Prevention and treatment technology of casing accident

1. Ferrule
01 Handling of ferrule
There are two reasons for CASING seizure:
One is adhesive card;
The second is shaft wall collapse or sand bridge jamming.
After the casing gets stuck, full pressure can be applied but no more lifting is allowed. The annular clearance between casing and well wall is small, so it is impossible to perform casing milling and backoff.
1. Sticking card
When the drilling fluid can be circulated, the method of injecting the releasing fluid is the same as that of treating sticking.
2. Collapse or sand jam
(1) Sand bridge has been formed in the well, but some drilling fluid has returned. Therefore, small displacement and low pump pressure circulation shall be insisted to improve the viscosity and shear of drilling fluid, and the well shall be cemented after normal circulation is restored.
(2) The casing has been run to the bottom of the well, and leakage occurs in case of collapse or sand sticking. Analyze the leakage layer, cement the well rapidly, and squeeze the cement slurry into the leakage layer.
(3) If the casing is not run to the bottom of the well but is not far from the target layer, the well can be cemented first, then the cement plug and casing shoe can be drilled through, the well can be circulated to the bottom, and the oil and gas reservoir can be sealed by hanging the tail pipe.
02 Measures to prevent casing pipe from sticking
The measures to prevent casing pipe from sticking mainly include the following:
(1) Before running the casing, circulate and adjust the performance of the drilling fluid to ensure that there is no leakage or blowout. If necessary, mix crude oil or plastic balls, and run the casing only when the downhole is safe.
(2) Before running the casing, the wellhead shall be calibrated to make the crown block, rotary table and wellhead in a vertical line, so as to ensure that it is not easy to make wrong connections.
(3) When running the casing, the drilling fluid must be filled regularly and fully according to the technical requirements to prevent crushing the pressure valve or casing body. The automatic grouting equipment can be installed on the lower accessories of the casing, but it is necessary to judge whether the automatic grouting equipment is working according to the hanging weight of the casing and the amount of drilling fluid discharged from the well at any time.
(4) When manual grouting is used, the casing string shall be moved continuously, and the upper and lower moving range shall not be less than 2m. In case of any indication of downhole obstruction, grouting shall be stopped, the casing shall be moved for a long distance immediately, and grouting shall be carried out after the downhole condition is normal.
(5) If the wellhead casing is wrongly buckled for many times and the downhole casing is stationary for a long time, the downhole casing shall be moved first and then another casing shall be replaced.
(6) In case of lost circulation, well collapse and other phenomena during casing running, the casing shall be pulled out and run in for treatment. The casing shall not be run until the downhole conditions are normal. If the casing has been run to the design depth, whether to cement or pull out the casing shall be determined according to the depth of the leakage layer.
(7) For deep wells, the casing can be circulated in sections to break the structural force of the drilling fluid. Each time the pump is started, the pump should be started from small to large and gradually to the normal flow rate to prevent pressure excitation and leaking formation.
(8) The casing lowering speed shall be controlled, especially when passing through the known leakage layer. Each single casing shall be controlled within 1.5~2min. After running the casing, the drilling fluid must be filled first, and then the pump can be started for circulation to prevent air from mixing.
2. No circulation after casing running
01 Back pressure valve blocked
1. Treatment measures
Immediately perforate near the choke ring, resume circulation, and then cement the well. Cementing bump can be measured by displacement to retain cement plug, which will solidify after fracturing.
2. Preventive measures
(1) Prevent gloves, brushes and other small objects from falling into the well.
(2) When running the casing, special personnel shall check the casing one by one, and there shall be no objects in the casing.
02 No circulation due to well collapse or sand blockage
1. Treatment measures
After running the casing, it is found that the well is collapsed or sand blocked. After the pump is started, the pump pressure rises. The drilling fluid only enters but does not return. It is impossible to pull out the casing. Different remedial measures can only be taken according to the downhole situation.
(1) If the leakage layer is in the upper soft formation, the pump pressure is not too high, and there is a large absorption capacity, the cement can be directly injected. If the leakage layer is in the medium hard formation, there is also a certain amount of absorption, but the pump pressure is high, the cement slurry can also be squeezed, but the thickening time and initial setting time of the cement slurry should be appropriately extended.
