What are the hazards of casing cracks? With the deepening of exploration and development, casing damage wells have increased year by year, and casing damage has become more and more serious, which has seriously affected the stable production in this area and needs to be repaired as soon as possible. What are the hazards of the following shared casing cracks?
What are the hazards of casing cracks? Cracks in 1 casing will reduce the insulation strength and further damage the insulation until it is completely broken down. When the water in the fracture freezes, it may also expand and break the casing. It can be seen that the casing crack is a great threat to the safe operation of transformer.
Common defects in waterproof casing welding zone
1, bubble
Bubbles often appear in the center of the weld, while hydrogen is still hidden in the weld metal in the form of bubbles. The main reason is that there is moisture on the surface of welding wire and flux, so they can be used directly without drying treatment. In addition, low current and high welding speed will also accelerate the solidification of metal.
Step 2 bite the edge
The weld edge has a groove along the weld centerline, mainly due to improper welding speed, current, voltage and other conditions, and the edge undercut defect is caused by too high welding speed.
3. Hot cracking
The cause of hot cracking is high welding stress or high silicon content in welding metal. The other is sulfur cracking. The blank is a strong sulfur segregation zone (soft boiling steel). In the process of welding, sulfide enters the weld metal and produces cracks.
4. Insufficient penetration
Internal and external welding metal lap is not enough, and sometimes it is not fully welded.
Therefore, welding is an important factor to determine the service life of flexible waterproof casing. Good welding technology makes the flexible waterproof shell more durable.
What are the hazards of casing cracks? Cause analysis of surface cracks in rigid waterproof casing.
Rigid waterproof casing seals the casing and pipeline with rigid materials to achieve the sealing effect. The shorter the light wave, the greater the energy. Multiply the construction efficiency and save labor; The construction surface in waterproof casing is smooth and flat, and the products are closely combined with the ground. Waterproof casing is directly bonded to the pipeline, which improves the construction quality standard and enhances the image of the enterprise in construction quality.
After construction, the walls and pipes are clean and tidy, and there is no need to put in artificial hole filling in the later period; Various specifications and sizes are in stock, which is convenient for construction and ready to use. Technical Support waterproof casing's application field will not be damaged. In order to prevent the load from being crushed, waterproof casing such as large-diameter concrete large-diameter steel pipe is installed outside to avoid external force (the load directly acts on the external wall to cause damage).
Dirty surface of rigid waterproof casing will reduce flashover voltage (i.e. low voltage at which flashover occurs). If the dirty surface is wet, the flashover voltage will drop even lower. At this time, if a certain amount of overvoltage invades the line, it will cause flashover. Flashover has the following hazards:
(1) causes power grid grounding fault, causing protection action and circuit breaker tripping;
(2) The damage to the surface of the bushing will become a factor that may lead to insulation breakdown in the future.
After the dirt on the surface of the bushing absorbs moisture, the conductivity increases, the leakage current increases, and the insulating bushing is heated, which may lead to cracks in the bushing and lead to breakdown. Cracks in the rigid waterproof casing will reduce the electrical strength. Because the gap is filled with air, the dielectric coefficient of the air is small, and the dielectric coefficient of the porcelain part of the porcelain bushing is large.
The distribution law of electric field intensity is that the electric field intensity with small dielectric coefficient is large, and the electric field intensity with large dielectric coefficient is small. When the electric field intensity in the crack reaches a certain value, it will release air, cause partial discharge, and further destroy the insulation until it is completely broken down. When water enters the fracture and freezes, it may also crack the casing.
What is the harm of casing crack? 3. Cause analysis and repair technology of casing damage
1, casing damage reason analysis
There are many reasons for casing damage, which can be generally divided into four categories: geological factors, engineering factors, corrosion factors and wellbore factors. Among them, sand production is the main cause of comprehensive oilfield, and corrosion is the main cause of fault block reservoir. Thermal recovery and gas injection, poor cementing quality, long casing life, many operations and complicated reservoir reconstruction measures are also the main reasons for casing damage.
In the process of multi-round steam huff and puff, the casing is subjected to alternating thermal stress load caused by high temperature and high pressure. During steam injection, high temperature causes thermal expansion and elongation of casing, resulting in local plastic deformation; When the injection is stopped, the shrinkage of the casing leads to the structural damage of the plastic deformation site. It can be seen that high temperature steam injection caused casing damage.
1. 1 Thermal stress is the main influencing factor of casing damage in thermal recovery wells.
High temperature steam injection makes the thermal stress of casing in the whole well reach 375MPa, which exceeds the allowable stress of N80 casing (352MPa). In some areas, the casing temperature reached above 270℃, and the thermal stress reached 545 MPa ~ 800 MPa, which was close to or exceeded the yield limit of N80 casing (552 ~ 655 MPa), and the yield limit of P 1 10 casing was 758 ~ 965 MPa, causing casing damage.
1.2 geostress change is the main reason for casing damage in thermal recovery wells.
