A device for testing the cathodic protection potential of an oilfield downhole casing and a method of use

Abstract

The application discloses a kind of oilfield downhole casing cathodic protection potential testing device and use method, including test cable, probe fixing device and potential test table, test cable includes from outside to inside outer plastic layer, shielding metal layer and inner plastic layer, inner plastic layer is provided with potential cable core, second potential cable core and reference cable core, test cable lower end passes through from cable through-hole, potential connection terminal is connected with the lower end of second potential cable core, spare potential connection terminal and the lower end of second potential cable core are connected, reference electrode is further connected to the lower end of reference cable core;With outer plastic layer, formation is insulated from casing outer wall, then shielding metal layer is used for electromagnetic shielding to prevent formation potential interference, the potential of two cable core ends on ground is equal to the potential of downhole potential connection terminal and solid reference electrode, that is, the protection potential and natural potential of casing in the position to be measured in downhole are directly detected on ground.

Description

Technical Field

[0001] This invention belongs to the field of corrosion detection, specifically relating to a cathodic protection potential testing device and its usage method for oilfield downhole casing. Background Technology

[0002] Currently, oil wells in oilfields worldwide widely utilize casing cementing completion for production. However, formation groundwater causes significant external corrosion damage to the casing. To protect the integrity of the casing string and ensure the long-term safe operation of oil and water wells, oil and gas producers typically employ impressed current cathodic protection technology for casing. This technology has been maturely applied in oilfields for over a century with remarkable results.

[0003] Regarding the evaluation of cathodic protection effectiveness, the national standards GB / T21447-2018 "Code for External Corrosion Control of Steel Pipelines", GB / T21448-2017 "Technical Code for Cathodic Protection of Buried Steel Pipelines", and GB / T33791-2017 "Technical Code for Cathodic Protection and Corrosion Resistance Operation of Steel Well Casing" stipulate that the cathodic protection criterion is that the metal's protection potential should have a negative offset of 100mV, or reach 850mV after removing the IR drop. The measurement method is to use the near-ground method or the far-ground method. Specifically, the reference electrode is buried near the measurement point or at a relatively far location without electric field interference. At this time, the potential difference between the reference electrode and the location point is the absolute potential value.

[0004] For vertical pipelines—oil and water well casings—because the formation is sealed with cement outside the casing, the reference electrode cannot be directly lowered into the annular space between the casing and the formation. Currently, there is no direct method to measure the cathodic protection potential of the casing outer wall at deep well locations. It is usually indirectly estimated and detected through numerical simulation calculations, downhole potential profile logging tools, or the E-LogI method to assess whether the cathodic protection effect meets the standards. This method is recommended as a national standard method in GB / T33791-2017 "Technical Specification for Cathodic Protection Corrosion Resistance Operation of Steel Well Casing".

[0005] The literature “Research on the Calculation Method of Cathodic Protection Potential of Oil Well Casing” (Chinese Journal of Corrosion and Protection, December 2001, Vol. 21, No. 6) proposes a method to calculate the cathodic protection potential at a certain depth of the downhole casing by detecting protection parameters on the surface and based on the basic completion parameters of the casing. However, the calculation model is complex and belongs to the indirect measurement of theoretical calculation.

[0006] The literature "Casing Well Potential Profile Logging Tool" (Logging Technology, 2002, Vol. 26, No. 4) proposes using a downhole potential profile logging tool to detect potential. This tool has two elastic contact probes less than 10m apart. Under the condition of protective current, it is lowered downhole to detect the potential difference between the two probes. This potential difference is used to calculate whether cathodic protection meets the protection standard (i.e., elimination of the anodic zone). This technology cannot detect the protection potential of cathodic protection and still belongs to indirect measurement effect estimation.

[0007] Direct potential measurement is a fundamental method for evaluating the effectiveness of cathodic protection of surface pipelines. Currently, the effectiveness of cathodic protection of oilfield casing can only be evaluated indirectly due to the limitations of well structure. Seeking direct potential measurement is the goal pursued by corrosion workers and a practical need faced by production. Summary of the Invention

[0008] The purpose of this invention is to provide a cathodic protection potential testing device and method for oilfield well casing, so as to directly evaluate the effect of cathodic protection of oilfield casing.

