1. The production string cannot be lowered into the well normally;

2. When the casing is in the position of water layer or sand layer, the oil well will produce a lot of water or sand;

3, oil well stimulation measures cannot be implemented;

4, easy to cause blowout outside the casing;

5. The oil well is scrapped. ?

Second, the casing maintenance and repair:

There are several ways to repair casing cracks, cracks or holes:

1, squeeze cement slurry

When the reservoir pressure is low and the cracks and leakage are not serious, cement slurry can be squeezed to repair it. The process method is as follows: firstly, measure the borehole diameter with a small diameter which is 8 ~ 10 mm larger than the casing inner diameter, then place a floating plug (tool name is bridge plug) at the appropriate position of the fracture, temporarily close the wellbore below the fracture, and then inject a certain amount of cement slurry above the bridge plug to solidify it into a cement plug. After the cement plug is solidified, drill the cement plug in the casing, and conduct pressure test to check the quality of cement sealing cracks until the sealing quality is confirmed. The casing repaired by this method can generally bear the pressure of 40 ~ 80 MPa. In the future, the packer should be used to protect the well to prevent this section from bearing high pressure.

2. Replace the casing

When the fracture is located in the upper part of the oil well and the casing above the fracture can be taken out by back-screwing, all the casing above the accident section can be lifted by back-screwing, and the new casing can be lowered again, and the casing can be properly fastened. The advantage of this method is to ensure the consistency of casing inner diameter, and downhole tools can pass smoothly in future operations; The disadvantage is that the tightening degree of downhole casing thread is not as good as that of wellhead.

3. Subsidy law

It is to stick a thin-walled tube on the inner wall of the damaged casing to achieve the purpose of repair. Its technological principle is: after a special high-pressure resistant rubber cylinder is sheathed with corrugated pipe (thin-walled pipe), it is lowered to the damaged position of casing in the well, and the rubber cylinder is expanded by pressing pressure, and at the same time, the corrugated pipe is expanded, so that the corrugated pipe is close to the inner wall of the damaged position of casing, and the casing and corrugated pipe are bonded into a whole by adhesive. After the adhesive is cured, move the drilling tool to take out the high-pressure resistant rubber cylinder. This subsidy process is simple and safe to operate. ?

Casing damaged wells are repaired in three situations. The first one is broken without dislocation; The second type is fracture but dislocation is not serious; The third type is broken and severely dislocated, and the casing can't be found.

In order to obtain data such as casing displacement, dislocation depth, relative distance between upper and lower cracks, and whether cracks are deformed, lead printing and instrument testing can be used to find out the situation.

For the damaged casing without dislocation, water injection mud can be used to repair it; For the damaged but not seriously misaligned casing, if it is allowed to be replaced, the casing replacement method will be used to repair it; If the casing cannot be replaced, it can be repaired by running the joint, that is, after grinding off a broken casing, the upper and lower casings are clamped and pulled by the joint tool to ensure the normal production of the oil well.

Well with serious dislocation after being folded in half and no casing can be found at the lower part can be treated by sidetracking. That is, the broken borehole is cemented, a small drill pipe is lowered from the upper casing to drill a borehole again, and the next casing smaller than the original casing is cemented and completed.