Cementing Complex Problems

Cementing operation is not only related to the successful completion of oil and gas wells, but also affects the quality of oil and gas wells, the life span of oil and gas wells and the production of oil and gas wells after putting into production. Moreover, its cost also accounts for a large density (20% - 30%) in the whole drilling engineering. The goal of cementing technology development has been around how to further improve cementing quality and reduce cementing accidents.

Cementing is also a system engineering, with complex and diverse influencing factors and its particularity, which is mainly manifested in the following aspects:

(1) Cementing operation is a one-time project. If the quality is unqualified, even if the remedial methods such as squeezing cement are used, it is difficult to achieve good results.

(2) Cementing operation is a systematic and concealed operation, involving materials, fluids, chemistry, machinery, mechanics and other disciplines. During construction, there are many unknown factors and great risks.

(3) Cementing operation is characterized by short construction time, large workload, strong technicality and high cost.

Therefore, it is required that the cementing operation should be carefully designed, prepared and constructed, and there should be a complete pretreatment scheme to prevent the complex situation of cementing, so as to ensure the completion of cementing operation with high quality and efficiency. Cementing operation involves casing, slurry performance design, cement injection site construction, cement cementation quality and other aspects. Therefore, cementing complex problems and accidents can also be divided into the following categories.

The first type: casing and running casing complex situation, including running casing jam, casing fracture, casing leakage, casing collapse, failure of casing accessories and tools, leakage or circulation failure after running casing, etc.

The second category: the performance accidents of cement slurry, including cement slurry flash setting, cement slurry thixotropy, cement slurry excessive retarding, etc. 

The third type: the complex situation of cement injection site construction, including the leakage of cement injection, the blockage of annulus, the displacement of cement injection and other complex situations and accidents.

The fourth type: the complex situation of cement bond quality, including oil-gas and water leakage, poor cement bond quality, air (water) channeling, etc.

In the following, the main causes, prevention and treatment methods of the above-mentioned cementing complications and accidents are discussed respectively.

1. Complex situation of casing running           

1.1casing stuck

Casing sticking can be generally divided into the following three categories: first, casing sticking, second, wellbore shrinkage, third, wellbore collapse or sand bridge.            

1) Causes and influencing factors of tube jamming           

1. Casing sticking is due to the fact that the outer diameter of casing is often larger than the outer diameter of drill pipe, the contact area between casing and well wall is larger than the contact area of drill pipe, the make-up time is larger than the make-up time of drill pipe, and it is difficult to rotate when running casing, so the probability of casing sticking is relatively high.           

2. Wellbore shrinkage and casing sticking is caused by wellbore instability, especially when drilling in peristaltic rock salt layer or thick fake mud cake due to poor drilling performance, which results in wellbore shrinkage and casing sticking accident.           

3. Wellbore collapse or sand bridge casing sticking is a casing sticking accident caused by wellbore collapse or sand bridge during or after casing running.           

4. Before running the casing, the well was not drilled carefully, and the reducing section was not well reamed, which is easy to cause casing sticking accident.           

5. The casing operation is not carefully prepared (including organization, tools, etc.), resulting in too long casing running time or intermediate pause, which is prone to casing sticking accident.

6. It is easy to get stuck when running casing directly without passing the well after midway test, coring and electrical test.           

7. The performance of drilling fluid is not good, no good filter cake is formed, and the wellbore friction coefficient is large, especially for high-density and dispersed drilling fluid, which has a high probability of casing sticking.           

8. The leakage layer is not well plugged before casing running. In addition, the speed of casing running is too fast, which is easy to kill the lost formation, resulting in casing sticking accident caused by well collapse.          

 9. There is no stable pressure before running the casing in the high pressure layer, overflow occurs in the process of running the casing, and the pressure of the night column in the annulus drops, which is prone to well collapse, Cause casing sticking accident.           

10. If the wellhead is not, the casing is repeatedly and wrongly buckled when running the casing. If the downhole casing is stationary for a long time and there is no moving casing when running the casing, it is prone to casing sticking.

11. The density design of drilling fluid is unreasonable, for example, the low density design causes borehole collapse or no pressure stable peristaltic formation causes borehole shrinkage and casing sticking accident.          

12. When running the casing, it is blocked and pressed blindly, resulting in casing stuck instead of being blocked.

2) Technical measures to prevent casing jamming           

1. Before running the casing, the well shall be shut in carefully and the reduced section shall be reamed repeatedly.          

2. Design a reasonable drilling fluid density to ensure the formation stability, prevent borehole collapse, and reduce the creep speed and borehole shrinkage of the creep formation.    3. The casing can only be run after the well has been drilled carefully after the midway test, coring and electrical test.

