1.2 research status at home and abroad

1.2.1 current situation of cementing technology in foreign sealing section in China

At present, low density cement slurry system is used for cementing long cemented section wells in foreign countries, mainly in the United States, Russia and the Middle East, especially foam cement, and it can meet the requirements of cementing quality. At the same time, two-stage and multi-stage cementing technology is adopted.

The oil fields in the former Soviet Union, the United States and Canada and offshore oil fields in some deepwater areas have been well cemented with foam cement for many times and achieved good results. Foam cement is an ultra low density cement. It is filled with gas (nitrogen or air) in cement slurry, and surfactants are added to stabilize the foam. Compared with other low density cement, foam cement has the characteristics of low density, low permeability, low thermal conductivity and high strength, so it has been widely applied at home and abroad for many years.

There are two ways to make froth: one is the mechanical aeration method, which can fill any amount of gas in cement slurry according to need, and form uniform and fine bubbles through manual intervention. But the equipment required is huge, and the process design and construction are more complex. The other is chemical gas generation. It uses chemical agents to produce nitrogen foam in cement slurry to form foam cement. Due to the small volume of gas, it is necessary to add microbeads in cement paste to reduce density. But no auxiliary equipment is needed, the design is simple and the construction is convenient. Foam cement slurry is mixed with a certain proportion of bubbles in cement slurry to reduce the density of cement slurry.

The traditional long seal cementing technology in China mainly adopts the following two ways: ordinary low density cement slurry and staged water injection Mud M

(1) Ordinary low density cement slurry

There are two ways: increasing water cement ratio and adding lightening agent.

Low density cement slurry was obtained by increasing water cement ratio. For example, by adding Bantu or chemical silicate type additives as fillers, increasing water consumption and reducing the density of cement slurry. However, with the increase of water cement ratio, the settlement stability of cement slurry becomes worse, the free water increases, and the compressive strength decreases with the decrease of density.

Low density cement slurry obtained by adding low density inert material. For example, add floating beads to reduce the density of cement slurry. The maximum liquid column pressure should be controlled within a certain range when the floating bead reducing agent is used. When the bottom hole pressure exceeds its damage resistance, the floating beads will be damaged and the density of cement slurry will increase.

(2) Staged cementing

Staged cementing is to divide the cementing work of a layer of casing into two parts. It is mainly realized by staged cementing device (coupling), which runs into the bottom of the second stage cementing section along with the casing. The utility model is mainly composed of an outer jacket, an upper inner jacket, a lower inner jacket, a closing jacket, etc; Its accessories have three rubber plugs: lower rubber plug, gravity plug and closing plug.