Cathode protection potential testing device of oil field underground casing and use method

Abstract

The invention discloses a cathode protection potential testing device of an oil field underground casing pipe and a using method, the cathode protection potential testing device comprises a testing cable, a probe fixing device and a potential testing meter, the testing cable comprises an outer plastic layer, a shielding metal layer and an inner plastic layer from outside to inside, and a potential cable core, a second potential cable core and a reference cable core are arranged in the inner plastic layer; the lower end of the test cable penetrates through the cable through hole, the potential wiring terminal is connected with the lower end of the second potential cable core, the standby potential wiring terminal is connected with the lower end of the second potential cable core, and the lower end of the reference cable core is further connected with a reference electrode; the outer plastic layer is adopted to insulate the stratum from the outer wall of the sleeve, then the shielding metal layer is used for electromagnetic shielding to prevent the potential interference of the stratum, and the potentials of the two cable core ends on the ground are measured to be equal to the potentials of the underground potential wiring terminal and the solid reference electrode. Namely, the protection potential and the natural potential of the casing pipe at the underground position to be detected are directly detected on the ground.

Description

Technical Field

[0001] The invention belongs to the field of corrosion detection, and in particular relates to a cathodic protection potential testing device for downhole casing in an oil field and a use method thereof. Background Art

[0002] At present, casing cementing and completion are widely used in oil and water wells in oil fields around the world for production. The formation groundwater causes significant external corrosion damage to the casing. In order to protect the integrity of the casing column and ensure the long-term safe service of oil and water wells, oil and gas producers usually use casing impressed current cathodic protection technology, which has been maturely applied in oil fields for hundreds of years with remarkable results.

[0003] In terms of cathodic protection effect evaluation, the cathodic protection criteria stipulated in national standards such as GB / T21447-2018 "Specifications for External Corrosion Control of Steel Pipelines", GB / T21448-2017 "Technical Specifications for Cathodic Protection of Buried Steel Pipelines", and GB / T33791-2017 "Technical Specifications for Cathodic Protection Corrosion Resistance of Steel Well Casings" are that the metal's protective potential is negatively offset by 100mV, or reaches 850mV after removing the IR drop. The measurement method is the near-ground method or the far-ground method. The specific operation is that the reference electrode is buried near the measuring point or at a relatively remote location without electric field interference. At this time, the potential difference between the reference electrode and the position point is the absolute potential value.

[0004] For vertical pipelines - oil and water well casings, since cement is used to seal the formation outside the oil and water well casing, the reference electrode cannot be directly lowered into the annular space between the casing and the formation. At present, there is no direct measurement method for the cathodic protection potential of the outer wall of the casing at deep wells. It is usually indirectly calculated and tested through numerical simulation calculations, or downhole potential profile logging instruments or E-LogI methods to evaluate whether the cathodic protection effect meets the standards. This method is recommended as a national standard method in GB / T33791-2017 "Technical Specifications for Cathodic Protection Corrosion Resistance of Steel Well Casing".

[0005] The document "Study on Calculation Method of Cathodic Protection Potential of Oil Well Casing" (Journal of the Chinese Society for Corrosion and Protection, December 2001, Vol. 21, No. 6) proposed a method of detecting protection parameters on the ground and calculating the cathodic protection potential of the casing at a certain depth based on the basic completion parameters of the casing. The calculation model is complex and it is an indirect measurement of theoretical calculation type.

[0006] The document "Cased Hole Potential Profile Logging Instrument" (Well Logging Technology, 2002, Vol. 26, No. 4) proposed the use of an underground potential profile logging instrument to detect potential. The instrument has two elastic contact arm probes less than 10m apart. When the protection current is passed, the potential difference between the two probes is detected underground. The potential difference is used to infer whether the cathodic protection meets the protection standard (i.e., the anode area is eliminated). This technology cannot detect the protection potential of the cathodic protection and still belongs to the indirect measurement effect inference.

