In recent years, a well-logging survey of a particular block project revealed that 45 wells, accounting for approximately 28.5% of the total, exhibited varying degrees of casing damage. This issue has shown a significant upward trend. The primary forms of damage include casing bending, shrinkage, faulting, rupture, and corrosion perforation, with casing shrinkage deformation and faulting being the most prevalent. The overall trend indicates that wells with casing damage tend to have a short production lifespan, and the damage typically occurs at greater depths.
Casing Damage Trends
Vertical Distribution: Deep casing damage is predominantly caused by shrinkage deformation, while shallow casing damage is mainly due to faulting or rupture, with damage points often concentrated near the perforation sections.
Horizontal Regional Distribution: Damage is concentrated near faults, structural highs, and areas with steep wing inclination angles.
Causes of Casing Damage
1. Geological Factors
Geological factors are the primary contributors to casing damage, including tectonic stress, interlayer sliding, mudstone expansion, salt rock layer creep, oil layer sanding, ground subsidence, and oil layer compaction.
Mudstone Expansion and Creep, Salt Rock Creep: Rocks have inherent creep and stress relaxation properties, varying by type. Clay minerals in mudstone, such as montmorillonite, illite, and kaolinite, expand and creep when exposed to water, exerting shear stress on the casing, increasing the external load. Over time, if the casing's compressive strength is surpassed, it may become squeezed, flattened, or even break.
Surrounding Rock Pressure: Drilling disrupts the equilibrium in the surrounding rock, leading to stress concentration and plastic deformation. This deformation, restricted by the casing and cement sheath, causes casing deformation and damage due to the reaction force from the surrounding rock.
Fault Activity, Modern Crustal Movement, Earthquakes, and Landslides: Faults cause pressure imbalances and water flow between rock layers. The presence of mud-salt layers exacerbates instability, leading to casing deformation and damage. Fault activity and crustal movement can severely damage oil layer casings.
2. Engineering Factors
Human factors can also contribute to casing damage, which can occur during the design, production, and use of the casing.
Improper Casing Design: Casing damage can result from materials and strength not meeting construction requirements, poor casing combinations leading to local extrusion damage, or unreasonable wellbore designs causing resistance during casing lowering.
Manufacturing Issues: Improper production, such as the absence of a protective film, large gaps between connecting threads, tiny cracks during processing, or uneven casing wall thickness, can lead to poor sealing and fluid leakage during use.
Improper Use: Inadequate operation or management, friction, and collision of the casing, or the use of inappropriate downhole tools can cause squeeze, stutter, and scratch damage. Additionally, unreasonable control of formation fracture pressure during fracturing can exceed the casing's rated pressure, leading to damage.
3. Chemical Factors
Chemical reactions between substances in the formation and casing materials can cause corrosion, leading to casing damage.
Casing Corrosion: This can occur both during use and while the casing is idle. Corrosion is more pronounced at points where the casing surface is worn or damaged, especially if it contacts water in the formation.
Corrosion Agents: Key factors include hydrochloric acid reducing bacteria, CO2, H2S, and saline substances. Hydrochloric acid reducing bacteria in formation water can cause electrochemical reactions that accelerate corrosion. CO2 dissolves in formation water, forming HCO3- and CO3- ions, which react with metal ions in the casing, causing corrosion. Hydrogen sulfide can lead to hydrogen embrittlement, while dissolved salts chemically react with damaged areas on the casing surface, furthering corrosion.
4. Environmental Factors
Climate change, such as seasonal floods and heavy rains, can also impact casing integrity, leading to damage or rupture.
