ABOUT CEMENTING COMPLEX PROBLEMS

1. Complexity of pipe accessories and tools

1) Complex situation of float hoop and float shoe

1. The float collar and float shoe are blocked: after running the casing, the circulation is blocked, the pump opening pressure continues to rise, and the wellhead is not reversed. Preventive measures: conduct strict drift diameter for casing entering the well and strictly prevent falling objects in the casing. The solution is to perforate the casing above the float collar and float shoe and re establish the circulation before cementing.

2. Failure of float collar and float shoe: after casing running or cement injection, float collar and float shoe fail or are not sealed tightly. Preventive measures: A. conduct strict drift diameter for the casing entering the well, and strictly prevent the damage of the floating hoop float shoes caused by falling objects in the casing; B. if the floating hoop float shoes have failed, the conventional cementing method can be used to close the well and wait for setting; for liner cementing or bipolar cementing, the balanced pressure cementing method of liquid column inside and outside the pipe is used. 

2) Complex situation of bipolar hoop

1. Double stage hoop cannot be opened

The failure to open the double stage collar means that the secondary cementing circulation hole of the double stage collar cannot be opened smoothly after the completion of the primary cementing, resulting in the failure of the secondary cementing.

The possible reasons for the failure of the two-stage hoop opening are as follows: 

① the opening plug and the opening plug seat of the discontinuous two-stage hoop are not tightly sealed, and the pressure can not be applied, resulting in the failure to open the two-stage hoop;

② the processing quality and design of the two-stage hoop are defective, the body of the two-stage hoop changes shape under the effect of gravity or the clearance between the two-stage hoop body and the opening sleeve is too small, resulting in the large downward resistance of the opening sleeve of the two-stage hoop It is impossible to open the double stage hoop;

③ the performance design of the first stage cementing slurry is improper, such as the short thickening time, the cement slurry has been thickened when it returns to above the double stage hoop, or the compatibility between the cement slurry and the drilling fluid is poor, which causes the cement slurry at the double stage hoop to be cemented and the double stage hoop cannot be opened smoothly;

④ the annulus is blocked after the first stage cementing, which causes the double stage hoop to be unable to open;

⑤ the location of the double stage hoop is not suitable It is suitable, the well angle is large and the dog leg degree is large, and the opening plug is not firmly seated, resulting in the failure of opening the double-stage hoop;

⑥ when the wellhead is connected with the double-stage hoop, the position of the clamp is not correct, and the inner casing of the double-stage hoop is slightly deformed.            

The technical measures to prevent the two-stage hoop from breaking are as follows:

① it is forbidden to clamp on the two-stage hoop body to prevent the deformation of the two-stage hoop body;

② it is necessary to select the high-quality and reasonable two-stage hoop products;

③ it is necessary to design the cement slurry as far as possible not to return to the above position of the two-stage hoop, for example, the first stage cementing slurry must return to the above position of the two-stage hoop, and its thickening time should be added with the falling time of the gravity plug, which is optional Good cementing isolation fluid can prevent the cement slurry from gelling at the double-stage collar;

④ before double-stage cementing, the drilling fluid shall be fully recycled to ensure the wellbore stability;

⑤ the proper location of the double-stage collar shall be selected. For conventional mechanical opening of the double-stage collar, the well inclination shall not be greater than 60-80 generally; for highly deviated wells, the hydraulic double-stage collar shall be used.

The treatment methods for the failure to open the double-stage hoop are as follows:

① if the cement slurry does not return to the double-stage hoop, it is in the casing Run in the small drilling tool, press down the opening sleeve of the double-stage hoop, and open the double-stage hoop mechanically;

② if the cement slurry has returned to above the double-stage hoop, first measure the sound amplitude, perforate about 50m above the cement slurry surface, establish circulation, and carry out the second-stage cementing;

③ if there is no special ground layer above the double-stage hoop and there is no high-pressure formation, run in the special tool to close the double-stage hoop, and then drill the inner sleeve for test Pressure, if it meets the requirements of next spud in or oil and gas production test, cement slurry can be injected from the wellhead for cementing.

2. Double stage hoop cannot be closed

Failure to close the double stage collar refers to the failure of the closing plug to close the secondary cementing circulation hole of the double stage collar smoothly after the secondary cementing, resulting in poor sealing at the double stage collar.

