Oil-water alternative wetting corrosion simulation device and method
Technical Field
The invention belongs to the field of corrosion risk evaluation of oil and gas pipelines, and particularly relates to an oil-water alternative wetting corrosion simulation device and method.
Background
The evaluation of the corrosion risk of the oil and gas transmission pipeline is important for ensuring safe production and stable operation. In order to reveal the corrosion rule and realize the prediction of the corrosion life, various research methods are established in the field to simulate or reproduce the field corrosion working condition environment of the oil and gas pipeline. The functional differences achieved by the various devices are mainly summarized as follows:
(1) common three-electrode electrochemical test systems: the experimental method mainly focuses on researching the influence of various ions in the aqueous solution on the corrosion of the carbon steel pipeline, such as Cl-、CO2And H2S is equal in concentration or partial pressure, and the dynamic process of carbon steel corrosion can be researched by virtue of the characteristic of quick response of an electrochemical testing technology, so that the method is an important means for revealing a microscopic corrosion mechanism. The method generally requires a solution medium to have higher conductivity, so that the obtained test data have certain discreteness in a medium containing crude oil. The method is widely applied to simulating the corrosion behavior of the material in the aqueous solution medium extracted from the oil field, and is difficult to simulate the actual oil-water conveying mediumCharacteristics and corrosion rules.
(2) High-temperature high-pressure kettle simulation experiment system: the experimental system provides a closed environment for the corrosion medium of the oilfield produced liquid, has obvious advantages for researching the carbon steel corrosion behavior under the conditions of high pressure and high temperature, but has great limitation for simulating the corrosion test of the oil-water two-phase medium. Research shows that the corrosion rule of the water-in-oil type emulsion can be researched by utilizing a high-temperature high-pressure kettle, but the premise is that the homogenization of the medium in the kettle is easily realized at a very high rotating speed. In addition, Nesic and the like adopt a specially designed horizontal kettle body, so that a rotary sample can realize corrosion simulation of oil-water two-phase alternate wetting, and the application range of the high-pressure kettle body in corrosion research is expanded by the attempts. Generally, corrosion rate evaluation is obtained by a corrosion coupon weight loss method, and meanwhile, research is also carried out to introduce technologies such as a high-temperature high-pressure electrochemical test method and a corrosion probe into a high-pressure reactor simulation test so as to comprehensively reflect the corrosion performance of the material in a special extreme environment.
(3) Small-size loop formula oil water dispersion medium corrodes test system: to understand the corrosion law of carbon steel in oil-water two-phase media, NACE proposes a method for evaluating the corrosivity of emulsions or oil-water dispersed phases. The method is based on a set of small loop devices, including a stirred tank, a pump, a test pipeline and the like. The method can determine the phase inversion point of the emulsion by changing the water content of the mixed medium; meanwhile, the local wetting characteristics and the like of the pipe wall can be determined by means of a test probe, for example, three conditions of oil wetting, water wetting, oil-water alternative wetting and the like can be distinguished, but detailed description of the oil-water alternative wetting behavior cannot be realized. In such loop systems, corrosion rate testing typically requires the use of sophisticated corrosion monitoring probe technology.
(4) Large-scale loop-type multiphase flow corrosion simulation system: some well-known research institutions and oil companies build large oil-gas-water multiphase flow corrosion simulation experiment loop devices, and the large oil-gas-water multiphase flow corrosion simulation experiment loop devices are mainly used for directly evaluating the corrosion rule of the field oil-gas production environment and are generally called as an internal corrosion direct evaluation technology. The device is characterized by maximum simulationThe flow pattern and the oil-water wetting state in the pipeline on site, and the corrosion rate of a specific position is monitored. According to the report at home and abroad, the corrosion probe, the corrosion coupon and the Fe can be used on a large-scale test loop2+The method has the advantages that the change of the corrosion rate in the flowing process is monitored by various technologies such as ion detection and the like, various process parameters such as the water content, the gas-liquid ratio, the flow rate and the like of the fluid medium can be controlled, and the measurement of fluid parameters such as the wetting type, the fluid flow pattern, the pressure difference, the near-wall shear stress and the like can be obtained by means of an auxiliary testing means. At present, the important significance of the technology on the research of the corrosion problem of the oil and gas gathering and transportation pipeline is highly agreed by the academic community. In order to achieve a high degree of coincidence with the actual situation on site, the test loop is large in scale, the diameter of the pipeline is 2-4 inches, and the length of the straight pipeline generally needs to be more than 200 times of the diameter of the pipeline, so that a large laboratory space is occupied. Crude oil, solution medium, CO required for a single test2The consumption of gas and the like is large, and the power consumption and the manpower demand are large. In addition, waste liquid treatment and other problems need to be considered in the experimental process.
