Cause of well casing pipe damage and repairing technology
How to prevent well casing pipe damage? How to repair well casing to restore its production? These are important questions in the development of oil field.
Cause of well casing pipe damages
Improper production method
Excessive differential pressure in production. Over-fast exploitation will destroy the stratum structure, and cause large quantities of sand flow into the wellbore.
Improper stimulation measures
High pressure operation will damage the low-strength casing. High pressure in fracturing, acidizing will cause formation channeling. The intrusion of foreign water and cool gas injected water will damage the stable cementation structure and cement sheath outside the casing pipe. The water minerals will cause corrosion to the casing and lower its strength.
Too frequent workover treatment
In the middle and later period of oil field production, the formation pressure decrease and have serious leakage. During the well flushing and sand washing works, large quantities of workover fluid flow into the formation and cause damage casing corrosion.
Work in unrepaired status
The casing pipe damages are not repaired in time and work in poor state. The formation water and injected water will flow into the fault formation and cause the formation to wiggle. These will aggravate the damages of well casing pipes.
High pressure injection
High-pressure water injection, steam injection, high-temperature stimulating treatment are the main cause of casing damages. High-pressure water injection is an important measure to increase the production of oilfield, and steam injection is the mean method for thick oil extraction.
Practical casing pipe damage repairing technology
With the increasing of exploitation time, the deformation and damages of casing pipes become serious. It is necessary to expand the diameter of the damaged casing and dredge the pipeline.Casing pipe repairing technology includes casing diagnosis technique, casing pipe opening technique, tie back and casing replacing technology. Although casing repairing technology is increasingly perfect, the repairing effect is unsatisfactory.
Casing diagnosis technique
Casing diagnosis technique. This technology is used to detect and inspect the casing leak, reducing, and breaking. Nowadays, there are many advanced casing diagnosis instruments, such as borehole televiewer, borehole imaging, and electronic multipoint monitor. But in practice, they usually diagnose the casing condition by lead stamp print or experience, which are easy to cause misjudge and failure of construction.
Casing pipe opening technique
Casing pipe opening technique. This is the basis of casing pipe repairing technology. The repairing work can be carried out only when the pipes are effectively opened. There are 3 opening methods, mechanical shaping, explosive shaping, and milling.
Mechanical shaping includes casing inflation and casing swage 2 types. The instruments are pear tube expander and roll shaper. Under the gravity and downward impact, the pear tube expander wedge into the deformed case to swell it up and restore the original size.
Explosive shaping is to deal with those seriously deformed casings. Send a shaping boom to the preset position of the well casing. The boom explosion will generate shock waves and high pressure gases, which transmit through the oil and gas in the pipe and act on the inner surface of the casing. The casing and the outer cement sheath will expand and the inner diameter will increase. After explosive shaping, they usually use casing pipes to reinforce the lower lining tubes and the restored parts.
Milling pipe opening technology
For the crimping and burrs cause by the corrosion and damage of perforation and packer seat sealing, we can use milling cone to grind the deformed parts.
For those seriously damaged well casings, we usually use milling to break through. The tools should get to the deformed casing and won’t deviate. The tools for finding casing and expansion includes: eccentric tube expander, movable guiding mill cutter, repairing milling drum, ball reshaper, probe-type milling cone.
For well casings deformed seriously and the diameter changes a lot, milling cones are harmful. They may cause the casing body thin, leak, and facture. We should choose matched milling tools with the casing pipes.
Casing mending and patching technology
Casing mending and patching is the major casing repairing technology. It is only 1/3-1/2 the cost of casing window sidetracking or casing replacing. Currently, it has 2 technologies, one for bellows, the other is for hydraulic expansion pipes. The mended and patched pipes can withstand 350 ℃ high temperature and 15MP high pressure. High strength ensure it works well in thermal wells. The inner diameter remain nearly the same, and the hole shrinkage is less than 10mm.
Currently, the casing pipe repairing technology is increasingly perfect, but it still have many problems. Only when we diagnose the casing damages accurately can we make reasonable repairing plans. It’s risky to rely on experience or rough to make judgment.
To get the whole picture of the damaged casing pipes, we need advanced technology and instruments. Draw up a repairing plan ahead of time rather than discover problems during operation. Otherwise, the construction would be complicated and even cause underground accident.