[0007] Direct potential measurement is a basic means of evaluating the effectiveness of cathodic protection of surface pipelines. At present, the effectiveness of cathodic protection of oil field casing can only be evaluated indirectly due to the limitations of the wellbore structure. Seeking direct potential measurement is the goal pursued by corrosion workers and is also a practical need faced by production. Summary of the invention

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

[0009] The object of the present invention is achieved by the following technical means: a cathodic protection potential test 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 clamping tooth, which is connected to the outside of the sleeve, and the sleeve clamping tooth is also provided with a cable through hole for the test cable to pass through, and the outer side of the sleeve clamping tooth is connected to a locking screw cap, and the locking screw cap is connected to a potential wiring terminal and a spare potential wiring terminal;

[0011] The test cable comprises an outer plastic layer, a shielding metal layer and an inner plastic layer from outside to inside, a first potential cable core, a second potential cable core and a reference cable core are arranged in the inner plastic layer, the lower end of the test cable passes through the cable through hole, the potential terminal is connected to the lower end of the first potential cable core, the standby 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, and the reference electrode is fixed outside the casing 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, and 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, through which the test cable is fixed to the outside of the casing, and the cable fixing devices are nylon tie or insulating tape.

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

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

[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 the same 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 test device for an oilfield downhole casing comprises the following steps:

[0019] Before the casing is lowered, the probe fixture is connected to the outside of the casing, 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 wiring terminal and the spare potential wiring terminal are connected to the locking screw cap, and the other ends of the potential wiring terminal and the spare potential wiring terminal are connected to the first potential cable core and the second potential cable core respectively;

[0020] The casing is lowered, and the test cable is lowered synchronously along the casing. During the lowering process, the test cable is fixed on the casing by a cable fixing device at a certain interval;

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

[0022] When the cathodic protection current does not pass through the casing, the potential between the reference electrode cable end and the first potential cable end is measured. This potential is the natural corrosion potential of the casing position at the corresponding depth when it is not protected. Then, when the cathodic protection current is turned on, the potential between the reference electrode cable end and the first potential cable end is measured. This is the protection potential.

[0023] The outside of the casing is connected to multiple probe fixtures located at different depths. Each probe fixture is connected to a ground potential tester through a test cable to measure the natural corrosion potential and protection potential at different depths of the casing.

[0024] The beneficial effects of the present 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, and 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 simultaneously grounded and connected, and 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 the two are the same, and the cable core resistance is many orders of magnitude smaller than the input impedance of the potential testing instrument. According to the potential testing principle, the potentials of the two cable core ends on the ground are equal to the potentials of the potential wiring terminals and the solid reference electrodes in the well, that is, the protective potential and natural potential of the casing at the position to be tested in the well are directly detected on the ground, and the online real-time direct working potential measurement under the cathodic protection state of the deep well casing is realized, and the problem that the existing theoretical calculation or indirect potential difference test is not intuitive, definite or accurate in evaluating the cathodic protection effect is solved. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

[0027] Figure 3 It is a schematic diagram of the casing clamping tooth structure;

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

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

[0030] In the figure, 1-1 probe fixture; 1-2 test cable; 1-3 cable fixture; 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 bushing; 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 bushing clamping teeth; 3-2 device through hole; 3-3 locking screw cap;

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

[0032] [Example 1]

[0033] like Figure 1As shown, a cathodic protection potential test device for oilfield downhole casing includes a test cable 1-2, a potential test meter 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 clamping tooth 3-1, which is connected to the outside of the sleeve 1-10. The sleeve clamping tooth 3-1 is also provided with a cable through hole 3-2 for the test cable 1-2 to pass through. The outer side of the sleeve clamping tooth 3-1 is connected to a locking screw cap 3-3, and the locking screw cap 3-3 is connected to a potential wiring terminal 1-12 and a spare potential wiring terminal 1-13;

[0035] 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 outside to inside, a first potential cable core 2-4, a second potential cable core 2-5 and a reference cable core 2-6 are arranged in the inner plastic layer 2-3, 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, and the reference electrode 1-11 is fixed to the outside of the sleeve 1-10 with 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 terminal 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 terminal 1-6 and the second potential cable terminal 1-14 respectively, and the first potential cable terminal 1-6, the second potential cable terminal 1-14 and the reference electrode cable terminal 1-4 are all connected to the potential test table 1-5.