The possible reasons for the failure of the two-stage hoop to close smoothly include:

① the large static pressure difference between the inside and outside of the pipe results in the high pressure of the two-stage hoop to close;

② the processing quality and design of the two-stage hoop are defective, the body of the two-stage hoop is deformed under the action of gravity or the clearance between the two-stage hoop body and the closure sleeve is too small, resulting in the large downward resistance of the two-stage hoop closure sleeve and the failure to close the two-stage hoop;

③ the position of the clamp connecting the two-stage hoop is not correct, and the body of the two-stage hoop is slightly deformed, which makes the two-stage hoop unable to close; ④ the closing pressure applied for the first time is not enough, and when the closing pressure is applied again, the sealing between the closing plug and the plug seat is not tight.        

The technical measures to prevent the double stage hoop from failing to close are as follows:

① it is forbidden to clamp on the double stage hoop body to prevent the deformation of the double stage hoop body;

② improve the processing quality of the double stage hoop itself and design a reasonable closing sleeve fit gap;

③ replace the slurry with heavy slurry to reduce the pressure difference between the inside and outside of the pipe as much as possible and reduce the final closing pressure value;

④ follow 0.5-1.0m3 cement after the second stage cementing of double stage cementing is put into closing block In case that the two-stage hoop cannot be closed normally, the sealing capacity of the two-stage hoop closure sleeve shall be improved;

⑤ the first closing pressure shall be increased.

The treatment methods for the double stage hoop can not be closed are as follows:

① continue to increase the closing pressure and try to close the double stage hoop;

② if it still cannot be closed under high pressure, shut in the well waiting for condensation;

③ for the well with the double stage hoop not closed, pay attention to try to close the double stage hoop closure sleeve with the drilling tool when drilling the accessories of the double stage hoop.

3) Tailpipe complexity

The requirements for liner hanger in liner cementing process are "down going, hanging, tight sealing, inverted opening and lifting out". Its complexity mainly includes the following:    

(1) obstruction in the middle of lower tail pipe

There are two kinds of situations in the middle of the lower tail pipe, one is in the upper casing, the other is in the open hole section. If there is resistance in the upper casing, it is generally caused by the early setting of the liner hanger's slips. In addition to the hanger, the resistance in the open hole section may also be caused by the formation.

The reasons for the advance seating of the liner hanger (hydraulic type) slip are as follows:

① for the hydraulic liner hanger, because the liner is blocked, the pump opening circulating pump pressure exceeds the shear pressure of the hanger seat seal pin, resulting in the advance seating of the liner hanger slip;

② the speed of the lower liner is too fast, which can also cause the advance seating and hanging of the slip and the resistance;

③ the design of the cone standard outer diameter of the liner hanger body is too large If the inner wall of the upper casing is not clean, slightly deformed or the wellbore is reduced, it may cause the liner to encounter resistance in the middle.

The technical measures to prevent the liner hanger's slips from setting up in advance are as follows:

① if the lower liner is blocked, the drilling fluid needs to be circulated, and the circulating pump pressure to start the pump shall not exceed the shear pressure of the hanger's pin;

② the lower liner speed shall be controlled, generally, the running time of a casing shall not be less than 20s, and the running time of a column shall not be less than 45s;

③ A rigid centralizer with an outer diameter larger than the cone shall be added on the upper and lower cone of the liner hanger body respectively;

④ the shear pin pressure of the hanger seat shall be properly increased.

The handling method for the advance setting and hanging of the liner hanger's slips is as follows: generally, the hydraulic liner is equipped with a return spring, the liner is lifted up to make it return, then the liner is lowered slowly, and the change of the weight of the weight indicator is noticed.

(2) the liner hanger seat cannot be hung

Failure to hang the liner hanger seat means that the liner weight cannot be effectively hung on the upper casing at the liner hanger.