(2) If the leakage layer is the production layer, squeezing cement will seriously damage the production layer. If the absorption capacity of the formation is very small and the conditions for cement squeezing are not met, the wellhead must be fixed. Perforate at an appropriate position above the production layer below the collapsed layer section, run the small drill pipe or TUBING packer into the casing, set the packer below the perforation position, and then inject cement slurry to seal the production layer after circulation is smooth.
2. Preventive measures
(1) Before running casing, the drilling fluid must be adjusted and treated during the well trip circulation. The drilling fluid must be thoroughly circulated to remove sand accumulation and consolidate the well wall. The return speed must meet the requirements for the return speed during cementing. If there are any problems in the well, it is not necessary to run casing for cementing.
(2) It is necessary to master the collapse law of the formation. The collapse of some formations has obvious periodicity, so casing cementing should be carried out in the stable period of the formation.
(3) Before tripping out, pay special attention to filling drilling fluid, which shall be continuously filled.
(4) The time from tripping out to the start of casing running shall be as short as possible. It is not allowed to run casing directly without swabbing after electric logging and sidewall coring.
(5) In some wells, there are many centralizers and mud scrapers connected to the casing string, and a lot of filter cakes are accumulated during casing running. If these filter cakes are located in the well section with small diameter, forming a blockage, the circulation is blocked. Therefore, the number of centralizers should be reasonably designed, and mud scrapers should be used with caution.
(6) When running the casing, the drilling fluid in the pipe must be filled regularly according to the technical requirements to prevent well collapse caused by crushing the pressure valve.
03 No circulation due to lost circulation
1. Treatment measures
In this case, the following measures should be taken instead of hastily cementing:
(1) If it is known that the pressure of the oil and gas reservoir is not high, the leakage layer is above the oil and gas reservoir, and reliable well control equipment is available, which can be used for cementing. After cement injection and pressure bump, close the BOP and pump drilling fluid from the annulus to maintain the annulus pressure.
(2) If the location of the leakage layer is not known, and the pressure of the oil and gas reservoir is high, there is a danger of downhole blowout, or the leakage layer is an oil and gas reservoir, it should be plugged before cementing. The plugging slurry can be replaced into the annulus and squeezed in sections. After the downhole circulation is restored, the well will be shut in and squeezed in a part of the plugging slurry. After a period of inactivity, a part of the plugging slurry will be squeezed in. When the pressure bearing capacity of the formation meets the requirements of cementing, the drilling fluid will be recycled before cementing.
2. Preventive measures
(1) During pigging circulation, the flow rate required by the design return speed during cementing shall be used for circulation. If the circulation is abnormal, the drilling fluid has poor performance, and the casing has to be run.
(2) The casing running speed shall be strictly controlled to avoid excessive exciting pressure and formation leakage.
(3) During casing running, keep the annular drilling fluid flowing. After running to the designed deep well, first use small displacement to jack up. After the structural strength of downhole drilling fluid is completely destroyed, gradually return to normal displacement circulation. The maximum circulating flow rate shall not be greater than the circulating flow rate during well trip.
(4) Use fewer centralizers and mud scrapers. Because the filter cake has a certain effect on stabilizing the well wall and preventing leakage.
(5) There is no leakage during pigging circulation. If leakage occurs after running casing, the leakage layer is usually the formation that has been lost during drilling. To be prudent, the formation that has been lost can be plugged once before running casing.
3. Collapsed casing or back pressure valve
During casing running, the main reason for casing or back pressure valve collapse is insufficient drilling fluid filling.
01 Treatment measures
(1) During various operations before and after cementing, the safety of casing must be considered. No matter what causes casing collapse, it is difficult to remedy it.
(2) If only the back pressure valve is destroyed, the well can be cemented by manually measuring the replacement amount of drilling fluid.
02 Preventive measures
(1) Fill drilling fluid regularly according to design requirements.
