Causes and preventive measures for OCTG casing damage

Causes and preventive measures for OCTG casing damage:

Casing damage status:

In recent years, a total of 45 wells were found to have varying degrees of casing damage in well logging in a certain block, accounting for about 28.5% of the total number of wells, and the number of casing damage wells has shown a sharp increase. The main forms of OCTG casing damage are casing bending, shrinkage, staggered fracture, rupture, casing corrosion and perforation, among which casing shrinkage deformation and staggered fracture are the main ones. The overall trend of casing damage is that the casing damage well has a short pumping life and a large depth of casing damage points. Vertical casing damage mode: deep casing damage is mostly shrinkage deformation, shallow casing damage is mostly staggered fracture or rupture, and casing change points are concentrated near the perforation section. Horizontal regional distribution mode: casing damage well points are concentrated near faults, structural highs and areas with large wing inclination angles.

Causes of oil well casing damage:

1. Geological factors of oil well casing damage

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

a. Mudstone expansion creep, salt rock creep

Rocks have the characteristics of creep and stress relaxation, and different types of rocks have different creep types and corresponding creep degrees. Rocks creep even under natural geological conditions. Clay minerals in mudstone, especially montmorillonite, illite and kaolinite, swell and creep when they come into contact with water. Because the casing prevents this creep and expansion, shear stress is generated on the casing. This increases the external load on the casing. Over time, 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.

b. Surrounding rock pressure

After drilling, free surfaces appear in the rock formations around the wellbore, destroying the original equilibrium state. When the stress at the stress concentration point reaches the yield limit of the surrounding rock, plastic deformation will occur. This deformation is restricted by the casing and the cement shell outside the casing. At the same time, the casing is also reacted by the surrounding rock, causing casing deformation and damage.

c. Fault activity, modern crustal movement, earthquakes, and landslides

The existence of faults causes pressure imbalance between faults, allowing water to form channels between rock formations. When mud layers, gypsum salt layers, and faults coexist, the stress on the casing will be unstable, causing casing deformation and damage. Fault activity is caused by a variety of factors. The formation moves horizontally along the fault, causing large-scale damage to the oil layer casing.

2. Engineering factors of oil well casing damage

a. Downhole reasons

The main reasons include insufficient casing design strength, poor thread sealing during construction, and poor cementing quality, which lead to casing wear during drilling. When the casing is lowered into the well, the outer surface of the casing will inevitably rub violently with the hard rock formation. If it is forced into the well, the casing will suffer obvious wear and tear, and the casing string is prone to instability, bending, and deformation. In addition, after the casing is worn, the inner surface area increases, the contact surface with the corrosive medium increases, the passivation layer on the inner wall of the casing disappears, and the inner metal is in direct contact with the corrosive medium, which accelerates the corrosion rate of the casing.

b. Production reasons

High-pressure water injection will cause the volume of the formation to expand. When the pressure spreads outward from the water injection well and the permeable rock formation, a large pressure will be generated at the interface between the oil layer and the boundary layer. If this force exceeds the interface strength, slip will occur, causing casing damage to adjacent wells.
The permanent volume change caused by solid filling during fracturing will also cause the formation to slide along the bedding.
Sand production in the oil layer will cause the combined effect of the overlying rock mass sinking and the underlying rock mass overlying, shortening the reservoir section, affecting the lateral support of the casing, and causing the casing to deflect.

3. Corrosion factors that cause oil well casing damage

Oil well casing corrosion refers to the reaction of sulfur, CO2, HS contained in crude oil and natural gas, and various corrosive substances contained in formation water and injection water with iron or ferrous ions in the casing, corroding the pipe body. The conditions for casing corrosion include a certain temperature, pressure, ferrous ion concentration and reducing bacteria in the formation water, most of which are related to the action of sulfate-reducing bacteria. Casing corrosion mostly occurs in casing solutions with high mineralization and low pH. The impact of corrosion on casing damage is mainly manifested in the following two forms of casing damage:

(1) Corrosion causes upper casing leakage. From the perspective of mechanics, casing damage caused by external forces will not manifest as casing leakage, but the final result of corrosion will cause casing leakage.

(2) Corrosion weakens the casing wall thickness, reduces the casing strength, and causes casing damage. Under the action of corrosion, the casing wall gradually becomes thinner overall or partially, the casing strength is reduced, and it is more likely to be damaged under the action of external forces.

Casing damage inspection method:

During normal logging, we can use electromagnetic flaw detection logging, isotope full-well leak detection, caliper logging and other logging methods to inspect the downhole tubing to check whether the casing is damaged or deformed.

(1) Electromagnetic flaw detection logging can measure the wall thickness change and damage of the casing in the tubing under normal production conditions of oil and water wells, saving the cost of checking the casing condition and running the tubing. This feature makes it possible to conduct a survey of structural damage in oil and water wells. The tubing has a transition between thick and thin walls, and the casing is deformed or scaled.

(2) Isotope five-parameter combined logging can record five curves at the same time. In this method, the isotope tracer curve, the flow rate and well temperature data in the tubing, and the pressure anomaly points can be mutually verified to detect casing leakage.

(3) Multi-arm caliper logging is the most widely used method in the casing inspection logging process. This instrument is a contact measuring instrument, that is, the measuring arm of the instrument contacts the inner wall of the casing, converting the change of the inner wall of the casing into the radial displacement of the caliper measuring arm, and then into the vertical displacement of the push rod through the mechanical design and transmission inside the caliper instrument; the differential displacement sensor converts the vertical displacement change of the push rod into an electrical signal, thereby indicating the casing leakage.

Preventive measures for oil well casing damage:

1. Reasonable control of water injection pressure

During the high-pressure water injection process of the oil field, as the water injection pressure continues to increase, the formation pressure level also continues to increase. When the formation pressure exceeds the critical pressure, relative sliding will occur between the formations, causing casing deformation, especially in areas with mudstone interlayers or faults, casing damage may occur in pieces. In addition, long-term high-pressure water injection will increase the casing damage rate under the action of long-term high-pressure load. Controlling the formation pressure within the critical formation pressure or within 70% of the casing collapse strength can slow down the occurrence of casing damage.

2. Reasonably control the production pressure difference and maintain the balance of injection and production

Due to the imbalance of injection and production, the well network is strengthened in a large area in the later stage, and the interlayer connectivity is poor, forming additional stress from the high-pressure area to the low-pressure area, inducing formation sliding, causing casing deformation, especially in the high-pressure area. Casing damage is particularly serious.

3. Optimize perforation and reservoir transformation plan

Any acid fracturing and perforation process and quality have an important impact on the life of the casing. Repeated perforation or acid fracturing will greatly reduce the extrusion strength of the casing and accelerate the damage and deformation of the casing. Perform fracturing and acid fracturing on the reservoir and determine the construction pressure. The construction pressure should not exceed 80% of the casing anti-extrusion strength.

4. Improve downhole tools to avoid casing grinding

Due to the design or construction reasons of the plan, objects often fall off, pipe jams, pipe blockages, etc., which need to be salvaged and ground, which will inevitably cause wear of the casing wall and reduce the strength of the casing. It is recommended to improve the drilling tools and install a straightening device during the drilling and grinding construction process to reduce eccentric wear and protect the casing.

Read more: Casing Pipe VS Drill Pipe or Anti-corrosion Measures for Well Casing Pipe