The reasons for the failure to seal the liner hanger seat are as follows:

① the inner wall of the upper casing is not scratched cleanly, the inner wall of the casing is seriously worn, or the wall thickness of the casing is small and the strength is low, or the seat hanging position is just in the coupling, etc., which may cause the failure to hang;

② the design defects of the hanger itself, such as the improper design of the taper of the seat hanging slip, the failure to realize self-locking, and the design gap of the hydraulic cylinder of the liner hanger seat seal It is suitable to cause that the piston can not effectively go up, etc.;

③ the liner hanger seat slip is damaged in the process of running down the liner;

④ the suspension weight is large, the hanger body is deformed, and the upward resistance of the piston is large;

⑤ the solid content of the drilling fluid is high, and the performance is unstable, resulting in the plug of the hydraulic cylinder.

The technical measures to prevent the liner hanger from failing to seat include:

① scraping the inner wall of the upper casing before running the liner, especially the wells with long drilling period or sidetracking the old wells;

② selecting a reasonable seat hanging position, avoiding the positions such as serious abrasion of the inner wall of the casing and casing coupling;

③ controlling the running speed of the liner, preventing the seat hanging slips of the liner hanger from being damaged during running the liner;

④ reasonable Design clearance of hydraulic cylinder for liner hanger seat, and carry out tension pressure test on the ground;

⑤ improve the stability performance of drilling fluid, and design a reasonable hydraulic cylinder anti blocking structure;

⑥ once the hanger passes the seat, it is not suitable to lift and unhook it again, and then seat it again;

⑦ the float shoe under the hydraulic liner hanger shall be designed with a bypass hole, in case the seat fails to hang, the bottom of the well can be reversed to complete cementing construction.

The treatment methods for the failure of liner hanger seat hanging are as follows:

① if the liner hanger cannot seat effectively at the design pressure, first check the position of hanger seat hanging, such as the position of seat hanging is at the position of serious abrasion of the inner wall of the casing and casing coupling, etc., release the pressure, change the position of seat hanging and hold the seat hanging again;

② if the liner hanger cannot seat effectively at the design pressure, gradually increase it In the way of setting up pressure, try setting up again and again, do not boost pressure blindly, so as not to break through the setting up ball socket at one time;

③ if the setting up ball socket has been broken through and has not been successfully set up, use the method of circulating drilling fluid with large displacement to set up the liner hanger;|

④ if the final hanger can not be set up, and the weight of the lower liner is not very large, choose the way of setting down the bottom of the well to back up and inject cement, otherwise, The casing had to be lifted.

(3) seal failure of liner hanger

The seal failure of liner hanger refers to the loss of sealing capacity of the seal between the liner hanger center pipe and the sealing core, resulting in the "short circuit" of liner cement injection. 

The reasons for the seal failure of the liner hanger are as follows:

① the seal ring in the seal core is damaged during assembly;

② the seal ring is not resistant to high temperature;

③ the seal ring is damaged by lifting up and down the central pipe when judging whether it has been reversed.

The technical measures to prevent the seal failure of the liner hanger are:

① assemble the seal ring carefully to prevent reverse rotation or damage during assembly;

② improve the finish of the central pipe to prevent the damage of the seal ring during reverse buckle or judgment of reverse buckle;

③ conduct pressure test for sealing performance before the liner hanger enters the well;

④ the seal ring shall be resistant to high temperature.

Treatment method after seal failure of liner hanger: generally, the delivery tool can only be put forward, and the packer is placed in the liner to inject cement.

(4) the liner hanger cannot be opened or lifted out

If the liner hanger fails to open or lift out, it means that after the liner is lowered to the bottom of the well, the back buckle part of the hanger's back buckle device and the liner can't be opened, or the delivery and release tools can't be raised after the back buckle, resulting in the hanger unable to release.

The reasons why the liner hanger can not be opened are as follows:

① the force is applied on the back fastening nut, which makes it difficult to make the back fastening;

② there is dirt at the back fastening nut, which causes the sticking of the buckle;

③ the design strength of the back fastening nut is low, which has been deformed when running the liner;

④ the well angle is large, and the dog leg degree of the well is large, so the back fastening torque cannot be transmitted to the bottom of the well normally during the back fastening.

The technical measures to prevent the liner hanger from failing to open are as follows: the liner hanger shall be strictly and carefully inspected before entering the well.