[0037] like Figures 3 to 5As shown, the probe fixing device 1-1 includes a sleeve clamping tooth 3-1 and a locking screw cap 3-3. The sleeve clamping tooth 3-1 is a conical clamping tooth with internal and external threads, wherein the internal thread is used to clamp the sleeve 1-10, and the minor diameter of the internal thread is the same as the outer diameter of the sleeve 1-10. The locking screw cap 3-3 is screwed and fixed with the external thread. The external thread is conical, and the smaller end of the external thread nominal diameter cooperates with the locking screw cap 3-3. As the locking screw cap 3-3 is axially screwed into the external thread clamping tooth, the internal thread clamping tooth of the sleeve clamping tooth 3-1 will be linked to produce radial movement, resulting in the internal thread clamping tooth tightly eating into the sleeve, so as to be fixed to the sleeve 1-10 and fix the measuring end of the test cable 1-2, and realize 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 device 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 with the lower end of the reference cable core 2-6 to prevent the reference electrode 1-11 from being impacted by the fluid in the well during the casing lowering process and moving longitudinally, thereby causing installation failure. After the casing 1-10 is lowered into the well, cement cementing is carried out, and the downhole part of the entire potential testing device is solidified in the annulus between the casing and the formation.

[0038] The potential terminal 1-12 and the spare potential terminal 1-13 are welded to the locking screw cap 3-3, the other end of the potential terminal 1-12 is connected and fixed to the first potential cable core 2-4, and the other end of the 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 fixture 1-1 from top to bottom along the outer wall of the casing 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, and the upper end of the second potential cable core 2-5 is led out as the second potential cable end 1-14.

[0040] The reference electrode cable terminal 1-4, the first potential cable terminal 1-6 and the second potential cable terminal 1-14 are all connected to a potential test table 1-5 for measuring potential. The potential test table 1-5 has a differential circuit unit for removing the in-phase common mode interference signal of the two input terminals of the reference electrode cable terminal 1-4 and the potential cable terminal 1-6.

[0041] It also includes several cable fixing devices 1-3, and the test cable 1-2 is fixed to the outside of the sleeve 1-10 through the cable fixing devices 1-3. The cable fixing devices 1-3 are nylon tie bands or insulating tapes.

[0042] like Figure 2As shown, in the test cable, the three first potential cable cores 2-4, the second potential cable core 2-5 and the reference cable core 2-6 are all three-core aluminum metal or copper metal with the same wire diameter, the same material properties and produced in the same batch. The outer plastic layer 2-1 of the outermost layer of the cable is an insulating plastic sheath, the middle shielding metal layer 2-2 is a metal mesh layer, and the inner plastic layer 2-3 of the inner layer is an insulating plastic layer covering the metal cable core. The outer diameter of the cable 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 for fixing the test cable 1-2 to the outer wall of the casing 1-10 is a nylon tie and a waterproof insulating tape, and the distance between adjacent cable fixing devices 1-3 is 10m.

[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 for the first potential cable core 2-4, and can also be removed in actual use, and only the first potential cable core 2-4 and the reference cable core 2-6 are used.

[0044] Two outer insulated wires are twisted together to improve anti-interference capabilities and achieve long-distance weak signal collection.

[0045] The outer plastic layer 2-1 is made of polyester polyethylene terephthalate or polybutylene terephthalate material, the inner plastic layer 2-3 is made of polyvinyl chloride material, and the shielding metal layer 2-2 is made of ordinary carbon steel belt or steel mesh, or 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 18 mm.

[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 the same material properties.

[0049] Reference electrode 1-11 is a solid reference electrode suitable for semi-dry concrete structure, with an outer diameter of less than 30 mm and a length of less than 100 mm.

