The graded cement injection is used in the following situations:

1. Seal two main layers far away, and do not inject cement in the interval; 2. Weak layer is in the lower part. The cementing pressure can be reduced by grading cement injection to avoid well leakage

3 in the long sealing of deep wells, the construction pressure is high, the capacity of cementing equipment for one time sealing is not up to or long construction time, and the thickening time of cement slurry can not meet the construction requirements.

At present, a series of technical measures have been adopted in cementing of long sealed section wells in domestic oilfields, and remarkable results are achieved 110. The specific situation is as follows:

The following technical measures have been taken in Changqing Oilfield and Tarim Oilfield: first, the whole pressure and plugging are adopted to improve the pressure bearing capacity of the formation, and the low density cement slurry is injected into the well section above the leakage layer; The technology of cement injection cementing is adopted; Reasonable determination of cementing section, sealing length and construction parameters, nd reduction of the equivalent liquid column pressure in the upper part of the leakage layer.

Shengli Oilfield mainly adopts the cementing technology of fly ash low density cement slurry system.

Sichuan Oilfield mainly adopts the technology of two-stage micro bead low density cement slurry and the combination of positive and negative injection.

Cementing of long sealing section wells in Xinjiang oilfield has developed foam cement, different kinds of low density cement slurry system, balanced pressure cementing technology, etc., and has achieved certain results in some blocks, but it has not fundamentally solved the cementing quality problem of long sealing section well.

The classification cement injection technology and micro bead low density cement slurry system are mainly used in Tuha oilfield. The main use of North China Oilfield is the outer pipe packer, double-stage cement injection technology and low density cement slurry system. Other oilfields also basically adopt the above corresponding cementing technology.

But in general, graded cement injection has its limitations:

1. Selection of grading hoop position: the diameter of the well where the grading hoop is located shall be regular and in the relatively straight well section. Too high slope will affect the sealing of the gravity plug and the lower inner sleeve, which is easy to cause: a. the circulation hole cannot be opened; b. The grading hoop is close to the side, one or more circulation holes are close to the well wall, and the circulating pressure is high after opening the circulation hole; C. The opening is unreliable, which leads to the replacement of the first class cement slurry.

Reliability of grading hoop tool: the key to success or failure of graded cement injection is whether the quality of grading hoop is reliable. The opening of the stop pin, the lower rubber plug, the through hole, the first stage pressure, the opening, the secondary pressure and the closing of the hole must be successful. It is necessary to avoid the situations such as opening the grading hoop in advance, not going under the rubber plug, not opening the circulation hole, and not closing tightly. Any accident will have serious consequences.

Quality of floating hoop and floating shoe: if the sealing of float hoop and floating shoe is not strict after the first stage cementing, and the cement slurry is flooded with the grading hoop due to the deviation of well diameter data, it is easy to cause secondary cementing failure. In addition to the pressure test of floating hoops and floating shoes, double safety string structure is adopted in the field to ensure that gravity plug can be put into the hole immediately after the first stage cementing is finished.