The handling methods for the tail pipe hanger not to be opened are as follows: if the reversal is serious, the neutralization point shall be calculated carefully to ensure that there is no force on the reversal nut and less friction of the reversal; when increasing the reversal torque, the number of turns of one reversal shall not exceed the allowable torsional strength of the drill pipe to prevent the drill pipe from twisting; if it is judged that the buckle has been opened, the method of lifting and lowering it up and down quickly shall be adopted , release the hanger.

2. Complex performance of cement slurry

The complex situation of cement slurry performance refers to the complex situation of cementing construction caused by improper design of cement slurry performance or change of cement slurry performance. It mainly includes: flash setting of cement slurry, excessive retarding of cement slurry, strength decline of cement stone, etc.

2.1 flash setting of cement slurry

Flash setting of cement slurry refers to the failure of cementing due to the sudden change of cement slurry performance and the thickening or setting of cement slurry in advance.        

(1) causes of cement slurry flash setting

① Material reasons. The cement, admixture or water used to prepare the cement slurry is inconsistent with the laboratory test or the thickening time or setting time of the cement slurry is inconsistent with the laboratory test results due to the poor quality control of the materials.

② The experimental conditions are different. Because the laboratory test conditions are not consistent with the actual situation, especially the temperature has a great influence on the thickening time and setting time of the cement slurry. If the experimental temperature is far lower than the actual temperature or the abnormal high temperature layer is encountered, the cement slurry flash setting is easy to occur.

③ Reasons for site construction. The density of cement slurry prepared in the field construction process is much higher than the design value, which may cause cement slurry flash setting. On the other hand, uneven mixing of cement slurry additives may also cause flash setting of cement slurry.

④ When the well fluid is mixed into the cement slurry, especially the high salinity brine, the thickening time and setting time of the cement slurry will be greatly shortened.

⑤ The solubility of cement slurry and drilling fluid is poor, and drilling fluid is mixed into cement slurry.

(2) technical measures to prevent flash setting of cement slurry

① Control the quality and stability of cementing materials. The cement, admixture or water for slurry preparation shall be consistent with the material requirements for laboratory experiments, and the admixture shall be evenly mixed.

② Accurately calculate the cementing parameters, especially the bottom hole circulation temperature.

③ The density of cement slurry shall be controlled within the range of 0.03kg/l up and down of the design value during the field construction.

④ Pay attention to the pressure balance in the process of cement injection to ensure the formation fluid is stabilized.

⑤ High quality flushing fluid and isolation fluid are used to effectively separate drilling fluid and cementing slurry.

⑥ Do a good job in the water aging experiment of cement slurry preparation and on-site detail review.

(3) treatment method of cement slurry after flash setting

In case of flash setting of cement slurry, it is necessary to use high pump pressure to replace the roof under the condition of ensuring the safety of equipment and underground. If possible, it is necessary to connect the cement roof to replace the roof as soon as possible, replace the cement slurry into the annulus as much as possible, and then use the method of squeezing cement to fill the cement.

2.2 thixotropy of cement slurry

The thixotropy of cement slurry refers to the fact that the cement slurry has a good flow performance when flowing, but it forms a cementitious structure quickly when it is slightly still, and the cement slurry loses its flow ability.

(1) causes of thixotropy of cement slurry

① Material reasons. For example, the cement slurry with ultra-fine materials or additives with high calcium content is easy to form a strong network structure, resulting in a strong thixotropy of the cement slurry.

② Well fluid mixed with cement slurry, especially formation water with high calcium ion content, will also cause cement slurry thixotropy.

③ High solid content, small water cement ratio and thixotropy in high density drilling fluid.

④ The mixing ability of cement truck is low, the mixing energy is small, and the thixotropy of mixed cement slurry is generally strong.

(2) technical measures to prevent thixotropy of cement slurry

① High efficient dispersant is used to improve the fluidity of cement slurry.

② Pay attention to the pressure balance during the cementing operation to ensure the formation fluid is stable and prevent the formation fluid from invading into the cement slurry.    ③ The cement truck with high power and high mixing capacity shall be used for construction.

④ Keep the cement injection construction continuous and prevent the pump from stopping.

(3) treatment method after cement slurry thixotropy

After the thixotropy of the cement slurry occurs, the dispersant can be added into the mixing water according to the site construction situation, and the continuous construction can be ensured.