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

[0051] The material of the probe fixture 1-1 and the locking screw cap 3-3 is the same as that of the casing 1-10 body, so as to achieve locking with the deep well casing. The probe fixture 1-1 is coated with insulating paint, and the probe fixture 1-1 eats into the outer wall of the casing 1-10 through the locking teeth of the casing teeth 1-3 to achieve excellent electrical connection with the casing 1-10.

[0052] The connection method of the first potential cable core 2-4 and 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: the exposed metal of the first potential cable core 2-4 is inserted into the holed end of the potential terminal 1-12 and the copper tube is flattened with a pressure pliers, and then 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. The other end of the potential terminal 1-12, i.e. the metal sheet end, is welded to the locking screw cap 3-3, and the welding area is sealed with an underwater curing agent. The connection method of the second potential cable core 2-5, the spare potential terminal 1-13 and the locking screw cap 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 screw cap 3-3 of the probe fixing device 1-1.

[0053] The electrode lead terminal of the reference electrode 1-11 is connected to one of the cable cores of the test cable 1-2, namely the reference cable core 2-6. The electrode lead terminal and the cable core are connected with a wiring copper tube. The electrode lead terminal and the exposed metal of the cable core are inserted into the wiring copper tube facing each other at the same time, and then the copper tube is flattened with a pressing pliers, and then sealed with an underwater curing agent. The outside is treated with three layers of insulating tape, waterproof tape and sealing tape, or the cable joint is treated with insulating putty and tape to achieve electrical connection and waterproof insulation sealing function.

[0054] The casing of oil and water wells is inserted thousands of meters underground. The outer wall of the casing is electrically connected to the formation. The natural potential of the formations at different depths varies greatly, resulting in a difference of hundreds of millivolts in the longitudinal potential of the casing at a depth of more than one kilometer. The conventional method of measuring the cathodic protection potential and natural potential at the wellhead can only reflect the polarization protection characteristics of the wellhead. The use of conventional armored cables to measure the potential directly underground is also affected by the natural potential of the formation. Since the outer metal shielding layer of the conventional armored cable has been in electrical contact with the solution formation in the longitudinal direction, and the outer metal shielding layer must have a non-negligible longitudinal resistance in the long longitudinal direction, this resistance will inevitably make The metal shielding layer has a potential difference in the direction vertical to the depth of the oil and water well, which causes an induced potential difference in the depth direction of the cable core conductor inside (although the shielding metal layer and cable core of the horizontal cable are of the same potential, the premise of the so-called metal electromagnetic shielding in daily life is that in a limited physical space, the resistance of the metal shielding layer at close distances is small enough to be ignored, so that the potentials on the surface of the metal layer are equal), thereby affecting the potential measurement between the ground cable cores, that is, the outer metal layer of the cable cannot play the role of metal shielding when it is in direct contact with the solution formation, and the downhole potential measured in this environment cannot reflect the actual protection status of the downhole casing.

[0055] This application solves the influence of natural potential fluctuations of different electrolyte formations on the detection of small potential differences at the end of the long-axis cable based on the electromagnetic shielding effect of metal. At the same time, the electromagnetic induction offset principle of twisted pair wires is used to eliminate the current noise interference between wires and improve the detection and resolution of long-distance weak signals.

[0056] First, a non-metallic layer, i.e., an outer plastic layer 2-1, is used to insulate the formation from the outer wall of the casing 1-10 to eliminate direct contact between the formation conductive layer and the metal shielding layer;

[0057] The shielding metal layer 2-2 is then used for electromagnetic shielding to prevent formation potential interference, thereby improving the device's ability to resist external coupling noise. During measurement, the shielding metal layer 2-2 is grounded and connected to the ground terminal of the potential test instrument at the same time, thereby reducing signal attenuation and noise, and ensuring that the downhole protection potential is not distorted after being transmitted through a longer cable. When measuring the potential at the casing wellhead, the twisted shielding layer must be grounded, that is, connected to the ground terminal of the potential detection 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, only because it is difficult to take out and reinstall after being lowered, it is used as redundancy to avoid the cable core 2-4 from being unable to be detected after it is broken). The resistivity and total impedance of the two are the same. The cable core resistance is many orders of magnitude smaller than the input impedance of the potential testing instrument. According to the potential testing principle, the potential of the two cable core ends on the ground is equal to the potential of 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 underground test position are directly detected on the ground.