(2) For cementing in the formation with salt rock creep, the collapse strength of casing shall be designed according to the creep stress of the creep layer.
(3) Cement must be filled evenly around the casing in the creep layer, and channeling is not allowed.
(4) The fishing depth during well testing shall be strictly limited to the allowable range of casing collapse strength and shall not be exceeded.
(5) When it is necessary to run the packer to squeeze cement, the packer shall be at least 35m away from the perforated well section.
4. Casing fracture
01 Treatment measures
(1) If the upper casing slips from the coupling and the coupling thread is still intact, a new casing butt joint can be run in.
(2) If the surface casing or technical casing is broken from the lower part, the tapered guide shoe can be lowered to centralize and be fixed with cement.
(3) If the lower broken casing is very short, or only one casing shoe can not be straightened with the tapered guide shoe, the lower grinding shoe shall be milled.
(4) If the surface casing or technical casing is disconnected from the middle and the fracture is misaligned, the bit shall be lowered to a smaller level for drilling. If the bit cannot be lowered, the milling cone shall be lowered to trim the lower fracture until the upper and lower passage is unimpeded, and then a layer of casing shall be lowered to separate the fracture. If the milling cone cannot be run in, sidetrack.
02 Preventive measures
(1) Sulfur proof casing and wellhead devices shall be used for sour oil and gas wells. The production layer containing hydrogen sulfide must be stabilized, the drilling fluid shall be fully recycled and treated, and the casing can be run only after the hydrogen sulfide mixed in the drilling fluid is removed.
(2) When connecting the casing, it is not allowed to make a mistake. After making a mistake, it must be re installed, and it is not allowed to use electric welding after making a mistake.
(3) In case of a snap, the casing can be fully pressed, but not lifted more. The lifting tension shall not exceed 80% of the tensile strength of the weakest casing in the casing string or the anti slip strength of the thread.
(4) The surface layer or the lower part of the technical casing shall be connected with 3~6 pieces of anti loose thread grease, and the threads must be cleaned without oil stain.
(5) The surface layer or technical casing shoe shall be located in the stratum that is not easy to collapse.
(6) The surface casing cement shall be returned to the ground. The cement return depth of the technical casing depends on the situation, and it is better to return to the ground or within the upper casing.
(7) It is better to use double rubber plugs for cementing, or leave more cement plugs in the pipe. For large-diameter casing, the internal pipe can be used for cementing, and the sealing quality of the casing tail shall be ensured.
(8) When drilling with rotary table in a well that has been cased, the rotary table speed shall be limited. Before the drill collar is out of the casing, the speed shall not exceed 60r/min, and after the drill collar is out of the casing, the speed shall not exceed 150r/min. For wells with a long construction period, protective measures shall be taken for technical casing, such as adding rubber hoops or anti-wear joints on drill pipes.
(9) The waiting time of cement shall be appropriately extended, and the cement plug shall be drilled when the cement stone has sufficient strength. When drilling the cement plug, the drilling tool shall not be equipped with a stabilizer, and the pressurization shall be uniform.
(10) When unloading the cement head and the landing joint, the casing at the bottom of the well must be fixed, and the casing shall not be reversed.
5. Casing leakage
01 Remedies
Find out the leakage position and plug the sleeve leakage with superfine cement. The superfine cement is finely ground cement with an average particle size of 6 μ m. The maximum particle size is 15 μ m. It is 1/5-1/7 of the standard cement particle size. The superfine cement used for squeeze injection is composed of 20%~30% finely ground cement and 70%~80% hydraulic slag.
02 Preventive measures
(1) Hydraulic pressure test and flaw detection inspection shall be carried out for all casing running in the well one by one.
(2) Sealing grease or adhesive for thread grease.
(3) Tighten according to the specified torque.
(4) Gas tight casing shall be used for gas well.
(5) The casing cement slurry of each layer of the gas well shall be returned to the ground.
(6) During shut in pressure test or well control operation, the shut in pressure shall not exceed 80% of the internal pressure resistance strength of the weakest casing.
(7) If rotary drilling is used in technical casing, anti-wear measures shall be taken for casing.