The quality of
well pipe casing cementing is crucial to the success of exploratory wells and is key to oil and gas testing. For production wells, the quality of the casing directly affects their lifespan and economic benefits. In the production process of oil fields, once the well pipe casing is damaged, it can disrupt the injection-production well network, severely impacting normal production. To restore normal production, the damaged casing must be repaired to avoid shutdown issues. This article discusses the causes of well pipe casing damage, types of damage, and repair methods, and proposes preventive measures to enhance the economic benefits of oil field development.
Causes of Well Pipe Casing Damage
As a fundamental element of oil and gas wells, the quality and performance of the well pipe casing are crucial. Casings endure high pressure and various corrosive media over the years, and are subject to the external forces from workover operations and production enhancement measures. The main causes of casing damage include:
Poor Casing Quality and Low Strength: Inferior casings are prone to deformation or rupture during use.
Poor Cementing Quality: Inadequate sealing between the casing and the wellbore leads to gaps and leaks.
Corrosion Effects: Water, chemicals, and microorganisms accelerate the aging and damage of the casing.
Geological Movements: Changes in formations and lithology around the casing cause damage.
High-Pressure Water Injection: High-pressure water injection operations impact and damage the casing.
Sand Production: Sand production in oil, gas, and water wells causes wear and impact on the casing.
Improper Operations: Incorrect procedures during workover operations damage the casing.
Types of Well Pipe Casing Damage
The typical damage forms of well pipe casings include:
Casing Deformation: The internal diameter of the casing reduces, affecting normal production and workover operations.
Casing Cracking: Cracks or holes develop in the casing, leading to media leakage.
Casing Breakage: The casing breaks, disrupting the wellbore channel.
Repair Methods for Well Pipe Casing
Different repair methods are employed based on the location, extent, and condition of the casing damage:
Repairing Casing Deformation
The primary method to repair casing deformation involves using gradually expanding roller reamers. By applying pressure through the drill pipe, the internal diameter of the casing is gradually expanded. This method is simple and effective.
Repairing Casing Cracks
Cement Squeezing: For minor cracks and leaks with low reservoir pressure, cement squeezing can be used. The process is as follows: first, run a gauge 8-10mm smaller than the casing's internal diameter, then place a bridge plug at an appropriate position below the crack to temporarily seal the lower wellbore. Next, inject a certain amount of cement slurry above the bridge plug, allowing it to set into a cement plug. Once the cement plug has set, drill out the cement plug inside the casing and pressure test the seal quality. If the sealing is confirmed to be of good quality, drill out the bridge plug and wash down to the bottom of the well. This method generally allows the repaired casing to withstand pressures of 40-80 MPa.
Casing Replacement: If the crack is in the upper section of the well and the casing above the crack can be retrieved, the casing can be replaced by unscrewing the joint and pulling out the damaged section. A new casing is then run in and connected tightly. This method ensures uniform casing internal diameter, allowing downhole tools to pass smoothly. However, the connection tightness may not be as good as when tightened at the surface.
Patch Method: This method involves attaching a thin-walled pipe inside the damaged casing to restore functionality. The process is as follows: a corrugated pipe (thin-walled pipe) is placed over a specially designed high-pressure rubber sleeve and lowered to the damaged position. The rubber sleeve is then inflated, expanding the corrugated pipe against the casing wall, and an adhesive bonds the corrugated pipe to the casing. After the adhesive sets, the high-pressure rubber sleeve is retrieved. This patching method is simple and safe to operate.
Repairing Casing Breakage
Different methods are used based on the breakage condition:
Breakage Without Misalignment: Cement squeezing can be used to repair.
Slight Misalignment: If conditions allow, replace the casing; otherwise, use a bridging tool to clamp and hold the upper and lower casing sections together after milling off a segment at the break.
Severe Misalignment: For severe misalignment where the lower section cannot be found, sidetracking can be employed. This involves cementing the broken wellbore, drilling a new wellbore from the upper casing, and running a smaller casing to complete the well.
Preventive Measures for Casing Damage
To effectively prevent casing damage, the following measures should be taken:
Optimizing Casing Design: Strictly adhere to API 5CT standards when selecting and manufacturing casing. Before running the casing, design its compression and tensile strength according to the formation conditions. In special cases, use double-layer casings for added strength or technical casings in complex formations for sealing.
Improving Drilling Quality: Focus on wellbore quality, casing quality, and cementing quality. During drilling, strictly control the wellbore trajectory, thoroughly gauge the casing before running, and ensure proper makeup torque. Prohibit using defective casing and ensure cementing quality meets standards.
Controlling Operational Parameters: Reasonably control parameters during perforation, fracturing, and other operations to avoid excessive impact on the casing.
Implementing Corrosion Prevention: Adopt effective corrosion prevention measures to protect the casing from water, chemicals, and microorganisms.
In summary, well pipe casing plays a critical role in the production of oil and water wells. The factors causing casing damage are complex and varied, with physical geological factors being the most significant and unpredictable. Accurate analysis of casing damage causes and the selection of appropriate repair techniques can effectively resolve casing damage issues. By strengthening construction management and corrosion prevention, the risk of casing damage can be minimized, maximizing its lifespan and ensuring the sustainable development of oil field enterprises.