[0059] Finally, the reference cable core 2-5 and the potential cable core 2-4 are respectively coated with an insulating layer, and a wiring structure in which twisted pairs are wound around each other is adopted to offset the internal electromagnetic noise interference between the two conductors when testing the protection potential. That is, when one wound conductor is conducting, the electromagnetic radiation emitted is offset 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 locations underground.

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

[0061] Before the casing 1-10 is lowered, the probe fixture 1-1 is connected to the outside of the casing 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 screw cap 3-3, and the other ends 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, the probe fixture 1-1, the reference electrode 1-11, the potential terminal 1-12 and the spare potential terminal 1-13 in the probe fixture 1-1 are connected with the corresponding parts of the test cable 1-2, and are ready for lowering;

[0063] The casing 1-10 is lowered, and the test cable 1-2 is lowered synchronously along the casing 1-10 when the casing 1-10 is lowered, and the test cable 1-2 is fixed on the casing 1-10 by the cable fixing device 1-3 at a certain interval during the lowering process;

[0064] When lowering, the test cable 1-2 is lowered synchronously with the casing 1-10, and the test cable 1-2 is fixed to the casing 1-10 by the cable fixing device 1-3 at a certain interval, such as 10m.

[0065] After the casing 1-10 is lowered, 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 are connected to the potential test table 1-5, and the shielding metal layer 2-2 of the test cable 1-2 is connected to the grounding end of the potential test table 1-5;

[0066] After the lowering is completed, 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 left on the ground and connected to the potential test table 1-5, and the shielding metal layer 2-2 is grounded.

[0067] When the cathodic protection current does not pass through the casing 1-10, the potential between the reference electrode cable terminal 1-4 and the first potential cable terminal 1-6 is measured. This potential is the natural corrosion potential of the casing position at the corresponding depth when it is not protected; then the potential between the reference electrode cable terminal 1-4 and the first potential cable terminal 1-6 is measured when the cathodic protection current is turned on. This is the protection potential.

[0068] A plurality of probe fixtures 1-1 located at different depths are connected to the outside of the casing 1-10. Each probe fixture 1-1 is connected to a ground potential test meter 1-5 via a test cable 1-2 to measure the natural corrosion potential and protection potential at different depths of the casing 1-10.

[0069] Reference Figure 1 Schematic diagram of protection potential test system, illustrating the test device and test method of cathodic protection potential of downhole casing. After the cathodic protection power supply 1-7 energizes the downhole casing, the negative current forms a closed loop along the cathodic protection power supply 1-7 → cathode line point A → casing 1-10 → bottom of the well B → auxiliary anode body 1-8 → cathodic protection power supply 1-7, and current 1-9, so that the casing 1-10 is protected from top to bottom by cathodic polarization and stops corrosion.

[0070] First, turn off the cathodic protection system, and use the potential tester 1-5 to measure the potential difference between the reference electrode 1-11 and the potential terminal 1-12. Because the potentials of the two cable core ends on the ground are equal to the potentials of the potential terminal 1-12 and the solid reference electrode 1-11 in the well, it is sufficient to measure the potential between the reference electrode cable end 1-4 and the first potential cable end 1-6. The measurement result is the natural corrosion potential of the casing position at the corresponding depth when it is not protected.

[0071] The cathodic protection system is turned on, and the potential between the reference electrode cable end 1-4 and the first potential cable end 1-6 is measured. The measurement result is the protection potential of the casing position at the corresponding depth under the protection state.

[0072] The current of the cathodic protection system can also be changed, the potential between the reference electrode cable terminal 1-4 and the first potential cable terminal 1-6 can be measured to obtain the protection potential corresponding to different currents; the series of potential values ​​when the current is disconnected can be measured to obtain the IR drop, and the natural potential and protection potential series values ​​can be processed secondary data to obtain various cathodic protection characteristic curves of deep well casing.