[0009] The objective of this invention is achieved through the following technical means: a cathodic protection potential testing device for oilfield downhole casing, comprising a test cable, a potential test meter, and a probe fixing device connected to the outside of the casing.

[0010] The probe fixing device is a conical sleeve clamp, which is connected to the outside of the sleeve. The sleeve clamp also has a cable through hole for the test cable to pass through. A locking nut is connected to the outside of the sleeve clamp, and a potential terminal and a spare potential terminal are connected to the locking nut.

[0011] The test cable consists of an outer plastic layer, a shielding metal layer, and an inner plastic layer, from the outside to the inside. The inner plastic layer contains a first potential cable core, a second potential cable core, and a reference cable core. The lower end of the test cable passes through a cable through-hole. The potential terminal is connected to the lower end of the first potential cable core, and the spare potential terminal is connected to the lower end of the second potential cable core. The lower end of the reference cable core is also connected to a reference electrode, which is fixed to the outside of the sleeve through the reference cable core.

[0012] The upper end of the reference cable core is the reference electrode cable end. The upper ends of the first potential cable core and the second potential cable core are the first potential cable end and the second potential cable end, respectively. The first potential cable end, the second potential cable end and the reference electrode cable end are all connected to the potential test meter.

[0013] It also includes several cable fixing devices. The test cable is fixed to the outside of the sleeve by the cable fixing devices, which are nylon cable ties or insulating tape.

[0014] The probe fixing device is made of the same material as the sleeve.

[0015] The vertical distance between the probe of the reference electrode and the outer wall of the sleeve is within 18mm.

[0016] The first potential cable core, the second potential cable core, and the reference cable core are double-core aluminum or copper metals with the same wire diameter and material properties.

[0017] The first potential cable core, the second potential cable core, and the reference cable core are all covered with a plastic layer, and the first potential cable core and the reference cable core adopt a twisted pair winding structure.

[0018] A method for using a cathodic protection potential testing device for oilfield downhole casing includes the following steps:

[0019] Before the sleeve is lowered, the probe fixing device is connected to the outside of the sleeve. The lower end of the test cable passes through the cable through hole. The reference electrode is connected to the reference cable core at the lower end of the test cable. The potential terminal and the spare potential terminal are connected to the locking nut. The other end of the potential terminal and the spare potential terminal are connected to the first potential cable core and the second potential cable core, respectively.

[0020] When lowering the sleeve, the test cable is lowered synchronously along the sleeve. During the lowering process, the test cable is fixed to the sleeve at certain intervals using a cable fixing device.

[0021] After the sleeve is lowered, connect the reference electrode cable end, the first potential cable end, and the second potential cable end of the test cable left on the ground to the potential test meter, and connect the shielding metal layer of the test cable to the grounding terminal of the potential test meter.

[0022] When the bushing is not carrying cathodic protection current, measure the potential between the reference electrode cable end and the first potential cable end. This potential is the natural corrosion potential of the bushing at the corresponding depth when it is not protected. Then measure the potential between the reference electrode cable end and the first potential cable end when the cathodic protection current is connected. This is the protection potential.

[0023] Multiple probe fixing devices located at different depths are connected to the outside of the casing. Each probe fixing device is connected to a ground potential tester via a test cable to measure the natural corrosion potential and protection potential at different depths of the casing.