(5) The evaluation method of the corrosion inhibitor of the oil field gathering and transportation system comprises the following steps: the laboratory verification of the effectiveness of a corrosion inhibitor is often based on two test methods, namely an electrochemical method and a corrosion coupon weight loss method. Generally, the electrochemical method for testing the corrosion inhibition effect of the corrosion inhibitor in the aqueous solution is simple and quick; the corrosion weight loss principle can simulate the action effect of the corrosion inhibitor in high temperature and high pressure or in a simulated field environment. However, the field application of corrosion inhibitors is often very different from laboratory tests. Wherein, the oil gas gathering and transportation system or the sewage system contain a certain amount of crude oil, which leads to the obvious change of the corrosion inhibitor effect and troubles the evaluation of the indoor corrosion inhibition effect for a long time. Certainly, by building a large multiphase flow corrosion simulation loop device, the butt joint and rule exploration of the corrosion inhibitor from a laboratory to a field can be realized, but a simple and quick evaluation method for the corrosion inhibitor in the laboratory is still lacked.
From the existing experimental research methods, the pipeline corrosion simulation for oil-water two-phase mixed transportation in an oil field still needs to be improved. The most outstanding problem is that the oil-water alternate wetting behavior cannot be quantitatively represented and corresponds to the corrosion monitoring signal in real time. And the information or the relevance has a decisive significance for comprehensively understanding the corrosion rule and the corrosion risk prediction of the oil and gas transmission pipeline. Therefore, the current technology mainly has the following defects:
(1) the traditional three-electrode electrochemical system needs a solution medium with higher conductivity, so that the test is difficult to carry out in an oil-containing medium, and the understanding of a micro corrosion mechanism is limited;
(2) the corrosion weightlessness test method controls the environment of the test piece in an oil-water alternate wetting or oil-water dispersion medium, and only can obtain final corrosion data or morphology, and lacks necessary environmental parameters such as oil-water contact wetting with the test piece and the like;
(3) the corrosion simulation loop device can realize the research on the relation between an oil-water two-phase flow pattern or wetting characteristic and corrosion, but the identification of the oil-water two-phase wetting state is too macroscopic, particularly the quantitative research on the oil-water alternate wetting state is difficult to realize, and the acquisition of corrosion signals by means of technologies such as a corrosion probe and the like is also difficult to form a contrast with the alternate wetting state of the inner wall of the pipeline;
(4) the operation and maintenance difficulty of the large-scale oil-gas-water multiphase flow corrosion simulation device is large, the manpower and material resources are consumed greatly, and the large-scale oil-gas-water multiphase flow corrosion simulation device also becomes one of the reasons for restricting the quantitative research on the oil-water two-phase or oil-gas-water three-phase flow corrosion rule.
The evaluation of the effect of the corrosion inhibitor on the oilfield site needs to develop a technology suitable for the quick response of a corrosion signal in a medium containing crude oil and the evaluation of the corrosion rate.
Disclosure of Invention
The technical problem to be solved by the invention is as follows: the inner wall of the oilfield produced fluid pipeline is always in an oil-water alternate wetting state, which is one of the main reasons for causing the non-uniform corrosion and perforation of different parts of the pipeline. However, there is currently a lack of knowledge of the corrosion rules. The main difficulty lies in (1) the simulation of the oil-water alternate wetting state; (2) and acquiring a transient corrosion signal in the environment containing the crude oil. In order to research the oil-water alternative wetting process, the wetting behavior and the corrosion behavior are generally researched respectively, and the clarification of the corrosion mechanism is hindered due to the lack of understanding of the correspondence and the correlation.
