Damage and repair of oil and gas well casing

Oil casing is the lifeline to maintain the operation of oil wells. Due to different geological conditions, the downhole stress state is complex, and the combined action of tensile, compressive, bending and torsion stresses on the pipe body, which puts forward higher requirements on the quality of the casing itself. Once the casing itself is damaged for some reason, the entire well may be reduced or even scrapped.

 

Oil and gas wells are the fixed assets of oil companies, and oil layer casing is the most basic element of oil and gas wells. Once the oil layer 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 layer casing is subjected to the high pressure of the oil layer and the erosion of gas, liquid and other media throughout the year, and it is also repeatedly subjected to external forces such as various workover operations and stimulation measures. The main reasons for the damage are as follows:

 

(1) The casing itself is of poor quality and low strength;

 

(2) The cementing quality is poor and the sealing is not firm;

 

(3) Corrosion effects of water, chemical agents and microorganisms;

 

(4) Geological tectonic movement and lithology around the casing;

 

(5) Damage to the casing caused by high-pressure water injection;

 

(6) The impact of sand production in oil, gas and water wells;

 

(7) Damage to casing caused by improper workover operation.

 

There are generally three types of casing damage, namely, casing diameter reduction, casing rupture, and casing breakage.

 

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

 

For the repair method of casing diameter reduction, the roller shaper which gradually expands is used during repair, and the inner diameter is gradually expanded by pressurizing the drill pipe. This method is relatively simple and easy to take effect.

 

There are several methods for repairing broken casing, cracks or holes:

 

(1) Squeeze cement slurry. When the pressure of the oil layer is not high, and the rupture and leakage are not serious, it can be repaired by squeezing cement slurry. The process method is: first use a bore gauge diameter 8-10 mm smaller than the inner diameter of the casing, and then place a suspended plugging device (the tool name is called a bridge plug) at the appropriate position of the breach to temporarily seal off the wellbore below the breach , And then inject a certain amount of cement slurry above the bridge plug to make it solidify into a cement plug. After the cement plug is solidified, drill the cement plug in the casing, and test pressure to check the quality of the fracture of the cement seal. The seal is 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 40-80 MPa, but a packer should be used to protect the well during high-pressure construction to prevent this section from being subjected to high pressure.

 

(2) Replace the casing. When the rupture position is in the upper part of the oil well and the casing above the breach can be taken out by the reverse buckle method, the reverse buckle method can be used to lift up all the casing above the accident section, reinsert a new casing, and fasten it tightly. The advantage of this method is to ensure the consistency of the inner diameter of the casing, and the downhole tools can be passed smoothly after operation; the disadvantage is that the tightness of the casing thread of the downhole butt is not as tight as on the wellhead.

 

(3) Subsidy Law. The supplementary rule is to paste a thin-walled pipe on the inner wall of the broken casing to achieve the purpose of repair. The process principle is: a special high-pressure resistant rubber tube is sleeved with a corrugated tube (thin-walled tube) down to the damaged position of the casing in the well, the pressure is held to expand the rubber tube, and the corrugated tube is expanded at the same time to make the corrugated tube tight. Stick the inner wall of the damaged part of the casing, and the casing and the bellows are bonded together by the adhesive. After the adhesive is cured, move the drill tool and take out the high-pressure rubber tube. This subsidy process is simple and safe to operate.

 

Wells with broken casing can be repaired separately in three cases. The first is broken but there is no dislocation; the second is broken but the dislocation is not serious; the third is broken and the dislocation is serious, and the lower casing can’t even be found.

 

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

 

For the broken casing without misalignment, it can be repaired by grout injection method; for the broken casing but the misalignment is not serious, use the replacement casing method to repair if the casing replacement is allowed; if the casing replacement conditions are not available, it can be repaired underneath. The repair method is to mill off a section of the broken casing, and clamp and pull the upper and lower casings with a repair tool in the middle to ensure the normal production of the oil well.

 

For wells that are seriously misaligned after fracture and can not find the lower casing, the sidetracking method can be used. That is, the wellbore at the fracture is cemented to death, a small drill pipe is lowered from the upper casing to re-drill a borehole, and a casing smaller than the original casing is inserted to cement the well.

 

The mechanics and environmental behavior of oil pipes have an important influence on the adoption of advanced technology and production and efficiency in the oil industry; oil pipes have a huge loss of failure, and their safety, reliability and service life are of great importance to the oil industry.