[0024] The beneficial effects of this invention are as follows: a non-metallic layer, i.e., an outer plastic layer, is used to insulate the formation from the outer wall of the casing. Then, a shielding metal layer is used for electromagnetic shielding to prevent formation potential interference. During measurement, the shielding metal layer and the grounding terminal of the potential testing instrument are grounded simultaneously. Finally, the same type of cable core produced by the same process is used as the reference cable core and the potential cable core. The resistivity and total impedance of both are the same. The resistance of the cable core is many orders of magnitude smaller than the input impedance of the potential testing instrument. According to the principle of potential testing, the potential at the two cable core ends on the ground is equal to the potential at the potential terminal and the solid reference electrode downhole. That is, the protective potential and natural potential of the casing at the test location downhole can be directly detected on the ground. This realizes online, real-time, and direct working potential measurement under cathodic protection of deep well casing, solving the problem that existing theoretical calculations or indirect potential difference tests for evaluating the cathodic protection effect are not intuitive, certain, or accurate. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of a cathodic protection potential testing device system for oilfield downhole casing.

[0026] Figure 2 This is a schematic diagram of the cross-section of the test cable;

[0027] Figure 3 This is a schematic diagram of the sleeve clamping structure;

[0028] Figure 4 This is a cross-sectional view of the cannula clamp teeth;

[0029] Figure 5 This is a schematic diagram of the locking screw cap structure;

[0030] In the diagram: 1-1 Probe fixing device; 1-2 Test cable; 1-3 Cable fixing device; 1-4 Reference electrode cable end; 1-5 Potential test meter; 1-6 First potential cable end; 1-7 Cathodic protection power supply; 1-8 Auxiliary anode body; 1-9 Current line; 1-10 Sleeve; 1-11 Reference electrode; 1-12 Potential terminal; 1-13 Spare potential terminal; 1-14 Second potential cable end; 2-1 Outer plastic layer; 2-2 Shielding metal layer; 2-3 Inner plastic layer; 2-4 First potential cable core; 2-5 Second potential cable core; 2-6 Reference cable core; 3-1 Sleeve clamp; 3-2 Device through hole; 3-3 Locking cap;

[0031] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Detailed Implementation

[0032]

Example 1

[0033] like Figure 1As shown, a cathodic protection potential testing device for oilfield well casing includes a test cable 1-2, a potential tester 1-5, and a probe fixing device 1-1 connected to the outside of the casing 1-10.

[0034] The probe fixing device 1-1 is a conical sleeve clamp 3-1. The sleeve clamp 3-1 is connected to the outside of the sleeve 1-10. The sleeve clamp 3-1 also has a cable through hole 3-2 for the test cable 1-2 to pass through. A locking nut 3-3 is connected to the outside of the sleeve clamp 3-1. A potential terminal 1-12 and a spare potential terminal 1-13 are connected to the locking nut 3-3.

[0035] The test cable 1-2 includes an outer plastic layer 2-1, a shielding metal layer 2-2, and an inner plastic layer 2-3 from the outside to the inside. The inner plastic layer 2-3 contains a first potential cable core 2-4, a second potential cable core 2-5, and a reference cable core 2-6. The lower end of the test cable 1-2 passes through the cable through hole 3-2. The potential terminal 1-12 is connected to the lower end of the first potential cable core 2-4. The spare potential terminal 1-13 is connected to the lower end of the second potential cable core 2-5. The lower end of the reference cable core 2-6 is also connected to a reference electrode 1-11. The reference electrode 1-11 is fixed to the outside of the sleeve 1-10 by a cable tie through the reference cable core 2-6.

[0036] The upper end of the reference cable core 2-6 is the reference electrode cable end 1-4. The upper ends of the first potential cable core 2-4 and the second potential cable core 2-5 are the first potential cable end 1-6 and the second potential cable end 1-14, respectively. The first potential cable end 1-6, the second potential cable end 1-14 and the reference electrode cable end 1-4 are all connected to the potential test meter 1-5.

