corrosion factor X as a variable, and take downhole corrosion factors Y and Z as fixed values respectively, and conduct a corrosion experiment on the downhole string material as the candidate material; (4)Measure the corrosion rate values Q1 - Qn of the candidate materials under different values X1~Xn of the downhole corrosion factor X through corrosion experiments, and obtain the formula for the variation law of the corrosion rate values of the candidate materials with the downhole corrosion factor X as the variable: Q x = f(Xn); Calculate the X corresponding to the limit safety corrosion rate Vd of the Qx value e which is the limit safety value X of the downhole corrosion factor X e1 ; (5) Change the values of other downhole corrosion factors and fix them. Take the downhole corrosion factor X as a variable and conduct the corrosion experiment again. Repeat step (4) to obtain the ultimate safety value X of the downhole corrosion factor X e2 ; (6) Each time the values of other downhole corrosion factors are changed and fixed, take the downhole corrosion factor X as a variable and conduct the corrosion experiment again. Repeat step (4) to obtain the ultimate safety value X of the downhole corrosion factor X e3 , ……, X en ; (7) Taking the downhole corrosion factor Y as a variable, and fixing the downhole corrosion factors X and Z respectively, conduct a corrosion experiment on the candidate materials; repeat steps (4)-(6) to obtain the ultimate safety value Y of the downhole corrosion factor Y e1 , Y e2 , Y e3 , ……, Y en ; Taking the downhole corrosion factor Z as a variable, and taking the downhole corrosion factors X and Y as fixed values respectively, conduct corrosion experiments on the candidate materials; repeat steps (4)-(6) to obtain the ultimate safety value Z of the downhole corrosion factor Z e1 , Z e2 , Z e3 , ……, Z en ; (9) According to the ultimate safety value X of the downhole corrosion factor X e1 , X e2 , X e3 , ……, X en , the ultimate safety value Y of the downhole corrosion factor Y e1 , Y e2 , Y e3 , ……, Y en , the ultimate safety value Z of the downhole corrosion factor Z e1 , Z e2 , Z e3 , ……, Z en A set of corrosion ultimate safety surfaces of the downhole corrosion factors X, Y, and Z of the to-be-selected material is obtained by fitting.

2. The method for determining the corrosion limit safety surface of downhole corrosion factors for downhole string material selection according to claim 1, characterized in that, In step (1), determine the ultimate safe corrosion rate V of the downhole string material in the well area to be selected d It refers to determining the ultimate safe corrosion rate of the downhole string material in the well area to be selected according to the method described in the standard document "NACE TM0177 - 2005".

3. The method for determining the corrosion limit safety surface of downhole corrosion factors for downhole string material selection according to claim 1, characterized in that For each group of downhole corrosion factors of each candidate material, the corrosion limit safety surface can be determined.

4. A method for determining the corrosion limit safety surface of downhole corrosion factors for downhole pipe string material selection according to claim 1 or 3, characterized in that, The candidate materials are selected from 15Cr, P110S, S13Cr, 25Cr, titanium alloy, and 028 alloy.

5. The method for determining the corrosion limit safety surface of downhole corrosion factors for downhole string material selection according to claim 1, characterized in that, The corrosion experiment in step (3) includes treating the candidate material specimens for more than 6 hours under the simulated downhole string working environment, then cleaning, drying, and weighing.

6. The method for determining the corrosion limit safety surface of downhole corrosion factors for downhole string material selection according to claim 1, wherein The calculation formula for the corrosion rate V of the candidate material specimens is as follows: ； Among them, V is the corrosion rate of the test piece of the material to be selected, with the dimension of mm / a; W1 is the weight of the test piece of the material to be selected before the test, in g; W2 is the weight of the test piece of the material to be selected after treatment, with the dimension of g; 87600 is the calculation constant; A is the surface area of the test piece of the material to be selected, with the dimension of cm 2 ; T is the treatment time, with the dimension of h; D is the density of the test piece material, with the dimension of g / cm 3 ; The corrosion rate value Qn of the material to be selected is the dimensionless value of the corrosion rate V.

7. The method for determining the corrosion limit safety surface of downhole corrosion factors for downhole string material selection according to claim 6, characterized in that, Simulating the downhole string working environment means: putting the prepared oilfield water sample into a high-temperature and high-pressure autoclave; the downhole corrosion factors simulated in the autoclave include: temperature of 30 - 210 °C, pressure of 20 - 200 Mpa, CO2 partial pressure of 0.1 - 12 Mpa, H2S partial pressure of 0.001 - 2 Mpa, and the downhole corrosion factors in the prepared oilfield water sample include: water cut of 5% - 100%, Cl - concentration of 1.0×10 4 mg / L ~ 15×10 4 mg / L, and elemental sulfur content of 10 - 200 ppm.

8. A method for selecting materials for a three-dimensional chart of downhole pipe strings, characterized in that, Use the method for determining the corrosion limit safety surface of the downhole corrosion factors for downhole string material selection described in any one of claims 1-7 to determine the corrosion limit safety surface of each group of downhole corrosion factors of each candidate material, and form a three-dimensional selection chart for the corresponding group; Measure the actual values of each group of downhole corrosion factors of the target well for which the corresponding candidate material is to be applied: x mk , y mk , z mk ; On the X, Y, and Z coordinate axes where the three-dimensional drawing board for material selection of the corresponding group is located, according to the measured values x mk , y mk , z mk Determine the Mk point of the downhole corrosion factors of the corresponding group of the target well in the three-dimensional coordinate diagram.

9. A method for selecting materials for a three-dimensional chart of downhole pipe strings according to claim 8, characterized in that, The inner side of the corrosion limit safety surface is the corrosion safety area, and the outer side is the corrosion danger area; If the Mk point falls within the corrosion safety area of the corrosion limit safety surface of the corresponding group of downhole corrosion factors of the corresponding candidate material, it indicates that the corresponding candidate material can be used for the downhole string of the target well.

10. A method for selecting materials for a three-dimensional chart of downhole pipe strings according to claim 8 or 9, characterized in that, The three-dimensional selection chart formed by the corrosion limit safety surface of the first group of downhole corrosion factors is Tu1, and the three-dimensional selection chart formed by the corrosion limit safety surface of the second group of downhole corrosion factors is Tu2; Measure the measured values of the first group of downhole corrosion factors of the target well where the candidate material is about to be applied: x1, y1, z1, and the measured values of the second group of downhole corrosion factors: x2, y2, z2; On the X, Y, and Z coordinate axes where the three-dimensional selection chart Tu1 is located, determine the M1 point of the corresponding group of downhole corrosion factors of the target well in the three-dimensional coordinate diagram according to the measured values x1, y1, z1, and on the X, Y, and Z coordinate axes where the three-dimensional selection chart Tu2 is located, determine the M2 point of the corresponding group of downhole corrosion factors of the target well in the three-dimensional coordinate diagram according to the measured values x2, y2, z2; For the candidate materials corresponding to the M1 point falling within the corrosion safety area of the corrosion limit safety surface, and the candidate materials corresponding to the M2 point falling within the corrosion safety area of the corrosion limit safety surface, take the intersection of the two to determine the candidate materials that are applicable to the downhole string of the target well.

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