Steam injection and sand production cause serious hole or even collapse of the formation, which leads to the change of in-situ stress field around the wellbore, which leads to the casing stress in the oil layer section increasing to 45 1MPa, and the casing stress in the cavity area can reach more than 530MPa, which makes the casing expand and contract in some areas near the oil layer, resulting in serious bending and dislocation.
1.3 local damage to casing wall caused by thermal recovery packer slip
Slip has obvious damage to casing wall. The depth of slip teeth embedded in casing wall is more than 65438±0mm, and the plastic deformation of casing is about 5 mm.
1.4 local damage of steam injection to casing in perforation section
During steam injection, the tail of steam injection string causes strong erosion and wear damage to oil layer casing.
2, casing damage repair technology
2. 1 casing inspection technology analysis
At present, the main methods of casing detection are packer leak detection and lead printing, which are mainly economical. Fast logging technologies such as neutron logging and multi-arm caliper logging have developed rapidly, but the application of downhole TV and electromagnetic flaw detection is limited.
2.2 characteristic casing repair technology
2.2. 1 Hydraulic variable diameter forming technology
To realize the rapid treatment of mild casing replacement wells, but its application scope needs to be clear. Using variable diameter rolling technology, the shaper is arranged by hundreds of variable diameter steel balls according to a certain rule, and the inner wall of the casing is rolled, so that the diameter of the expanding head is automatically increased to a predetermined size, and the casing is completely restored to its original diameter. Not only will it not damage the shell, but it will also strengthen it.
Characteristics of hydraulic shaping process: 1) Shaping diameter: single shaping diameter (6- 10mm), and the inner diameter of 5- 1/2 "casing is restored to14 ~1/kloc-0. 7 "set back to 146 ~ 158 mm; 2) It can handle multi-point nesting with any length, especially suitable for nesting, with high safety and high yield. 3) The construction is simple, and the requirements for workover equipment and well site are low, so minor repairs can be implemented. 4) Rolling forming can improve the repair strength.
The casings used in some oil wells are thick-walled N80, TP and BE high-strength casings with small inner diameter, and the hydraulic shaper currently used has a large diameter, which cannot be anchored and cannot be used for shaping construction. The construction of high-strength casing is difficult and the success rate is low. Multi-stage alloy steel shaper can be used, and the shaping effect is good.
2.2.2 Casing damaged pipe string releasing and fishing technology
The fishing technology of hydraulic force-boosting and sticking-releasing is mainly developed for oil and water wells because of the limited lifting load and the difficulty of normal sticking-releasing of minor repair machines.
In this technology, the stuck pipe string is released by pressing the cement truck on the ground, and the sticking releasing tool generates huge pulling force underground, which does not depend on the pulling load of the workover rig. It has the characteristics of large sticking force, fast workover speed and simple construction technology, and can be implemented in coordination with minor repairs and major repairs.
This technology has changed the stress mode of fishing pipe string, and the maximum pulling force provided by conventional sticking release and minor repair equipment has increased from 30 tons to 85 tons. The anchor shoe adopts a ring structure, which has a large contact area with the casing and will not damage the casing; The lifting force of the hydraulic release can be controlled by the ground pressure, which can directly lift the downhole sticking point.
At present, the success rate of hydraulic sticking and heat insulation pipe sticking is high, but the success rate is low because the hydraulic sticking tool is far from the sticking point due to the back-off operation of the sticking point on site. Can cooperate with fixed-point cutting tools to cut near the sticking point before hydraulic release. The sticking release of horizontal wells is restricted by wellbore structure, and the fishing efficiency and success rate are low, so there is no technical breakthrough in major oil fields.
2.2.3 Expansion pipe technology realizes casing repair, plugging and sand control.
By using the plastic deformation of metal materials, the reinforced pipe is expanded by expanding the head and applying external force, and it is tightly attached to the inner wall of the casing to achieve anchoring and sealing. The problems of short sealing section, small anchoring force and small diameter after reinforcement in conventional reinforcement technology are solved.
Technical features of expansion casing subsidy: expansion pipe technology is used to repair casing damaged wells, with water plugging and layer plugging: 1) The sealing bearing effect is good, and the diameter of the plugged interval is large; 2) It does not affect the running depth of the pump; 3) simple construction technology, high reliability and long validity period; 4) It can be re-punched to realize full or partial opening.
3. Conclusion and understanding
Preventing casing damage wells is a comprehensive project, involving drilling construction, completion technology, operation technology, gas injection technology, oil production management and so on. No matter which process is not well controlled or the matching process is not perfect, it will affect the service life of casing.
Therefore, from the beginning of drilling, it is necessary to take all-round and whole-process casing damage prevention measures to protect the casing to the maximum extent. Hydraulic variable diameter shaping technology can replace the mechanical shaping of overhaul equipment and realize casing shaping. Hydraulic power-assisted technology can improve the success rate of minor repairs and reduce the workload of major repairs. Expansion pipe subsidy technology can realize low-cost management of casing damaged wells and maintain the integrity of injection-production well pattern.