[0037] like Figures 3 to 5As shown, the probe fixing device 1-1 includes a sleeve clamp 3-1 and a locking nut 3-3. The sleeve clamp 3-1 is a tapered clamp with internal and external threads. The internal thread is used to clamp the sleeve 1-10. The minor diameter of the internal thread is the same as the outer diameter of the sleeve 1-10. The locking nut 3-3 is screwed to fix the external thread. The external thread is tapered. The smaller end of the nominal diameter of the external thread mates with the locking nut 3-3. As the locking nut 3-3 is screwed into the external thread clamp in the axial direction, the internal thread clamp of the sleeve clamp 3-1 will move radially in conjunction. As a result, the internal thread clamp is tightly engaged in the sleeve, achieving the purpose of fixing it to the sleeve 1-10 and fixing the measuring end of the test cable 1-2, and realizing the electrical connection between the first potential cable core 2-4 and the second potential cable core 2-5 and the sleeve 1-10. The through hole 3-2 of the probe fixing device 1-1 is used to allow the test cable 1-2 to pass through, and the reference electrode 1-11 is fixed at the lower end of the reference cable core 2-6 to prevent the reference electrode 1-11 from moving longitudinally due to the impact of the fluid in the well during the casing running process, which would lead to installation failure. After the casing 1-10 is run into the well, cementing is carried out, and the downhole part of the entire potential testing device is solidified in the casing and the formation annulus.

[0038] Potential terminal 1-12 and spare potential terminal 1-13 are soldered to locking cap 3-3. The other end of potential terminal 1-12 is connected and fixed to the first potential cable core 2-4, and the other end of spare potential terminal 1-13 is connected and fixed to the second potential cable core 2-5.

[0039] The test cable 1-2 is connected to the probe fixing device 1-1 from top to bottom along the outer wall of the sleeve 1-10. The upper end of the test cable 1-2 is located on the ground. The first potential cable core 2-4 at the upper end is led out as the reference electrode cable end 1-4. The upper end of the potential terminal 1-12 is led out as the first potential cable end 1-6. The upper end of the second potential cable core 2-5 is led out as the second potential cable end 1-14.

[0040] Reference electrode cable terminal 1-4, first potential cable terminal 1-6, and second potential cable terminal 1-14 are all connected to potential meter 1-5 for measuring potential. Potential meter 1-5 has a differential circuit unit to remove in-phase and common-mode interference signals from the two input terminals of reference electrode cable terminal 1-4 and potential cable terminal 1-6.

[0041] It also includes several cable fixing devices 1-3. The test cable 1-2 is fixed to the outside of the sleeve 1-10 by the cable fixing devices 1-3. The cable fixing devices 1-3 are nylon cable ties or insulating tape.

[0042] like Figure 2As shown, the test cable contains three aluminum or copper cores of the same diameter, material properties, and batch, consisting of the first potential cable core 2-4, the second potential cable core 2-5, and the reference cable core 2-6. The outermost plastic layer 2-1 is an insulating plastic sheath, the middle shielding metal layer 2-2 is a metal mesh layer, and the innermost plastic layer 2-3 is an insulating plastic layer covering the metal cable core. The cable outer diameter is <30mm. The cable is lowered into the casing 1-10 and led out from the casing head at the wellhead. The cable fixing device 1-3 that fixes the test cable 1-2 to the outer wall of the casing 1-10 consists of nylon cable ties and waterproof insulating tape, with a spacing of 10m between adjacent cable fixing devices 1-3.

[0043] The first potential cable core 2-4, the second potential cable core 2-5, and the reference cable core 2-6 are all covered with a plastic layer, and the first potential cable core 2-4 and the reference cable core 2-6 adopt a twisted pair winding structure. The second potential cable core 2-5 is only a spare of the first potential cable core 2-4, and can be removed in actual use, using only the first potential cable core 2-4 and the reference cable core 2-6.

[0044] Two insulated wires are twisted together to improve anti-interference capabilities and enable long-distance acquisition of weak signals.

[0045] The outer plastic layer 2-1 is made of polyester polyethylene terephthalate or polybutylene terephthalate, the inner plastic layer 2-3 is made of polyvinyl chloride, and the shielding metal layer 2-2 is made of ordinary carbon steel strip or steel mesh, or it can be an aluminum conductor material.

[0046] The probe fixing device 1-1 is made of the same material as the sleeve 1-10.

