Oil and gas well casing damage and repair

Oil and gas wells are fixed assets of an oil and gas company, and the formation casing is the most basic element of an oil and gas well. Once the oil formation casing is damaged and cannot be repaired, it means the scrapping of the oil and gas well and the loss of fixed assets. The oil formation casing is subjected to high pressure and erosion of gas and liquid media, and is repeatedly subjected to external forces such as well repair operations and production enhancement measures. The causes of damage are mainly the following.
(1) Poor quality and low strength of the casing itself.
(2) Poor cementing quality and poor sealing.
(3) Corrosion by water, chemical agents and microorganisms
(4) Geological movement and lithology around the casing
(5) damage to the casing by high-pressure water injection
(6) The influence of oil, gas and water wells out of sand.
(7) The damage to the casing caused by improper construction of well repair operations.

There are three general forms of casing damage, namely casing reduction, casing rupture, and casing breakage.

Due to the different casing damage location and damage degree and damage condition, some can not be repaired, some can be repaired.

The repair method for casing shrinkage is to gradually expand the inner diameter by adding pressure to the drill pipe through the roller shaper, which is enlarged step by step during repair. This method is simpler and easier to see results.

For the repair of casing rupture, with seam or hole, there are several methods as follows.

(1) Squeezing cement slurry. When the formation pressure is not large, rupture and leakage is not serious, the method of squeezing cement slurry can be used to repair. The process approach is: first 8 to 10 mm smaller than the casing inside diameter through the well gauge through the diameter, and then in the appropriate location under a suspended plugging device (tool name called bridge plug), the break below the wellbore temporary seal off, and then inject a certain amount of cement slurry above the bridge plug, so that it solidifies into a cement plug, cement plug after solidification, drilling open the cement plug in the casing, and test pressure to check the quality of the cement seal break, to confirm the quality of seal After confirming the quality of the seal, drill open the suspended plug (bridge plug) and flush the sand to the bottom of the well. The casing repaired by this method can generally withstand a pressure of 40 to 80 MPa, but the well should be protected by a packer to avoid high pressure in the section when the well is under high pressure construction.

(2) Casing replacement. When the rupture location in the upper part of the well, and can be inverted to remove the casing above the break, the inverted buckle method can be used to bring up all the casing above the accident section, re-enter the new casing, and tighten the good buckle. The advantage of this method is to ensure the consistency of the casing inside diameter, and the downhole tools can pass smoothly after operation; the disadvantage is that the tightness of the casing buckle downhole is not as tight as at the wellhead.

(3) Subsidy method. The subsidy method is to paste a layer of thin-walled tubing on the inner wall of bad casing to achieve the purpose of repair. The process principle is: in a special high-pressure rubber cylinder set with bellows (thin-walled tube) down to the well casing damage location, holding pressure to make the rubber cylinder expansion, while expanding the bellows, so that the bellows close to the casing damage at the inner wall, and by the adhesive to the casing and bellows adhesive into one, to be cured adhesive, active drilling tools, the high-pressure rubber cylinder removed. This subsidy process is simple and safe to operate.

The well with broken casing can be repaired separately in three cases. The first one is broken but not misaligned; the second one is broken but not seriously misaligned; the third one is broken and seriously misaligned, even the next section of casing cannot be found.

In order to obtain the displacement of the casing after fracture, misalignment depth, the relative distance between the upper and lower fracture and whether the fracture is deformed and other information, can be used to hit the lead seal, instrument testing and other methods to clarify the situation.

For broken casing without misalignment, water mud injection method can be used to repair; for broken but not seriously misaligned casing, such as allowing the replacement of casing, then use the replacement of casing method to repair; such as not having the conditions of the replacement of casing can be used under the repair of the connector, that is, the broken casing grinding and milling off a section, the middle of the upper and lower casing clamping, pulling the tool to ensure that the well can be normal production.

For wells with serious misalignment after fracture and cannot find the next section of casing, the side drilling method can be used to deal with. In other words, the borehole at the fracture is cemented shut, and a borehole is re-drilled with a small drill pipe from the upper casing, and a smaller casing than the original casing is put into the borehole to complete the well.