Oil and gas wells are the fixed assets of petroleum enterprises, and oil casing is the most basic element of oil and gas wells. Once the reservoir casing is damaged and cannot be repaired, it means the abandonment of oil and gas wells and the loss of fixed assets. The oil layer casing is subjected to high pressure, gas, liquid and other media erosion all the year round, and also to various workover operations, stimulation measures and other external forces repeatedly. The main causes of damage are as follows:

(1) The casing itself has poor quality and low strength; (2) poor cementing quality and poor sealing performance; (3) Corrosion of water, chemicals and microorganisms; (4) Geological tectonic movement and lithologic action around casing; (5) High pressure water injection damages the casing; (6) The influence of sand production from oil, gas and water wells; (7) Improper workover operation causes casing damage.

1- Schematic diagram of conduit well structure; 2— Surface casing; 3— Cement ring of surface casing; 4-technical shell; 5— Cement ring of technical casing; 6- high pressure gas reservoir; 7- high pressure water layer; 8-easily collapsed stratum; 9-drilling; 10- oil casing; 11-main reservoir (target reservoir); 12— There are generally three kinds of casing damage, namely casing shrinkage, casing fracture and casing rupture.

Due to the different damaged parts, degrees and conditions of casing, some casings cannot be repaired and some casings can be repaired.

The repair method of casing shrinkage is to run the gradually expanding roller shaper during the repair process, and gradually expand the inner diameter through the pressure of drill pipe. This method is simple and effective.

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, but when the well is under high pressure construction, it should be protected by packer to avoid this section from bearing high pressure.

(2) Replace the shell. 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 after operation; The disadvantage is that the tightening degree of downhole casing thread is not as good as that of wellhead.

(3) Subsidy law. The repair method is to stick a thin-walled tube on the inner wall of the bad casing to achieve the purpose of repair. Its technological principle is: the bellows (thin-walled tube) is sleeved with a special high-pressure resistant rubber tube and lowered to the damaged position of casing in the well, and the rubber tube is expanded by pressure pressing, and the bellows is expanded at the same time, so that the bellows is close to the inner wall of the damaged position of casing, and the casing and bellows 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.

There are three situations in which a well with damaged casing can be repaired. The first one is broken without dislocation; The second type is fracture but dislocation is not serious; The third kind is broken, severely dislocated, and even 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.