[0047] The vertical distance between the probe of the reference electrode 1-11 and the outer wall of the sleeve 1-10 is within 18mm.

[0048] The first potential cable core 2-4, the second potential cable core 2-5, and the reference cable core 2-6 are double-core aluminum or copper metals with the same wire diameter and material properties.

[0049] Reference electrodes 1-11 are solid reference electrodes suitable for semi-dry concrete structures, with an outer diameter of <30mm and a length of <100mm.

[0050] The reference electrode 1-11 does not contact the sleeve 1-10. The reference electrode 1-11 is fixedly connected to the reference cable core 2-6, which is equivalent to being fixed outside the sleeve 1-10. The vertical distance between the reference electrode 1-11 and the outer wall of the sleeve 1-10 is within 18mm.

[0051] The probe fixing device 1-1 and the locking nut 3-3 are made of the same material as the casing 1-10 body to achieve locking with the deep well casing. The probe fixing device 1-1 is coated with insulating paint. The probe fixing device 1-1 is inserted into the outer wall of the casing 1-10 through the locking teeth of the casing 1-3 to achieve excellent electrical connection with the casing 1-10.

[0052] The connection method for the first potential cable core 2-4, the second potential cable core 2-5 of the test cable 1-2 with the potential terminal 1-12 and the spare potential terminal 1-13 is as follows: After inserting the exposed metal end of the first potential cable core 2-4 into the perforated end of the potential terminal 1-12, the copper tube is flattened with crimping pliers, and then sealed with underwater curing agent. The external surface is treated with three layers of insulating tape, waterproof tape, and sealing tape, or with cable joint insulating putty and tape. The other end of the potential terminal 1-12, i.e. the metal plate end, is welded to the locking nut 3-3. The welded area is sealed with underwater curing agent. The connection method for the second potential cable core 2-5, the spare potential terminal 1-13, and the locking nut 3-3 is the same as above. The first potential cable core 2-4 and the second potential cable core 2-5 are electrically connected to the sleeve through the locking nut 3-3 of the probe fixing device 1-1.

[0053] The electrode lead of the reference electrode 1-11 is connected to one of the cores of the test cable 1-2, namely the reference cable core 2-6. The electrode lead and the core are connected by a copper conduit. The exposed metal of the electrode lead and the core are simultaneously inserted into the copper conduit and then flattened with a crimping pliers. Then, it is sealed with an underwater curing agent and treated with three layers of insulating tape, waterproof tape and sealing tape, or cable joint insulating putty plus tape, to achieve electrical connection and waterproof insulation sealing function.

[0054] Oil and water well casings extend thousands of meters underground, with the outer wall of the casing electrically connected to the formation. The natural potential of the formation varies significantly at different depths, resulting in a longitudinal potential difference of hundreds of millivolts for casings at depths exceeding one kilometer. Conventional methods of measuring cathodic protection potential and natural potential at the wellhead only reflect the polarization protection characteristics at the wellhead. Direct potential measurement using conventional armored cables is also subject to interference from the formation's natural potential. Because the outer metal shielding layer of a conventional armored cable is electrically in contact with the solution formation in the longitudinal direction, and this long longitudinal distance inevitably introduces a significant longitudinal resistance, this resistance will inevitably cause… The metal shielding layer has a potential difference in the direction of vertical well depth in oil and water wells. This potential difference causes an induced potential difference in the direction of well depth to the cable core conductor inside (although the shielding metal layer and cable core of the horizontal cable are at the same potential, the premise of the commonly mentioned metal electromagnetic shielding is in a limited physical space. The resistance of the metal shielding layer with a close distance is so small that it can be ignored, so that the potentials on the surface of the metal layer are equal). This affects the potential measurement between the cable cores on the ground. That is, when the outer metal layer of the cable is in direct contact with the solution formation, it cannot play the role of metal shielding. The downhole potential measured under this environment cannot reflect the actual protection status of the downhole casing.