The invention relates to a corrosion simulation test device in the oil-gas production process, which mainly quantitatively represents the quick response and record of a corrosion signal of the inner wall of a pipeline under the oil-water alternation action condition, thereby providing a corrosion evaluation method under the crude oil wetting condition. The device is used as a basic experimental device for revealing the corrosion inhibition effect and corrosion rule of the crude oil, and provides a reliable and convenient method for evaluating the effect of the corrosion inhibitor in the crude oil gathering and transportation pipeline.
The invention provides an oil-water alternate wetting corrosion simulation device, which comprises:
the electrochemical signal processing system is an electrochemical workstation capable of recording current signals, and the electrochemical workstation is provided with a working electrode terminal, a reference electrode terminal and a counter electrode terminal;
the oil-water alternative wetting simulation electrolytic cell comprises an electrolytic cell outer cylinder and an electrolytic cell inner cylinder sleeved in the electrolytic cell outer cylinder, wherein the electrolytic cell inner cylinder is communicated with the electrolytic cell outer cylinder; a platinum counter electrode electrically connected with the counter electrode terminal and a reference electrode electrically connected with the reference electrode terminal are arranged in the inner barrel of the electrolytic cell;
the rotary disk electrode system comprises a disk electrode and a rotary driving device for driving the disk electrode to rotate, wherein the disk electrode extends into the inner cylinder of the electrolytic cell; the platinum counter electrode is positioned below the disk electrode and is parallel to the bottom surface of the disk electrode; the tube tip of the salt bridge sharp tube of the reference electrode is arranged between the platinum counter electrode and the bottom surface of the disc electrode; the bottom surface of the disc electrode is a conductive surface and serves as a working electrode, the rest part of the outer surface of the disc electrode is insulated, and the working electrode is electrically connected with a working electrode terminal; or, the side surface of the disc electrode is provided with an annular conductive surface as a working electrode, the rest part of the outer surface of the disc electrode is insulated, and the working electrode is electrically connected with a working electrode terminal;
an oil-water interface control system comprising: the device comprises a floater and a vertical displacement control device for controlling the floater to move up and down, wherein the floater extends into a position between an outer cylinder of the electrolytic cell and an inner cylinder of the electrolytic cell;
the aeration system comprises an aeration head arranged in the outer cylinder of the electrolytic cell and outside the inner cylinder of the electrolytic cell and a gas supply device communicated with the aeration head through a ventilation pipeline;
and the temperature control system is used for heating the oil-water alternate wetting simulation electrolytic cell.
As a preferred embodiment, the vertical displacement control device includes: the stepping type electric floater cleaning device comprises a fixing frame, a stepping type motor, a motor control unit and a precise threaded guide rod, wherein the stepping type motor is connected with the electric control unit and then fixed on the fixing frame, and is connected with the floater through the precise threaded guide rod.
As a preferable technical scheme, the temperature control system comprises an electric heating belt wound on the outer wall of the outer barrel of the electrolytic cell and a temperature control unit electrically connected with the electric heating belt.
As the preferred technical scheme, the upper bottom surface of the inner cylinder of the electrolytic cell is sealed with an external end surface of a corrosion cell, and the external end surface of the corrosion cell is provided with a crude oil injection and recovery hole, a rotary disc electrode jack, a reference electrode jack, a counter electrode connecting hole and a breathing hole; a lead wire connected with the counter electrode terminal passes through the counter electrode connecting hole to be connected with the platinum counter electrode; the reference electrode is clamped in the reference electrode jack; the disc electrode is connected and fixed on the rotary driving device and is inserted into the inner barrel of the electrolytic cell through the rotary disc electrode jack.
Preferably, the cross-sectional area of the outer barrel of the electrolytic cell is more than 2 times of the drainage area of the floater.
As a preferred technical scheme, the disc electrode is composed of a lead, an insulating shell and a conductive inner core connected with the lead, and the lead is electrically connected with the working electrode terminal;
the bottom surface of the insulating shell is provided with an opening, the shape of the opening is the same as that of the bottom surface of the conductive inner core, and the bottom surface of the conductive inner core is clamped at the opening and is used as a working electrode; or the insulating shell is composed of an upper part and a lower part, the cross section of the upper part and the cross section of the lower part are the same as that of the conductive inner core, and the upper part and the lower part sandwich the conductive inner core, so that an annular conductive surface is formed on the side surface of the insulating shell and serves as a working electrode.
