RU2233911C1 - Method of cathodic protection of a casing string of a well against corrosion and a device for its realization - Google Patents

Abstract

FIELD: petroleum industry.

SUBSTANCE: the invention presents a method of the cathodic protection of the casing string of the hole against corrosion and a device for its realization. The invention is pertinent to the methods and devices used for protection of a casing string of an injection well or a producing oil well against corrosion with the help of application of a controlled potential difference and may be used in different mining industries, including petroleum industry. The technical result is an increased efficiency of the cathodic protection against corrosion of the casing strings located in the remote from the day-light zones of wells. The method provides for superposition of the difference of potentials between a point of a cable connection on a daylight surface to the casing string and the anodic earth electrode. Additionally they impose the difference of potentials between a point located below on the casing string of the well and the anodic earth electrode. The device contains a station of a cathodic protection (СКЗ) connected by a cable on a daylight surface to the casing string and to the anodic earth electrode. The station of the cathodic protection is additionally connected to the casing string point located below. In the capacity of the casing string point located below in the well they use a point located above the upper spacing of perforation or the lowest point of the casing string. The difference of potential between the point located above the upper spacing of perforation or the lowest point of the casing string of the well and the anodic earth electrode is imposed through the point of the cable connection on a daylight surface to the casing string with application of an isolated jumper.

EFFECT: increased efficiency of the cathodic protection against corrosion of the casing strings located in the remote from the day-light zones of wells.

10 cl, 3 dwg, 1 tbl, 3 ex

Description

The invention relates to methods and devices for protecting a casing of an injection or production well from corrosion with the application of a controlled potential difference (cathodic protection) and can be used in various industries, including oil.

Known methods of protecting the metal of the casing of injection and production wells in oilfield environments using chemicals - corrosion inhibitors by dosing into the formation and into the annulus of the well bounded by the casing and tubing [1, 2]. The main disadvantage of the known methods is the high consumption of chemicals with a low actual effectiveness of corrosion protection, which is associated with the deposition of sulfide films on the metal surface and the difficult access of inhibitors to it.

Known methods of protecting carbon steel, mainly onshore and underground pipelines, from general corrosion using treads of metals of various nature, including the imposition of a controlled potential difference [3, 4]. The main disadvantage of the known methods is the difficulty of their application in wells, for example, oilfield ones, since there is a separation of treads, the extraction of which requires laborious and expensive work to drill and raise the spent metal. The high cost of traditional tread protection of downhole equipment is also determined by the use of protectors made of expensive magnesium-zinc alloy, which remain during attempts to lift in the well and are practically destroyed by drilling.

Closest to the claimed one is a method of cathodic protection of casing strings of wells [5], which includes applying a potential difference between the cable connection point on the day surface to the casing and the anode ground electrode using a cathodic protection station (SKZ) equipped with a reference electrode and an ampervoltmeter. The result is a shift in the potential of the protected section of the casing and its passivation. At the same time, the method does not provide effective protection of the casing zones remote from the day surface due to a decrease in the protective potential difference in them.

Closest to the claimed is a device for the cathodic protection of the casing string of the well [5], which includes an SCZ equipped with a reference electrode and an ampervoltmeter and connected by a cable on the surface to the casing and to the anode ground electrode.

A disadvantage of the known device is that it does not provide effective protection of the casing zones remote from the day surface due to a decrease in the protective potential difference in them.

Solved by the invention, the task and the expected technical result consists in increasing the efficiency of the method and device for the cathodic protection of the casing zones remote from the day surface without decreasing the efficiency with distance from the day surface.

The problem is solved by the fact that in the method of cathodic protection of the casing string of the well, which includes applying the potential difference between the cable connection point on the day surface to the casing string and the anode earthing switch, a potential difference between the downstream point of the casing string of the well and the anode grounding conductor is additionally applied.

As the downstream point of the well casing, use a point above the upper perforation interval of the well casing.

As the lower point of the well casing, the lower point of the well casing is used.

The potential difference between the downstream point of the well casing and the anode earthing is applied through the cable connection point on the surface to the casing.

The potential difference between the downstream point of the well casing and the anode earthing is applied using an insulated connecting wire.

The task is also solved by the fact that in the device for cathodic protection of the casing string of the well, containing a cathodic protection station (SCZ), connected by a cable on the surface to the casing and to the anode ground electrode, the SKZ is additionally connected to the downstream point of the casing string of the well.

As the downstream point of the well casing, a point above the upper perforation interval of the well casing is used.

As the lower point of the well casing, the lower point of the well casing is used.

SKZ is additionally connected to the downstream point of the well casing through the cable connection point on the surface to the casing.

SKZ is additionally connected to the downstream point of the well casing with an insulated connecting wire. The proposed device is presented in figure 1, where 1 is the casing string; 2 - connection point of the cable on the surface to the casing; 3 - downstream point of the casing string; 4 - insulated connecting wire; 5 - cathodic protection station equipped with a reference electrode and an ampervoltmeter; 6 - anode grounding; 7, 8 - cable (connecting).

The device operates as follows. The cathodic protection station is turned on and the required protection current is set, which is selected by the copper-sulfate reference electrode. A uniform polarization of the metal occurs over the entire surface of the protected equipment due to the summation of the protection currents through the upper and lower points of the casing.

The method is carried out by the following sequence of operations.

1. Overlaying the potential difference between the cable connection point on the day surface to the casing and the anode earthing switch.

