Causes of Casing Pipe Damage

Current status of casing pipe damage

In recent years, during the well logging of a certain block project, a total of 45 wells were found to have varying degrees of casing pipe damage, accounting for about 28.5% of the total number of wells, and the number of casing damage wells has shown a significant upward trend. The main forms of casing damage include casing bending, shrinkage, faulting, rupture, and casing corrosion perforation, among which casing shrinkage deformation and casing faulting are the main ones. The overall trend of casing damage is that the casing damage wells have a short production life and the casing damage point is deep. Longitudinal damage law: deep casing damage is mainly caused by shrinkage deformation, shallow casing damage is mainly caused by faulting or rupture, and the casing change points are concentrated near the perforation section. Horizontal regional distribution law: casing damage well points are concentrated near faults, structural highs and areas with large wing inclination angles.

Causes of casing damage

1. Geological factors of casing damage

Geological factors are the main causes of casing damage, which include tectonic stress, interlayer sliding, mudstone expansion, salt rock layer creep, oil layer sanding, ground subsidence and oil layer compaction.

(1) Mudstone expansion and creep, salt rock creep

Rocks have the characteristics of creep and stress relaxation. Different types of rocks have different types of creep and corresponding degrees of creep. Even under natural geological conditions, rocks will creep. Clay minerals in mudstone, especially montmorillonite, illite, and kaolinite, will swell and creep when they come into contact with water. Since the casing prevents this creep and expansion, shear stress is generated on the casing. This increases the external load of the casing. As time goes by, the load will increase. When the compressive strength of the casing is lower than the external load, the casing will be squeezed, flattened, or even broken.

(2) Surrounding rock pressure

After drilling, an empty surface appears in the rock around the wellbore, and the original equilibrium state is destroyed. When the stress at the stress concentration point reaches the yield limit of the surrounding rock, plastic deformation occurs. This deformation is restricted by the casing and the cement shell outside the casing. At the same time, the casing is also affected by the reaction of the surrounding rock, resulting in casing deformation and damage.

(3) Fault activity, modern crustal movement, earthquakes and landslides

The existence of faults causes pressure imbalance in the fault zone, causing water to flow between rock layers. When mud-salt layers and faults coexist, the casing will be unstable, resulting in casing deformation and damage. Fault activity is caused by many factors, and the movement of strata along the fault plane causes a large amount of damage to the oil layer casing.

2. Engineering factors of casing damage

(1) Casing damage can also be affected by human factors. Casing damage can occur during the design, production and use of casing.

(2) Casing damage caused by improper casing design mainly includes three aspects: the material and strength of the casing do not meet the requirements of on-site construction, the unreasonable combination of casing leads to local extrusion damage of the casing, and the unreasonable wellbore design leads to resistance during the casing lowering process.

(3) Improper production mainly refers to the lack of protective film during the casing manufacturing process, too large gaps between connecting threads, tiny cracks during processing, or uneven casing wall thickness. These problems will lead to poor sealing effect and fluid leakage during the use of the casing.

(4) Improper use refers to the friction and collision of the casing due to inadequate operation or management, or unreasonable selection of downhole tools during downhole operations, resulting in squeeze, stutter and scratch damage to the casing, unreasonable control of formation fracture pressure during fracturing, exceeding the rated pressure of the casing, resulting in casing damage.

3. Chemical factors of casing damage

(1) Chemical factors affecting casing damage mainly refer to the chemical reaction between chemical substances in the formation and the casing material, causing corrosion of the casing, thereby forming casing damage.

(2) Under normal circumstances, casing corrosion is caused by the chemical reaction between the casing pipe and other substances. Casing corrosion not only occurs during use, but also during idleness. 

(3) If the casing comes into contact with water in the formation during use, the corrosion phenomenon will be more obvious at the location where the casing surface is worn and damaged.

(4) The factors affecting the corrosion of the casing are mainly hydrochloric acid reducing bacteria, CO2, H2S and saline substances. Among them, hydrochloric acid reducing bacteria mainly exist in formation water. Under anoxic conditions, they are easy to form S*, and in the process of cathode depolarization, they are easy to react electrochemically with the casing, thereby accelerating the corrosion rate of the casing. When CO2 comes into contact with formation water, CO2 will quickly dissolve in the water to form HCO3~ ions and CO3- ions, and react chemically with metal ions in the water, causing metal iron ions to fall off the casing surface, causing casing corrosion damage.

(5) Hydrogen sulfide will also corrode the casing, and in severe cases, it will cause hydrogen embrittlement of the casing. Salt substances dissolved in water will react chemically with the damaged location on the casing surface, thereby corroding the casing.

4. Weather reasons: Climate change can also affect the use of casing. For example, seasonal floods, heavy rains and other extreme weather conditions can cause casing damage or rupture.