As a preferable technical scheme, the inner cylinder of the electrolytic cell is provided with a liquid level scale device.
As a preferred technical scheme, the distance between the platinum counter electrode and the working electrode is less than 2 cm.
The invention provides a method for simulating oil-water alternate wetting corrosion by applying the oil-water alternate wetting corrosion simulation device, which comprises the following steps:
(1) preparation of the experiment: preparing an aqueous solution, filling the aqueous solution into the oil-water alternative wetting simulation electrolytic cell, controlling the liquid level above a salt bridge sharp tube of a reference electrode and below a working electrode, ventilating and deoxidizing through an aeration system, starting a temperature control system, heating to a set value, controlling a vertical displacement control device to press a floater into water to adjust the working electrode below the water surface and above the salt bridge sharp tube of the reference electrode, connecting a working electrode terminal and the working electrode, the reference electrode terminal and the reference electrode, starting a temperature control system for an electrode terminal and a platinum counter electrode, and heating to the set value;
(2) heating crude oil to a testing temperature in advance, ventilating and deoxidizing, and injecting the crude oil into an inner cylinder of the electrolytic cell through crude oil injection and recovery holes;
(3) starting a rotary driving device of a rotary disc electrode system, and enabling a disc electrode to rotate stably at a specific rotating speed; starting the electrochemical workstation, and testing the open-circuit potential to enable the open-circuit potential to be stable;
(4) and (3) carrying out constant potential corrosion test: when the test is initial, the working electrode is in the water phase, after the experimental set time is reached, the oil-water interface control system is started, the speed and the distance of the up-and-down movement of the floater in the vertical direction and the oil-water interface reciprocating motion period of the working electrode are set through the vertical displacement control device, the oil-water interface starts to reciprocate up and down periodically, an oil-water alternate wetting corrosion test stage is started, and a current-time change curve is recorded;
(5) and (3) data analysis: microscopic observation is carried out on the surface corrosion morphology of the working electrode; statistical analysis of the current-time response curves, t0To t1The average corrosion current density at that time is calculated as:
dt
wherein,for average current density, I (t) is the current-time response function obtained from experimental tests, t0Recording time, t, for selected oil-water wetting start1The etch time for the desired calculated average current density.
At present, the most advanced technical means for the international research on the corrosion of oil and gas pipelines is a large oil-gas-water multiphase flow corrosion simulation device, and the device can be used for simulating the actual transmission condition of the pipeline on site and reflecting the operation rule of the corrosion of crude oil on the inner wall of the pipeline. Compared with the technology, the device and the method provided by the invention have the following advantages:
1. the rapid response of carbon steel corrosion signals in an oil-water alternate wetting environment is realized, the main characteristics of an oil-water two-phase flow environment are grasped, and the corrosion rule can be disclosed more comprehensively and deeply;
2. the corrosion evaluation is rapid and has pertinence, and particularly has obvious advantages for the evaluation of the effect of the corrosion inhibitor in an oil-water two-phase environment;
3. the device is simple and small, the manufacturing cost is low, the operation is convenient, the liquid amount for experiment is little, and compared with a large multiphase flow corrosion loop, the experiment operation cost is very low;
4. the parameters such as experiment temperature, wetting frequency, simulation flow velocity, crude oil characteristics and the like are simply and accurately controlled.
Drawings
FIG. 1 is a schematic structural diagram of a water-alternating wetting corrosion simulator according to the present invention.
FIG. 2 is a schematic view of the cross-sectional structure of the simulated oil-water wetting cell of the present invention.
FIG. 3 is a schematic diagram of the top view of the simulated oil-water wetting electrolyzer of the present invention.
Fig. 4 is a first schematic diagram of the disc electrode structure of the present invention.
FIG. 5 is a second schematic diagram of the disk electrode structure of the present invention.
FIG. 6 is a graph showing the corrosion current signal as a function of time under the conditions that the disc electrode rotates at 1200 rpm and the oil-water interface reciprocates for a period of 2.15 s.
FIG. 7 is a graph showing the corrosion current signal as a function of time under the conditions that the disc electrode rotates at 600 rpm and the oil-water interface reciprocates for a period of 5.0 s.
Detailed Description
The present invention is further described with reference to the following drawings and specific examples so that those skilled in the art can better understand the present invention and can practice the present invention, but the examples are not intended to limit the present invention.