2.3 excessive retarding of cement slurry

Excessive retarding of cement slurry refers to the slow development or even non setting of cement strength due to the long thickening time of cement slurry, resulting in the inability to effectively seal the oil, gas and water layers.

(1) reasons for excessive retarding of cement slurry

① Excessive coagulant is added to the cement slurry.

② During construction, the density of cement slurry mixed is far lower than the design density.

③ The actual underground temperature is far lower than the experimental temperature, because the temperature has a great influence on the development of cement slurry strength, the lower the temperature, the slower the development of cement slurry strength.

④ The displacement efficiency of cement slurry is low, and the drilling fluid is mixed in the cement slurry, resulting in excessive retarding of cement slurry.

(2) technical measures to prevent excessive retarding of cement slurry

① Add a proper cement slurry retarder, under the condition of ensuring the construction safety, the thickening time is generally 30-60min longer than the construction time

② The development of early strength of cement paste is accelerated by using cement paste accelerator or early strength agent.

③ The density of cement slurry shall be controlled within the range of 0.03kg/l up and down of the design value during site construction.

④ Take effective measures to improve the displacement efficiency of cement slurry.

⑤ Accurately calculate the cementing parameters, especially the bottom hole circulation temperature, and carry out indoor cement slurry experiment according to the actual temperature.

⑥ Do a good job of cement slurry aging test and on-site detail review. Unqualified cement slurry is not allowed to enter the well.

(3) treatment of cement slurry after excessive retarding

After the cement slurry is excessively retarded, the waiting time of cement slurry can only be prolonged, and the next operation can only be carried out after the cement slurry is solidified.

2.4 cement stone strength decline

Cement strength decline refers to the phenomenon of strength degradation and sealing capacity reduction of cement under underground conditions. At high temperature, the strength of conventional oil well cement will decline when the temperature is higher than 110 ℃.

(1) reasons for strength decline of cement stone

① The conventional cement slurry is generally below 110 ℃, and the low permeability and high strength of the chert is formed after hydration. When the temperature is further increased, the strength of the chert decreases, the permeability increases, and the sealing ability decreases.

② The permeability of cement stone is high, and the formation fluid with high corrosion intrudes into cement stone, resulting in strength decline.

③ In the high temperature thermal recovery, the strength of cement stone in the well will decline due to the injection of steam.

(2) technical measures to prevent the strength decline of cement stone

① When the bottom hole static temperature is higher than 110 ℃, the cement slurry high temperature strength stabilizer (silica powder) is added. When the temperature is higher than 130 ℃, the dosage is 35%. When the temperature is higher than 130 ℃, the dosage is 35% ~ 45%. The higher the temperature is, the greater the silica powder dosage is.

② When the local layer is highly corrosive, the permeability of cement stone can be reduced by adding some non permeable agents and ultra-fine materials.

③ A proper amount of high temperature strength stabilizer should be added to the cement slurry of high temperature thermal recovery well.            (3) treatment method of cement stone after high temperature strength decline

After the high-temperature strength decline of cement stone, its sealing capacity is seriously reduced. At present, there is no better way to deal with it, so we should pay attention to the safety of underground operation.

3. Complex situation of cement injection construction

The complex situation of cement injection construction refers to the complex situation or failure of cement injection operation due to the performance of cement slurry, complex underground strata or construction technology. It mainly includes the leakage of cement injection, sausage injection and cement displacement.

3.1 cement injection leakage

Leakage of cement injection refers to that in the process of cement injection or slurry replacement, the sum of annular fluid column pressure and annular friction exceeds the formation leakage pressure, and the cement slurry leaks to the formation, resulting in insufficient return height of cement slurry, leakage of oil, gas and water layers, and poor quality of cement cementation.

(1) causes of cement injection leakage

① The reasons for formation are high permeability, cement slurry leakage, poor formation cementation, low formation pressure bearing capacity and low leakage breaking pressure, formation fracture and fault development, resulting in cement slurry leakage;

② The space gap between the casing and the wellbore is small and the circulating friction is large, which results in cement injection leakage;

③ The high density design of cement slurry and the long sealing section of cement slurry lead to the high pressure of annulus liquid column and the leakage of cement injection;           

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