[0055] This application addresses the impact of natural potential fluctuations in different electrolyte strata on the detection of minute potential differences at the ends of long-axis cables by utilizing the electromagnetic shielding effect of metals. Simultaneously, it leverages the principle of electromagnetic induction cancellation in twisted pairs to eliminate current noise interference between conductors and improve the detection and resolution of weak signals over long distances.

[0056] First, a non-metallic layer, namely the outer plastic layer 2-1, is used to insulate the ground layer from the outer wall of the casing 1-10, so as to eliminate direct contact between the conductive layer of the ground layer and the metal shielding layer.

[0057] Electromagnetic shielding is then used with shielding metal layer 2-2 to prevent formation potential interference and improve the device's ability to resist external coupling noise. During measurement, shielding metal layer 2-2 is grounded and connected to the grounding terminal of the potential testing instrument at the same time to reduce signal attenuation and noise, and ensure that the downhole protection potential is not distorted after being transmitted through a long cable. Furthermore, when measuring the potential at the casing wellhead, the twisted pair shielding layer must be grounded, i.e. connected to the grounding terminal of the potential testing instrument.

[0058] Finally, the same type of cable core produced by the same process is used as the reference cable core 2-6 and the first potential cable core 2-4 (the second potential cable core 2-5 is exactly the same as the first potential cable core 2-4, but it is used as a redundancy because it is difficult to remove and reinstall after being lowered, in order to avoid the inability to detect if the second potential cable core 2-4 is damaged). The resistivity and total impedance of the two are the same. The cable core resistance is much smaller than the input impedance of the potential testing instrument by many orders of magnitude. According to the potential testing principle, the potential at the two cable core ends on the ground is equal to the potential at the potential terminal 1-12 and the solid reference electrode 1-11 in the well. That is, the protective potential and natural potential of the casing at the test position in the well can be directly detected on the ground.

[0059] Finally, the reference cable core 2-5 and the potential cable core 2-4 are respectively covered with an insulation layer and a twisted pair wiring structure is adopted to cancel the mutual interference of internal electromagnetic noise between the two conductors when testing the protection potential. That is, when one of the twisted conductors is conducting electricity, the electromagnetic radiation emitted is canceled by the electromagnetic radiation emitted by the other twisted conductor. This method improves the ability to detect weak potential signals through long-distance cables at deep underground locations.

[0060] like Figure 1 As shown, a method for using a cathodic protection potential testing device for oilfield downhole casing includes the following steps:

[0061] Before the sleeve 1-10 is lowered, the probe fixing device 1-1 is connected to the outside of the sleeve 1-10. The lower end of the test cable 1-2 passes through the cable through hole 3-2. The reference electrode 1-11 is connected to the reference cable core 2-6 at the lower end of the test cable 1-2. The potential terminal 1-12 and the spare potential terminal 1-13 are connected to the locking cap 3-3. The other end of the potential terminal 1-12 and the spare potential terminal 1-13 are connected to the first potential cable core 2-4 and the second potential cable core 2-5, respectively.

[0062] Before lowering, connect the probe fixing device 1-1, the reference electrode 1-11, the potential terminal 1-12 and the spare potential terminal 1-13 inside the probe fixing device 1-1 to the corresponding parts of the test cable 1-2, and prepare for lowering.

[0063] Lower the sleeve 1-10. When lowering the sleeve 1-10, simultaneously lower the test cable 1-2 along the sleeve 1-10. During the lowering process, fix the test cable 1-2 to the sleeve 1-10 at certain intervals using the cable fixing device 1-3.

[0064] During the lowering process, test cable 1-2 is lowered simultaneously with sleeve 1-10, and at regular intervals, such as 10m, test cable 1-2 is fixed to sleeve 1-10 by cable fixing device 1-3.

[0065] After the sleeve 1-10 is lowered, connect the reference electrode cable end 1-4, the first potential cable end 1-6 and the second potential cable end 1-6 of the test cable 1-2 left on the ground to the potential test meter 1-5, and connect the shielding metal layer 2-2 of the test cable 1-2 to the grounding terminal of the potential test meter 1-5.