The oil-water alternate wetting corrosion simulation device comprises an electrochemical signal processing system; a rotating disk electrode system; oil-water alternate wetting simulation electrolytic cell; an oil-water interface control system; temperature control system and aeration system. Specifically, as shown in fig. 1:
the electrochemical signal processing system mainly completes the control of an external potential and the recording and analysis processing of corrosion current signals and is mainly realized by an electrochemical workstation 1 capable of recording the current signals and matched software. Three electrode terminals (a working electrode terminal, a reference electrode terminal, and a counter electrode terminal) of the electrochemical workstation 1 are connected to the working electrode 60, the reference electrode 18, and the counter electrode 19, respectively.
Referring to fig. 1 to 3, the simulated electrolytic cell 10 with oil-water alternative wetting comprises an outer cylinder 27 and an inner cylinder 28 sleeved in the outer cylinder 27. The inner cell cylinder 28 has no bottom surface, so that the inner cell cylinder 28 is communicated with the outer cell cylinder 27; the upper bottom surface of the inner barrel 28 of the electrolytic cell is sealed with an external connection end surface 5 of the corrosion cell, and the external connection end surface 5 of the corrosion cell is provided with a breathing hole 25, a crude oil injection and recovery hole 23, a rotary disc electrode jack 26, a reference electrode jack 22 and a counter electrode connection hole 24; a lead wire connected with the counter electrode terminal passes through the platinum counter electrode connecting hole 24 to be connected with the platinum counter electrode 19, the platinum counter electrode 19 is positioned in the inner cylinder 28 of the electrolytic cell and below the disk electrode 6, the platinum counter electrode 19 is parallel to the bottom surface of the disk electrode 6, and the distance is preferably less than 2 cm; the reference electrode 18 is clamped in the reference electrode jack 22, wherein the tip 180 of the salt bridge tip tube of the reference electrode 18 is placed between the platinum counter electrode 19 and the bottom surface of the disk electrode 9, and the reference electrode terminal is electrically connected with the reference electrode 18. The crude oil injection and recovery holes 23 are provided to prevent the crude oil from contacting the working electrode (the salt bridge tip tube 180 of the reference electrode 18, the disk electrode 6 and the platinum counter electrode 19) at the initial stage of the experiment; the breathing hole 25 is mainly arranged to lead the liquid levels of the inner electrolytic cell cylinder 28 and the outer electrolytic cell cylinder 27 which are communicated with each other at the bottom to change in a coordinated way; 27-the diameter of the outer cylinder of the electrolytic cell is designed to be large enough to ensure the liquid level to be stable, and the cross-sectional area of the outer cylinder is generally required to be more than 2 times of the drainage area of the floater 9. A liquid level scale 20 may be provided within the cell inner cylinder 28 to gauge and control liquid level.
The rotating disk electrode system achieves a simulation of fluid flow rate. The device comprises a disk electrode 6 and a rotary driving device 2 for driving the disk electrode to rotate, wherein the disk electrode 6 is connected and fixed on the rotary driving device 2 and extends into an inner barrel 28 of an electrolytic cell through a rotary disk electrode jack 26. Different flow rate simulations are realized by means of the rotation speed regulation of the disc electrode 6, and the linear speed can be obtained according to the rotation speed and the radius of the electrode. Specifically, the disc electrode 6 may be composed of a lead 29, a cylindrical insulating housing 30 and a conductive core 31 connected to the lead, wherein the lead 29 is connected to the working electrode terminal (the insulating housing may be made of any insulating material, such as plastic, etc., or the surface of the conductive core 31 may be coated with insulating paint). Among them, the disk electrode 6 can be designed into two structures as shown in fig. 4 and 5, thereby realizing the bottom surface test and the side surface test. The bottom surface test electrode can realize the instantaneous conversion of an oil-water interface, the operation of different linear velocity radiuses on oil-water wetting can be simultaneously considered, an opening is formed in the bottom surface of the insulating shell 30 of the disc electrode 6 for bottom surface test, the shape of the opening is the same as that of the bottom surface of the conductive inner core 31, the bottom surface of the conductive inner core 31 is clamped at the opening and serves as a working electrode 60, and the rest outer surfaces are insulated. The oil-water interface of the side surface test electrode is in non-instantaneous transition, but a single linear velocity can be obtained, and the disc electrode 6 for side surface test is designed into an annular working electrode, and the following embodiments are adopted: the insulating sheath 30 is formed of upper and lower portions having the same cross-section as the conductive core, and the upper and lower portions sandwich the conductive core, thereby forming a ring-shaped conductive surface as the working electrode 60 on the side of the insulating sheath. The specific experimental study can be selected or designed according to different purposes. The rotary driving device 2 is fixed on the fixing frame 4, the vertical support of the fixing frame 4 is made of carbon steel, the horizontal support is made of carbon steel, the vertical support can rotate in the horizontal direction and move in the vertical direction, and the fixing and locking are needed after the position is determined.
