What hazards will be caused by damage to oil well casing?

Damage to oil well casing pipe causes the following hazards to oil production:

1. The production string cannot be lowered into the well normally;
2. When the damaged part of the casing is in the water layer or sand layer, the oil well will produce a large amount of water or sand;
3. Measures to increase oil well production cannot be carried out;
4. It is easy to cause blowout outside the casing;

5. Cause oil wells to be scrapped.

Casing maintenance and repair:

There are several methods to repair casing cracks, seams or holes:

1. Squeeze cement slurry

When the pressure of the oil layer is not high and the rupture and leakage are not serious, repairs can be made by squeezing out cement slurry. The process is as follows: first use a well gauge diameter that is 8 to 10 mm smaller than the inner diameter of the casing, and then place a suspended plug (the name of the tool is called a bridge plug) at the appropriate position of the breach to temporarily seal the wellbore below the breach. , and then inject a certain amount of cement slurry above the bridge plug to solidify it into a cement plug. After the cement plug solidifies, drill out the cement plug in the casing, and perform a pressure test to check the quality of the cement-sealed breach. Wait for the sealing to be confirmed. After the quality is qualified, the suspended plug (bridge plug) is drilled and the sand is flushed to the bottom of the well. The casing repaired by this method can generally withstand a pressure of 40 to 80 MPa. In the future, the well should be protected by a packer to prevent the section from being subjected to high pressure during high-pressure construction.

2. Replace the casing

When the rupture location is in the upper part of the oil well, and the casing above the breach can be taken out by inversion, the inversion method can be used to lift up all the casing above the accident section, re-run the new casing, and tighten it properly. The advantage of this method is that it ensures the consistency of the inner diameter of the casing, so that the downhole tools can pass smoothly in future operations; the disadvantage is that the threaded threads of the casing buckled downhole are not as tight as those at the wellhead.

3. Subsidy Law

That is, a layer of thin-walled pipe is attached to the inner wall of the damaged casing to achieve the purpose of repair. The process principle is as follows: After putting a bellows (thin-walled tube) on a special high-pressure-resistant rubber cylinder and lowering it to the damaged position of the casing in the well, the pressure is suppressed to expand the rubber cylinder and at the same time expand the bellows to make the bellows close to each other. The inner wall of the casing is damaged, and the casing and the bellows are bonded together with an adhesive. After the adhesive solidifies, move the drilling tool to take out the high-pressure resistant rubber cylinder. This subsidy process is simple and safe to operate.

Wells with broken casings will be repaired separately in three situations. The first type is broken but not dislocated; the second type is broken but the dislocation is not serious; the third type is broken and seriously dislocated, and the lower casing cannot even be found.

In order to obtain information such as the displacement of the casing after it is broken, the depth of staggered fracture, the relative distance between the upper and lower edges of the fracture, and whether the fracture is deformed, methods such as printing and instrument testing can be used to clarify the situation.

For casings that are broken but not misaligned, cement slurry can be used to repair them; for casings that are broken but not misaligned, if the casing is allowed to be replaced, the casing replacement method can be used to repair it; if the conditions for casing replacement are not met, the casing replacement method can be used to repair the casing. The method of repairing the casing is to grind out a section of the broken casing, and then use a patching tool to clamp and pull the upper and lower casings to ensure normal production of the oil well.

For wells with severe dislocation after breakage and where the lower casing cannot be found, the sidetracking method can be used. That is, the broken wellbore is cemented, a small drill pipe is lowered from the upper casing, and a new wellbore is drilled, and a casing smaller than the original casing is lowered to complete the well.

Learn more: Difference between casing pipe and drill pipe