[0066] After the test cable is lowered, the reference electrode cable end 1-4, the first potential cable end 1-6, and the second potential cable end 1-6 are led out from the part of the test cable 1-2 that is left on the ground and connected to the potential test meter 1-5. The shielding metal layer 2-2 is grounded.

[0067] When the bushing 1-10 is not carrying cathodic protection current, measure the potential between the reference electrode cable end 1-4 and the first potential cable end 1-6. This potential is the natural corrosion potential of the bushing position at the corresponding depth when it is not protected. Then measure the potential between the reference electrode cable end 1-4 and the first potential cable end 1-6 when the cathodic protection current is connected. This is the protection potential.

[0068] Multiple probe fixing devices 1-1 located at different depths are connected to the outside of the sleeve 1-10. Each probe fixing device 1-1 is connected to the ground potential test meter 1-5 through a test cable 1-2 to measure the natural corrosion potential and protection potential at different depths of the sleeve 1-10.

[0069] Reference Figure 1 This diagram illustrates the protective potential testing system, showcasing the testing apparatus and method for the cathodic protection potential of the downhole casing. After the cathodic protection power supply 1-7 energizes the downhole casing, a negative current flows along the path from cathodic protection power supply 1-7 → cathode point A → casing 1-10 → bottom of the well B → auxiliary anode body 1-8 → cathodic protection power supply 1-7, forming a closed loop. This current 1-9 causes cathodic polarization of the casing 1-10 from top to bottom, thus protecting it and stopping corrosion.

[0070] First, shut down the cathodic protection system. Use potential tester 1-5 to measure the potential difference between the reference electrode 1-11 and the potential terminal 1-12. Since the potential at the two cable core ends on the ground is equal to the potential at the potential terminal 1-12 and the solid reference electrode 1-11 downhole, the potential between the reference electrode cable end 1-4 and the first potential cable end 1-6 can be measured. The measurement result is the natural corrosion potential at the corresponding depth of the casing when it is not protected.

[0071] Turn on the cathodic protection system and measure the potential between the reference electrode cable end 1-4 and the first potential cable end 1-6 at this time. The measurement result is the protection potential of the corresponding depth sleeve position under the protection state.

[0072] It can also change the current of the cathodic protection system, measure the potential between the reference electrode cable end 1-4 and the first potential cable end 1-6 to obtain the protection potential corresponding to different currents; measure the series potential values ​​when the current is disconnected to obtain the IR drop, and perform secondary data processing on the series values ​​of natural potential and protection potential to obtain various cathodic protection characteristic curves of deep well casing.

[0073] Multiple sets of cathodic protection potential testing devices for oilfield downhole casing can also be set at different depths of casing 1-10. For example, a probe fixing device 1-1 can be set at 100m and 200m downhole. Each probe fixing device 1-1 is connected to the ground through a test cable 1-2. In this way, the natural corrosion potential and protection potential of casing at different depths can be measured.

Claims

1. A cathodic protection potential testing device for oilfield downhole casing, characterized in that: It includes a test cable (1-2), a potential meter (1-5), and a probe fixing device (1-1) connected to the outside of the sleeve (1-10). The probe fixing device (1-1) is a conical sleeve clamp (3-1). The sleeve clamp (3-1) is connected to the outside of the sleeve (1-10). The sleeve clamp (3-1) also has a cable through hole (3-2) for the test cable (1-2) to pass through. A locking nut (3-3) is connected to the outside of the sleeve clamp (3-1). A potential terminal (1-12) and a spare potential terminal (1-13) are connected to the locking nut (3-3). The test cable (1-2) comprises an outer plastic layer (2-1), a shielding metal layer (2-2), and an inner plastic layer (2-3) from the outside to the inside. The inner plastic layer (2-3) contains a first potential cable core (2-4), a second potential cable core (2-5), and a reference cable core (2-6). The lower end of the test cable (1-2) passes through a cable through-hole (3-2). The potential terminal (1-12) is connected to the lower end of the first potential cable core (2-4). The spare potential terminal (1-13) is connected to the lower end of the second potential cable core (2-5). A reference electrode (1-11) is also connected to the lower end of the reference cable core (2-6). The reference electrode (1-11) is fixed to the outside of the sleeve (1-10) via the reference cable core (2-6). The outer plastic layer (2-1) is an insulating plastic sheath. The upper end of the reference cable core (2-6) is the reference electrode cable end (1-4). The upper ends of the first potential cable core (2-4) and the second potential cable core (2-5) are the first potential cable end (1-6) and the second potential cable end (1-14) respectively. The first potential cable end (1-6), the second potential cable end (1-14) and the reference electrode cable end (1-4) are all connected to the potential test meter (1-5).