An oil-water interface control system comprising: the device comprises a fixed frame 14, a stepping motor 8, a motor control unit 13, a precise threaded guide rod 7 and a floater 9, wherein the stepping motor 8 is connected with the motor control unit 13 and fixed on the fixed frame 14, the stepping motor 8 is connected with the floater 9 through the precise threaded guide rod 7, and the floater 9 extends into a space between an outer cylinder 27 of the electrolytic cell and an inner cylinder 28 of the electrolytic cell. The precise threaded guide rod 8 is matched with a rotor component of the stepping motor 8, so that the rotation of the motor is changed into translation of the guide rod, and the floater is driven to reciprocate up and down 9, and the oil-water interface is controlled. The thread density and the motor rotation speed determine the translation speed and precision of the guide rod. The motor control unit 13 mainly comprises a stepping motor driver and a stepping motor switch power supply box stepping motor pulse controller. In the non-overload condition, the rotation speed and stop position of the motor only depend on the frequency and pulse number of the pulse signal, and are not influenced by the load change, when the stepping driver receives a pulse signal, the stepping driver drives the stepping motor to rotate by a fixed angle in a set direction, namely a stepping angle, and the rotation of the stepping motor is operated by one step at the fixed angle. The angular displacement can be controlled by controlling the number of pulses, so that the aim of accurate positioning is fulfilled; meanwhile, the rotating speed and the rotating acceleration of the motor can be controlled by controlling the pulse frequency, so that the aim of speed regulation is fulfilled. Wherein, the fixing frame 14 is made of common carbon steel, the horizontal bracket can also be adjusted correspondingly in position and direction, and the bottom needs to be fixed in order to ensure bearing.
The aeration system comprises an aeration head 12 arranged in the outer cylinder 27 of the electrolytic cell and outside the inner cylinder 28 of the electrolytic cell, and a gas supply device 15 communicated with the aeration head 12 through a ventilation pipeline 11. The aeration head 12 is placed outside the inner barrel 28 of the electrolytic cell for the main purpose of avoiding escaping bubbles from affecting the stability of the oil-water interface of the inner barrel. The aeration system is mainly used for introducing specified gas into the solution in the electrolytic cell; n is generally required to be introduced in the experimental preparation stage2Deoxidizing, introducing CO in the test process according to the experimental requirements2Simulating the field working condition by using the corrosive gas; the pipeline needs to be provided with gas flow control and metering equipment.
The temperature control system mainly comprises an electric heating belt 3 and a temperature control unit 16. Wherein, the electric heating belt 3 is made of flexible material and can be wound on the outer wall of the outer cylinder 27 of the electrolytic cell, and the maximum temperature rise can reach 100 ℃. The temperature control unit 16 mainly comprises a thermocouple, a power switch, a temperature regulator and the like; when the temperature sensed by the thermocouple reaches a set value, automatic power off is realized; due to the model differences of the thermostats, they generally need to be temperature calibrated before the experiment.