4. Before running the casing, carefully handle the drilling fluid performance, reduce the viscosity, shear force and water loss of the drilling fluid, and fully recycle the drilling fluid before running the casing.           

5. For deep well, long open hole well, directional well and horizontal well, it is necessary to add plastic ball or mix 5% - 10% crude oil before running casing, so as to reduce wellbore friction coefficient.          

6. The casing operation shall be carefully prepared (including personnel organization, tools, etc.), which can only reduce the casing running time and intermediate stop.          

7. Before running the casing, it is required to plug the lost formation well, and control the running speed of the casing to prevent the formation from leaking.           

8. Before running casing in high pressure formation, it is required to keep stable pressure to prevent overflow during running casing and maintain well pressure balance.          

9. In case of well loss, well collapse and other complex situations during casing running, it is generally required to pull out the casing, download and handle the well bore, and then run the casing again after it is normal.           

10. If the casing is blocked during running, the casing shall be moved repeatedly, and Kelly or circulating head shall be connected to circulate the drilling fluid. Blind running down is not allowed to prevent the casing from being stuck.           

11. Before running the casing, the wellhead shall be calibrated, and the crown block, rotary table and the wellhead shall be aligned at three points to prevent the wrong connection when running the casing.           

12. If necessary, use the casing stabbing board and use manual or mechanical stabbing to prevent the casing from being screwed up by mistake and speed up the casing running.        13. Use automatic grouting equipment as much as possible to reduce the standstill time of running casing caused by grouting. Pay attention to its working condition in time when using automatic grouting equipment, and use manual grouting in case of failure.

14. After running the casing, it is required to fill the drilling fluid first, and then slowly start the pump for circulation. After the circulation is smooth, slowly increase the circulation displacement to prevent the difficulty of starting the pump and the formation of pressure leakage caused by air mixing.          

15. When manual grouting is used, the casing shall be moved continuously in the grouting gap, and the distance between the upper and lower moving casings shall not be less than 2 meters. If there is any sign of resistance in the well, grouting shall be stopped, and measures such as large distance moving casing or Kelly connection circulation shall be taken, and grouting and casing running shall be conducted after normal operation.         

16. Pay attention to mud return at the wellhead in time during casing running. If any abnormality is found, immediately stop casing running for treatment, and continue casing running until it is normal.         

3) Treatment of casing jamming

After the casing is stuck, the treatment work shall be carried out on the premise of ensuring that the casing string is not damaged. Moreover, different treatment methods shall be adopted according to different types of stuck casing, which is more difficult and less means than that of stuck drilling.         

 (1) Casing sticking

After casing sticking, the following steps are recommended:

1. Strong movable casing: in case of casing sticking, it is generally connected with Kelly or circulation head to start the pump for circulation, and then it is possible to move up and down the movable casing as far as possible under the safe condition of casing and equipment (derrick and lifting system). This method can generally eliminate casing sticking.            If the card still cannot be released after the number of strong activities (usually about 10 times), the strong activities shall be stopped generally. After that, move the non stuck casing within a certain range to prevent the stuck point from moving up.           

2. The main way to deal with the casing sticking is to deal with the casing sticking through the method of foaming and releasing agent after the strong movable casing is invalid. The basic steps are as follows:

The first is to choose the right releasing agent. Generally speaking, the releasing agent can be divided into water-based and oil-based. Its density should be selected according to the formation pressure in the well. For high-pressure wells, the releasing agent with high density should be selected. General oil-based card remover is suitable for most areas, but in some areas, water-based card remover has also achieved good application results.

Second, calculate the location of the card point. The location of the sticking point is generally calculated by calculating the casing elongation under certain tension conditions on site. 

Third: calculate the amount of unbucker. According to the calculated position of the sticking point, the appropriate releasing agent is injected into the position of the sticking point and the following parts. A certain additional amount is required, generally around 20%.

Fourth: Calculation of well pressure balance. According to the formation pressure, drilling fluid density, formation lithology, density and dosage of the releasing agent in the well, the well pressure balance calculation is carried out to ensure that no kick, blowout and well collapse accidents will occur.

Fifth: release the card. According to different types of releasing agent, formation characteristics and actual situation of casing stuck in the field, a similar strong active casing treatment method is adopted after the releasing agent is injected for a certain time. 

(2) Casing reducing card and borehole collapse or sand bridge card

1. When the casing is stuck, the drilling fluid can be circulated in the well, and it can be treated by the treatment method similar to casing sticking.