2. The cathodic protection potential testing device for oilfield downhole casing according to claim 1, characterized in that: It also includes several cable fixing devices (1-3). The test cable (1-2) is fixed to the outside of the sleeve (1-10) by the cable fixing devices (1-3). The cable fixing devices (1-3) are nylon cable ties or insulating tape.

3. The cathodic protection potential testing device for oilfield downhole casing according to claim 1, characterized in that: The probe fixing device (1-1) and the sleeve (1-10) are made of the same material.

4. The cathodic protection potential testing device for oilfield downhole casing according to claim 1, characterized in that: The vertical distance between the probe of the reference electrode (1-11) and the outer wall of the sleeve (1-10) is within 18mm.

5. The cathodic protection potential testing device for oilfield downhole casing according to claim 1, characterized in that: The first potential cable core (2-4), the second potential cable core (2-5), and the reference cable core (2-6) are double-core aluminum or copper metals with the same wire diameter and material properties.

6. The cathodic protection potential testing device for oilfield downhole casing according to claim 5, characterized in that: The first potential cable core (2-4), the second potential cable core (2-5), and the reference cable core (2-6) are all covered with a plastic layer, and the first potential cable core (2-4) and the reference cable core (2-6) adopt a twisted pair winding structure.

7. The method of using the cathodic protection potential testing device for oilfield downhole casing according to any one of claims 1-6, characterized in that, Includes the following steps: Before the sleeve (1-10) is lowered, the probe fixing device (1-1) is connected to the outside of the sleeve (1-10). The lower end of the test cable (1-2) passes through the cable through hole (3-2). The reference electrode (1-11) is connected to the reference cable core (2-6) at the lower end of the test cable (1-2). The potential terminal (1-12) and the spare potential terminal (1-13) are connected to the locking cap (3-3). The other end of the potential terminal (1-12) and the spare potential terminal (1-13) are connected to the first potential cable core (2-4) and the second potential cable core (2-5) respectively. Lower the sleeve (1-10). When lowering the sleeve (1-10), simultaneously lower the test cable (1-2) along the sleeve (1-10). During the lowering process, fix the test cable (1-2) on the sleeve (1-10) at certain intervals using the cable fixing device (1-3). After the sleeve (1-10) is lowered, connect the reference electrode cable end (1-4), the first potential cable end (1-6), and the second potential cable end (1-14) of the test cable (1-2) left on the ground to the potential test meter (1-5), and connect the shielding metal layer (2-2) of the test cable (1-2) to the grounding terminal of the potential test meter (1-5). When the bushing (1-10) is not carrying cathodic protection current, measure the potential between the reference electrode cable end (1-4) and the first potential cable end (1-6). This potential is the natural corrosion potential of the bushing position at the corresponding depth when it is not protected. Then measure the potential between the reference electrode cable end (1-4) and the first potential cable end (1-6) when the cathodic protection current is connected. This is the protection potential.

8. The method of using the cathodic protection potential testing device for oilfield downhole casing according to claim 7, characterized in that: Multiple probe fixing devices (1-1) located at different depths are connected to the outside of the casing (1-10). Each probe fixing device (1-1) is connected to the ground potential test meter (1-5) through a test cable (1-2) to measure the natural corrosion potential and protection potential at different depths downhole of the casing (1-10).

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