The simulation method of the oil-water alternate wetting corrosion simulation device is summarized by taking an oil-water alternate wetting corrosion simulation experiment as an example:
(1) preparation of the experiment: preparing aqueous solution (based on experimental design, in principle any aqueous solution medium can be used, generally oil field simulation produced water, in this embodiment, produced liquid simulating certain working condition of oil field is adopted, and the specific solute content is 0.1 mol/LNaCl + 0.01 mol/L NaHCO3+ 0.01 mol/L CaCl2) The electrolyte is contained in an oil-water alternate wetting electrolytic cell, the liquid level is controlled between identification limit lines (above a salt bridge sharp tube and below a working electrode), oxygen is removed by ventilation, a temperature control system is started (generally, an oil-water interface vibrates up and down at equal distance by taking the working electrode 60 as an original point, so that the maximum limit of the oil-water interface is equal to the distance from the working electrode to the salt bridge sharp tube of a reference electrode), and the temperature is raised to a set value; controlling the stepping motor 8 to press the floater 9 into water to adjust the testing end surface (working electrode 60) of the disc electrode 6 below the water surface, and connecting a reference electrode terminal and a reference electrode 18, a counter electrode terminal and a platinum counter electrode 19, the reference electrode 18 and the working electrode 60 of the electrochemical workstation; and installing and debugging the oil-water interface control system to enable the device to be in a standby state.
(2) Injecting a certain amount of crude oil into an inner barrel 28 of an electrolytic cell by using a dropper through a crude oil injection and recovery hole 23, wherein the crude oil needs to be preheated to a test temperature and is ventilated to remove oxygen; in order to ensure the stability of data in the test process, the oil-water interface is controlled between two limit lines of the high limit mark and the low limit mark of the oil-water interface, so that the salt bridge sharp tube 180 of the platinum counter electrode 19 and the reference electrode 18 is prevented from being polluted by crude oil.
(3) Starting a rotating disk electrode system, and enabling the disk electrode 6 to rotate stably at a specific rotating speed; electrochemical workstation 1 and associated software are turned on and the open circuit potential is first tested for a period of about 600 seconds or more to stabilize.
(4) Carrying out a constant potential corrosion test, and recording a current-time change curve; meanwhile, an oil-water interface controller is arranged, the translation speed and the distance of the floater 9 and the oil-water interface reciprocating motion period of the conductive surface of the disc electrode 6 are set (the conductive surface is wetted by water when contacting with a water phase, wetted by oil when contacting with an oil phase, and the alternate wetting period is controlled by the movement of the floater), and the initial position of the oil-water interface, the retention time in the oil phase and the water phase and the like are determined according to specific requirements when setting parameters; it should be noted that the potentiostatic corrosion current test should be started in the water phase at the beginning, and after about 1-2 minutes or the set time of the experiment is reached, the oil-water interface control system is started, and the oil-water interface starts to periodically reciprocate up and down to enter the oil-water alternate wetting corrosion test stage.
(5) After the experiment test is finished, firstly stopping the electrochemical test, closing the electrochemical workstation and dismantling the connecting wire; then stopping the oil-water interface controller and lifting the floater out of the water surface; the rotary disk electrode is lifted, and crude oil is completely sucked out by a suction pipe, so that the reference electrode and the counter electrode are prevented from being polluted in subsequent treatment; cleaning the electrolytic cell;
(6) and (3) data analysis: microscopic observation is carried out on the surface corrosion morphology of the conductive surface of the disc electrode 6; the current-time response curve is subjected to statistical analysis, and particularly, the corrosion current, the number and the shape of transient peaks and the like can be analyzed according to research needs.
Fig. 6 and 7 are graphs showing the time course of current measured by the above-described apparatus and method of the present invention. Wherein the experimental conditions of FIG. 6 are set as the rotating speed of the disc electrode is 1200 r/min, and the reciprocating period of the oil-water interface is 2.15s (adjusted by two variable parameters of the up-down moving speed and the distance); the experimental conditions of FIG. 7 were set to a disc electrode rotation speed of 600 rpm and an oil-water interface reciprocation period of 5.0 s. The experimental temperature is 30 ℃, the viscosity of the crude oil is 4.3 centipoises, and CO is continuously introduced in the corrosion process2A gas. The external potential is more than the open circuit potential and is +100 mV.
In the experimental process, the first 40s is that the working electrode 60 stays in the aqueous solution, a continuously and stably changed current signal is obtained through testing, the current shows transient fluctuation along with the start of an oil-water alternative wetting simulation experiment, the current transient peak duration depends on parameters such as alternative wetting frequency and crude oil viscosity, the current peak intensity depends on the stability of a water phase, and the change of the number of transient peaks is possibly related to the adsorption and desorption rules of crude oil on the surface, so that the crude oil corrosion inhibition effect can be deeply revealed.
