Causes Of Oil Casing Damage

During the production process of oil and gas wells, oil casing damage often occurs, which greatly affects the service life of oil casing. Next, Union Steel will introduce you to the causes of oil casing damage in detail.

1. External factors

The so-called external factors refer to the reasons for casing damage caused by oil and gas well production stimulation measures, oil well workover, etc. after the oil and gas well is put into production. Most oil and gas wells in my country require production stimulation measures such as fracturing or acidification before being put into production. After a period of production, fracturing and acidification will be carried out. Some wells also undergo refracturing. With the continuous improvement of process technology, the scale of transformation is also getting larger and larger. These transformation measures will greatly affect the service life of the casing.

(1) Perforating operations

Perforating is an important task before fracturing and acidizing. Improper engineering or operation can also cause casing damage. For example, too much hole density can reduce the strength of the casing, and perforations can cause cement to remain outside the casing. The ring ruptures, causing casing rupture, the perforation depth is too large or the design is inaccurate, and the shot penetrates the interlayer mudstone by mistake, causing the mudstone water to expand, resulting in changes in ground stress, causing casing deformation or dislocation.

(2) Fracturing and acidification operations

In the past decade or so, people have adopted large-scale fracturing measures for oil wells and considered the effect of transforming low-permeability oil layers. The wellhead pressure can reach 50-70MPa, and the oil layer casing pressure has reached 70-100MPa. The internal pressure resistance strength of commonly used N-80 casing is designed to be 64.6MPa, while the strength of J55 casing is only 21.93~28.4MPa. In this way, casing couplings and threaded parts, as well as well sections with poor cementing quality, are prone to rupture. In addition, when the oil well is acidified, casing corrosion will occur because the acid is not discharged in time. Some wells are acidified multiple times, which accelerates the corrosion rate of the casing, causing casing perforation and leakage.

(3) Oil well transfer and downhole tools

When the oil wells are put into production later, some oil wells will be converted into water injection wells. The original oil well cementing height is only about 200m above the oil layer, while the normal water injection well cementing height is required to reach the wellhead. Transfer wells are quite different from ordinary water injection wells. The cement return height is not high enough. The upper casing is not protected by a cement ring and is completely submerged in the shallow water above. On the one hand, the external corrosion of the pipeline is serious. On the other hand, after water injection, the upper casing is subject to water injection pressure, which promotes corrosion with the outside of the pipe and aggravates the damage of the casing. In addition, the collision damage of downhole tools to the inner wall of the casing will also aggravate the corrosion, especially when there is a packer downhole, the packer will cause extremely serious damage to the inner wall of the casing. Firstly, the strong stress expansion when the packer is seated will cause huge stress damage to the inner wall of the casing; secondly, due to the obstruction of the packer, dirt is easy to accumulate near the packer, causing corrosion under the scale, stress damage and scale Synergy. Under stress, the casing loss rate near the packer will increase significantly. When a packer fails and is unsealed, severe damage to the sealing location will be caused. A large amount of practice has proved that the damage to the casing when the packer is unsealed is extremely serious. Packers can cause extremely severe damage to the inner wall of the casing. Firstly, the strong stress expansion when the packer is seated will cause huge stress damage to the inner wall of the casing; secondly, due to the obstruction of the packer, dirt is easy to accumulate near the packer, causing corrosion under the scale, stress damage and scale Synergy. Under stress, the casing loss rate near the packer will increase significantly. When a packer fails and is unsealed, severe damage to the sealing location will be caused. A large amount of practice has proved that the damage to the casing when the packer is unsealed is extremely serious. Packers can cause extremely severe damage to the inner wall of the casing. Firstly, the strong stress expansion when the packer is seated will cause huge stress damage to the inner wall of the casing; secondly, due to the obstruction of the packer, dirt is easy to accumulate near the packer, causing corrosion under the scale, stress damage and scale Synergy. Under stress, the casing loss rate near the packer will increase significantly. When a packer fails and is unsealed, severe damage to the sealing location will be caused. A large amount of practice has proved that the damage to the casing when the packer is unsealed is extremely serious. The strong stress expansion when the packer is seated will cause huge stress damage to the inner wall of the casing. Secondly, due to the obstruction of the packer, dirt is easy to accumulate near the packer, causing corrosion under the scale. Stress damage and scale work together. . Under stress, the casing loss rate near the packer will increase significantly. When a packer fails and is unsealed, severe damage to the sealing location will be caused. A large amount of practice has proved that the damage to the casing when the packer is unsealed is extremely serious. The strong stress expansion when the packer is seated will cause huge stress damage to the inner wall of the casing. Secondly, due to the obstruction of the packer, dirt is easy to accumulate near the packer, causing corrosion under the scale. Stress damage and scale work together. . Under stress, the casing loss rate near the packer will increase significantly. When a packer fails and is unsealed, severe damage to the sealing location will be caused. A large amount of practice has proved that the damage to the casing when the packer is unsealed is extremely serious. When a packer fails and is unsealed, severe damage to the sealing location will be caused. A large amount of practice has proved that the damage to the casing when the packer is unsealed is extremely serious. When a packer fails and is unsealed, severe damage to the sealing location will be caused. A large amount of practice has proved that the damage to the casing when the packer is unsealed is extremely serious.

2. Internal factors

(1) Corrosion in pipelines

In-pipe corrosion mainly refers to the corrosion of the casing caused by the fluid entering the annulus of the oil casing. It mainly occurs in production wells because crude oil or natural gas contains some corrosive gases such as sulfur, carbon dioxide, chloride ions, and hydrogen sulfide. When these gases enter the oil and casing annulus and mix with water, they will cause varying degrees of corrosion to the casing. Injection wells will also contain some corrosive substances, mainly bacteria. However, compared with production wells, the incoming fluid of injection wells can be processed through the surface, minimizing corrosion of the pipe string.

(2) Corrosion outside the pipe

External corrosion mainly occurs when the cement sheath is damaged or there is no cement sheath in the upper part of the oil well. The casing is in direct contact with formation water. Corrosive gases such as carbon dioxide, chloride ions, and hydrogen sulfide in the formation water cause certain corrosion to the casing. When there are corrosion products or scale and any medium containing O2, H2S, Cl- and CO2, galvanic cell corrosion can occur under scale. Taking oxygen corrosion as an example, many oxygen atoms are easily adsorbed on the surface of corrosion products. The difference in oxygen concentration promotes cathode depolarization on the metal surface and accelerates corrosion of the metal surface.

In summary, the causes of oil and casing damage are divided into external factors and internal factors. In actual use, we must pay attention to avoid these problems to extend the service life of oil casing. If you have any other questions, please contact: sales@union-steels.com