[0073] It is also possible to set up multiple groups of cathodic protection potential testing devices for oilfield downhole casing at different depths of the casing 1-10. For example, a probe fixture 1-1 is set at 100m and 200m underground, and each probe fixture 1-1 is connected to the ground through a test cable 1-2. In this way, the natural corrosion potential and protection potential of the casing positions at different depths can be measured.

Claims

1. A cathodic protection potential test device for oilfield downhole casing, characterized in that: It comprises a test cable (1-2), a potential test meter (1-5) and a probe fixing device (1-1) connected to the outside of a casing (1-10). The probe fixing device (1-1) is a conical sleeve clamping tooth (3-1), the sleeve clamping tooth (3-1) is connected to the outside of the sleeve (1-10), the sleeve clamping tooth (3-1) is also provided with a cable through hole (3-2) for the test cable (1-2) to pass through, the outer side of the sleeve clamping tooth (3-1) is connected with a locking screw cap (3-3), and the locking screw cap (3-3) is connected with a potential wiring terminal (1-12) and a spare potential wiring terminal (1-13); The test cable (1-2) comprises, from outside to inside, an outer plastic layer (2-1), a shielding metal layer (2-2) and an inner plastic layer (2-3); a first potential cable core (2-4), a second potential cable core (2-5) and a reference cable core (2-6) are arranged in the inner plastic layer (2-3); the lower end of the test cable (1-2) passes through a cable through hole (3-2); a potential wiring terminal (1-12) is connected to the lower end of the first potential cable core (2-4); a standby potential wiring 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); and the reference electrode (1-11) is fixed to the outside of the sleeve (1-10) through the reference cable core (2-6); 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 respectively the first potential cable end (1-6) and the second potential cable end (1-14), and 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 table (1-5).

2. The cathodic protection potential testing device for oilfield downhole casing according to claim 1, characterized in that: It also includes a plurality of cable fixing devices (1-3), through which the test cable (1-2) is fixed to the outside of the sleeve (1-10), and the cable fixing device (1-3) is a nylon tie or an 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 18 mm.

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 the same 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. A method for using a cathodic protection potential test device for oilfield downhole casing, characterized in that: The following steps are involved: Before the casing (1-10) is lowered, the probe fixing device (1-1) is connected to the outside of the casing (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 wiring terminal (1-12) and the spare potential wiring terminal (1-13) are connected to the locking screw cap (3-3), and the other ends of the potential wiring terminal (1-12) and the spare potential wiring terminal (1-13) are respectively connected to the first potential cable core (2-4) and the second potential cable core (2-5); The casing (1-10) is lowered, and a test cable (1-2) is lowered synchronously along the casing (1-10) when the casing (1-10) is lowered, and during the lowering process, the test cable (1-2) is fixed to the casing (1-10) at a certain interval by a cable fixing device (1-3); After the casing (1-10) is lowered, 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) remaining on the ground are connected to the potential test meter (1-5), and the shielding metal layer (2-2) of the test cable (1-2) is connected to the grounding end of the potential test meter (1-5); When the casing (1-10) does not pass the cathodic protection current, the potential between the reference electrode cable end (1-4) and the first potential cable end (1-6) is measured, and the potential is the natural corrosion potential of the casing position at the corresponding depth in the unprotected state; then, when the cathodic protection current is turned on, the potential between the reference electrode cable end (1-4) and the first potential cable end (1-6) is measured, and this potential is the protection potential.

8. The method for using the cathodic protection potential test device for oilfield downhole casing according to claim 7, characterized in that: A plurality of probe fixtures (1-1) located at different depths are connected to the outside of the casing (1-10); each probe fixture (1-1) is connected to a ground potential test meter (1-5) via a test cable (1-2) to measure the natural corrosion potential and protection potential of the casing (1-10) at different depths underground.

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