2. When the wellbore collapses or sand bridge is stuck, if the drilling fluid can be circulated and the wellhead can still return the slurry, the low-pressure circulation drilling fluid in small batch shall be adhered to first, then the density and shear force of the drilling fluid shall be gradually increased, and the well shall be cemented after it is normal.

3. If the casing has run to the bottom of the well and the circulating drilling fluid is lost, it shall be treated according to the actual situation on site. In most cases, the method of small batch well cementation is selected to seal the lower formation, and if necessary, the upper formation is subject to cement squeezing operation.

4. If the casing does not go down to the bottom of the well, it can be well cemented first, and then the method of adding a layer of liner for well cementation and sealing the lower formation can be used for remedy.

1.2 Casing Fracture

1) Causes and influencing factors of casing fracture

1. In casing design, the safety factor design is low, and the influence of temperature change, casing bending and other factors on casing strength is not considered, resulting in insufficient casing strength and casing fracture.

2. The casing itself has quality problems, especially the screw thread processing quality is not up to standard, causing the screw thread to fall off.

3. The casing falls off when drilling cement plug because the joint of casing screw thread is not fixed above the casing float collar.

4. In case of drilling in hydrogen sulfide gas reservoir, hydrogen embrittlement will occur due to hydrogen sulfide in drilling fluid, which will cause casing fracture.

5. When drilling in the technical casing, effective protective measures are not taken, and the drill pipe joint wears the casing through, resulting in casing fracture.

6. Formation water contains corrosive substances, such as poor sealing quality of cement sheath, which is easy to cause casing corrosion and fracture.

7. When the casing is stuck, the applied tension is too large, causing the casing to fall off.

8. During the fracturing and cement injection construction, the construction pressure is too high, which exceeds the compressive strength of the casing, causing the casing fracture.      9. In the thermal production well, the casing is expanded by heat, but there is cement consolidation outside the casing, which limits the free extension of the casing and generates pressure stress inside the casing. When the pressure stress exceeds the yield limit of the material, the casing will break.

2) Technical measures to prevent casing fracture

1. When running the casing, it is not allowed to weld the casing in the wrong way.

2. After the casing is blocked, it shall not be forced to lift, and the lifting force shall not be greater than 80% of the tensile strength of casing body and screw thread.

3. The surface casing and the cement plug casing at the lower part of the technical casing shall be protected from thread loosening grease or riveted at the loose part. To prevent casing falling off during drilling of cement plug or lower part.

4. For wells containing hydrogen sulfide, the drilling fluid must be fully circulated before running the casing to stabilize the production layer and remove the sulfur from the drilling fluid hydrogen evolution. At the same time, sulfur visiting casing and wellhead equipment shall be used.

5. The cementing quality of surface layer and technical casing shoe should be improved as much as possible.

6. When drilling in a cased well, the rotating speed of the rotary table shall be controlled. When the drill collar is not out of the casing shoe, the rotating speed shall not be greater than 60R / min, and the rotating speed shall not be greater than 150r / min after the drill collar is out of the casing shoe. For deep wells and complex wells, the drilling cycle is long, and corresponding protective measures shall be taken for the casing.

7. For the cementing of thermal recovery wells, high-quality steel shall be used. During cementing, a certain amount of prestress shall be raised to eliminate the pressure stress caused by the thermal expansion of steel due to the rise of temperature.

1. 3 Casing Collapse

1) Causes and influencing factors of tube collapse

1. Casing strength design is unreasonable, resulting in casing collapse.

2. The drilling fluid is not filled in time, resulting in too long hollowing during casing running, resulting in casing collapse.

3. The casing processing quality is not good, such as uneven wall thickness, too long ovality or steel performance not up to standard.

4. In the process of cement extrusion, there is no cement packer, and the construction pressure of cement extrusion exceeds the internal pressure strength of the upper casing, resulting in the collapse of the upper large-diameter casing.

5. There is a special stratum, such as salt rock stratum. Because of the creep of salt rock stratum, the creep pressure is greater than the casing anti extrusion strength, which will cause casing collapse.

2) Technical measures to prevent casing collapse

1. When running the casing, grouting shall be conducted in time to control the hollowing depth of the casing.

2. The strength design of casing in the special section of peristalsis, such as rock salt layer, should adopt the creep pressure design, and consider the influence of uneven load.            3. The influence of casing compressive strength and external extrusion strength should be considered in the design of cement squeezing operation.

4. Control the quality of casing to prevent unqualified casing from entering the well.

6. Try to improve the cementation quality of cement stone in the sealing section, especially in the special creep formation, and improve the casing anti extrusion ability.