2. An additional overlay of the potential difference between the lower or lower point of the casing and the anode ground electrode, preferably through a point above the upper perforation interval and through the cable connection point on the surface to the casing. For cathodic protection of the casing, the potential difference between the downstream or lower point of the casing and the anode earthing is applied using an insulated connecting wire that is lowered behind the casing during the construction of the well.

The method is illustrated by examples.

Example 1. The cathodic protection of the casing of the well of the Sergeevskoye field of NGDU Ufaneft with a descent depth of 1800 m (according to the prototype).

The known method of cathodic protection of the casing string from corrosion with the application of a potential difference has efficiency limitations in connection with connecting a connecting cable from the negative pole of the casing to a point on the surface of the casing string. In this case, the effective protective potential decreases along the axis of the column, which is due to the presence of leakage currents through the electrolyte - mineralized oilfield water in contact with the metal throughout the well.

As a cable (connecting), a 3 × 16 KPBP cable was used. SK3 No. 51 type OPS2-50-24-U1. Anode ground electrode system - a steel pipe weighing 5 tons, immersed in the ground to a depth of 2 m in a radius of 100 m from the wellhead.

The measurement of the real protective potential with depth was carried out by a metal probe similar to the protected one by lowering it with a connecting cable, ensuring continuous electrical contact with the body of the column. The results are shown in the table, which implies the almost complete absence of a real protective component of the potential on the metal already at a depth of 1000 m, since its values are -0.797 V and then continuously decrease until the stationary corrosion potential, while according to the regulations of the cathodic protection stations (GOST 9.602-89, p. 11) the potential protection range is considered the potential range minus 0.87 ... minus 2.5 V.

For further analysis, on the basis of the obtained data, a curve of step-by-step change of potential is constructed, which is presented in FIG. The results of its comparison with a model construction based on laboratory studies are reflected in Example 2.

Example 2. The cathodic protection of the casing model (prototype).

In order to simulate the real protective potential of the casing string when it comes into contact with mineralized oil field waters, we experimentally studied the change in the potential difference along the axis of an extended rod 100 mm long while moving the connection point of the negative pole of the stabilized voltage source. The step-wise potential variation curve is shown in FIG. 2 for clarity, together with a similar casing curve obtained in the data of Example 1.

The approximation of the obtained data by exponential dependences indicates the similarity of the processes of leakage of the protective current in both cases and the practical disappearance of the effective protective component of the potential on the metal at point 5 with a further decrease to stationary corrosion values (-0.711 V).

Example 3. The effectiveness of the cathodic protection according to the claimed method (casing of Example 1 and the model of Example 2).

Connecting the protective potential difference according to the claimed method provides the practical equipotentiality of both the casing string and its model, which is reflected in figure 3, which shows the step-wise potential changes for both measurements along their axis. The results of the application of the method indicate greater efficiency of the proposed cathodic protection compared to the prototype.

Thus, the inventive method really allows you to increase the efficiency of the cathodic protection of casing strings of wells made of carbon steel, using as a soluble anode an electrode of similar steel placed outside the borehole in the zone of stable electrolytic contact. The method is not laborious, efficient and industrially applicable, because for its implementation use available equipment and materials.

Sources of information

1. Ibragimov G.Z., Khisamutdinov N.I. Reference manual on the use of chemicals in oil production. - M .: Nedra, 1977. -271 p.

2. Khisamutdinov NI, Ibragimov G.Z. Oilfield development. Volume IV - M.: VNIIOENG, 1994. -262 p.

3. Protection of metal structures from underground corrosion: Reference / Strizhevsky I.V. et al. - M .: Nedra, 1981. -293 p.

4. Corrosion resistance of chemical equipment: Methods for protecting equipment from corrosion. / Ed. B.V. Strokana, A.M. Sukhotina. - L .: Chemistry, 1987.280 s.

5. Dautov F.I. and others. Cathodic protection of casing strings from corrosion in oil fields. - M.: VNIIOENG, 1981, 55 S.

Claims (10)

1. A method of cathodic protection of a casing string of a well, comprising applying a potential difference between a cable connection point on a day surface to a casing string and an anode ground electrode, characterized in that a potential difference is additionally applied between a downstream point of a casing string of a well and an anode ground electrode. 2. The method according to claim 1, characterized in that as the lower point of the well casing string, a point above the upper perforation interval of the well casing is used. 3. The method according to claim 1, characterized in that the bottom point of the well casing is used as the lower point of the well casing. 4. The method according to any one of claims 1 to 3, characterized in that the potential difference between the downstream point of the well casing and the anode ground electrode is applied through the cable connection point on the surface to the casing. 5. The method according to any one of claims 1 to 4, characterized in that the potential difference between the downstream point of the well casing and the anode ground electrode is applied using an insulated connecting wire. 6. A device for cathodic protection of a casing string of a well, comprising a cathodic protection station (SCZ) connected by a cable on the surface to the casing and to the anode ground electrode, characterized in that the SCZ is further connected to a downstream point of the casing of the well. 7. The device according to claim 6, characterized in that the point above the upper perforation interval of the well casing is used as the lower point of the well casing. 8. The device according to claim 6, characterized in that the bottom point of the well casing is used as the lower point of the well casing. 9. The device according to any one of paragraphs.6-8, characterized in that the SCZ is additionally connected to the downstream point of the well casing through the cable connection point on the surface to the casing. 10. The device according to any one of claims 6 to 9, characterized in that the SCZ is additionally connected to the downstream point of the well casing with an insulated connecting wire.

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