By combining specific conditions and crude oil characteristics, various statistical analysis methods can be introduced aiming at corrosion current signals, and the action mechanism of the crude oil on carbon steel corrosion in an alternate wetting (controlled by the oil-water interface reciprocating motion period) environment is disclosed. Wherein, t0To t1The average corrosion current density at a time may be solved as follows:
dt
from the above equations, the average corrosion current density of the two sets of current-time response curves of FIG. 5 can be solved. Corresponding to the conditions of 1200 rpm and 2.15s of wetting period, the calculated average corrosion current is 96.2 microamperes; the calculated average corrosion current was 35.6 microamps corresponding to 600 rpm, 5.0s wet cycle. In the calculation process, the current integration time period is selected to be between 40 seconds and 300 seconds. Therefore, under the conditions of low rotating speed and low wetting frequency, the alternate wetting of the crude oil has better corrosion inhibition effect, and the crude oil is less prone to be desorbed from the surface of the carbon steel. Therefore, quantitative research on corrosion rules in an oil-water alternate wetting environment is realized through statistical analysis.
The invention has the beneficial effects that:
(1) the corrosion simulation research of the inner wall of the pipeline under different oil-water alternative action conditions is realized:
by utilizing the function of adjusting the rotating speed of the rotating disc electrode, the laboratory simulation of the metal corrosion condition under different fluid flow rates under the condition of oil-water alternate wetting can be realized, and then the influence of the fluid flow rate on the metal corrosion under the condition of oil-water alternate wetting can be researched. The adjustable rotating speed range is 0-2000 rpm.
By utilizing the accurate controllable function of the reciprocating motion frequency of the stepping motor, the laboratory simulation of the metal corrosion condition under the condition of different oil-water alternate wetting frequencies can be realized, and the influence of the oil-water alternate wetting frequency on the metal corrosion under the condition of oil-water alternate wetting can be further researched. The adjustable frequency range is 0.01-2.0 Hz and depends on the precision of the stepping motor and the power supply pulse setting.
By changing the oil product or temperature used in the experiment, the metal corrosion laboratory simulation under the condition of oil products with different viscosities can be realized, and further the influence of the oil product viscosity on the metal corrosion under the condition of oil-water alternate wetting can be researched. The adjustable temperature range is from room temperature to 80 ℃.
By changing the electrode material and the surface state (such as different roughness and surface pre-filming conditions) used in the experiment, laboratory simulation of corrosion conditions of different metal materials and different surface states under the condition of oil-water alternate wetting can be realized, and further the corrosion conditions of different metal materials and different surface states under the condition of oil-water alternate wetting can be researched.
(2) Rapidly evaluating the effect of the corrosion inhibitor under the condition of oil-water alternate wetting:
the corrosion inhibitor is widely applied to oil field gathering and transportation systems and sewage systems, and has the characteristics of rapidness, reliability and the like when the corrosion inhibition effect is evaluated by an electrochemical method in a laboratory. However, the current corrosion inhibitor effect evaluation result is mainly based on an aqueous solution environment test, and no mature research method exists for evaluating the effect of the corrosion inhibitor under the conditions of crude oil content and oil-water alternate wetting. The invention provides a rapid, simple and reliable electrochemical test method, which determines the reasonable addition of the corrosion inhibitor by simulating the carbon steel corrosion behavior in an oil-water alternate wetting environment and provides more direct reference data for a complex multiphase flow environment on site. The electrochemical test method has the characteristics of rapidness, real-time monitoring and the like, so that the research method disclosed by the invention is widely applied to corrosion inhibitor evaluation, a rapid and effective reference basis is provided for oil field production, and the production safety and efficiency are improved.
The invention provides a method for deeply disclosing the research on the surface corrosion of carbon steel under the alternate action of crude oil wetting and water wetting, and the obtained experimental data has important significance for developing the related knowledge in the corrosion field. More research support is needed for more detailed and profound analysis of experimental data.
The above-mentioned embodiments are merely preferred embodiments for fully illustrating the present invention, and the scope of the present invention is not limited thereto. The equivalent substitution or change made by the technical personnel in the technical field on the basis of the invention is all within the protection scope of the invention. The protection scope of the invention is subject to the claims.