1.1 purpose and significance of the study

Cementing operation is an important part of the whole drilling and completion of a well. The practical experience shows that the quality of cementing has a great influence on the productivity and life of oil reservoir. If the oil well can not achieve complete interlayer sealing after cementing, the oil well can not achieve full productivity and short life. Therefore, cementing is very important to protect productivity, and the cementing of long cementing section is one of the difficulties in cementing work.

At present, there is no unified definition of long sealing section at home and abroad. The cementing technology of long cementing section referred to in this study refers to the special cementing technology that applies unconventional slurry system and construction technology, and the one-time cementing section is more than 1500m or the total length of cementing section is more than 2600m after using staged cementing device. With the continuous exploration and development of the oilfield, more and more target layers are required to be sealed, and more and more complex layers are required. In order to save costs and provide treatment layers for the unpredictable pretreatment of complex situations, there are more and more methods to use long cementing section in the upper layer to save casing layers, and the cementing section is longer and longer; In addition, some special development wells need long or even full well cementing, such as the development wells in Qiudong gas field and the miscible flooding production wells in Pubei oilfield.

Two stage cementing technology is generally used in cementing operation of long cementing section. Two stage cementing technology is an effective method to improve cementing quality and reduce oil and gas reservoir pollution for long cementing section, but it also has many disadvantages that are difficult to overcome: 1. Grading hoop must be used for two-stage cementing. In cementing (especially inclined well cementing), after the first stage water mud is fixed, sometimes the circulation hole can not be opened, and the wellhead will hold high pressure, It brings danger to cementing operation and even causes cementing failure; 2. A grading hoop is worth tens of thousands or even hundreds of thousands of yuan, which increases the cost of cementing; 3. After the completion of two-stage cementing, the cement plug at the position of grading collar should be drilled, which not only increases the drilling cost, prolongs the well construction period, but also has the risk of drilling through the casing.

Therefore, in order to avoid the above problems, low-density cement slurry is often used for one-time cementing. For long cementing section, there are some problems in using one-time cementing.