With the continuous expansion of global energy demand, large onshore oil and gas fields are increasingly exhausted. People are gradually turning their attention to the ocean. The marine oil and gas resources are mainly distributed on the continental shelf, accounting for about 60% of the global marine oil and gas resources. The potential of oil and gas resources in deep and ultra deep waters is considerable, accounting for about 30%. Low temperature, soft bottom and shallow layer flow are the common characteristics of this kind of strata.

The low-density cement slurry system which is suitable for surface cementing in deep water can solve the problems faced by surface cementing in deep water, and it can be applied to wells with a depth of more than 500 m in China, which is conducive to improving the overall level of deep-water oil and gas well cementing in China.

1. Development of deep water low density cement slurry system

1.1 technical background

According to the geophysical survey, the well will encounter faults and there is a risk of lost circulation. In addition, for the formation with the same sedimentary thickness, with the increase of water depth, the fracture pressure gradient of the formation is decreasing, resulting in a narrow window between the fracture pressure gradient and the formation pore pressure gradient, which is prone to lost circulation.

The commonly used low density cement slurry at home and abroad mainly includes floating beads, bentonite, fly ash and other systems. From the field experience, the density of these low density systems has the lowest limit value. When the density is lower than this limit value, the performance of cement slurry changes greatly, especially the stability of slurry and compressive strength of cement stone. For example, the minimum density limit value of floating bead low density cement slurry system is 1.4g/cm3; the minimum density limit value of low density cement slurry on bentonite is 1.6g/cm3; the minimum density limit value of fly ash low density cement slurry is 1.55g/cm3. When the density of each cement slurry is lower than this density, the cement slurry will be stratified and the compressive strength will drop rapidly. 

1.2 selection of new low density lightening materials

The main reason for the poor pressure bearing capacity of the low-density floating bead system is that the density reducing material used is the by-product of coal combustion, with different particle size, irregular shape and inconsistent composition, resulting in its poor pressure bearing capacity. The hollow glass bead ZOC-P62S developed and improved by Zoranoc Oilfield Chemical Research Institute is a factory product with better performance Particle size distribution and strength. 

ZOC-P62S artificial glass beads are mainly composed of alkali lime borosilicate glass. They are transparent micron glass sealed hollow positive spheres with hard spherical shell. The spheres are filled with thin gas. Macroscopically, it is pure white powder. The minimum particle size is 2 mm and the average particle size is 3.5 mm. The wall thickness is about 10% of its diameter. As a new type of inorganic filler, it has the characteristics of low density, strong compression resistance and excellent stability.

The results show that this series of hollow glass beads are perfect spheres with small particle size, easy to mix and pump; they are incompressible and can be used for logging conveniently and accurately; they have a high strength density ratio and will not be broken in downhole operation; they have a fairly high sealing rate, water can not enter the sphere, so the density can be kept constant; they are chemically inert and will not mix with cement slurry The results show that the addition agent can react with other additives, so that it can be compatible with all cementing slurry systems; the isotropic stress of micro beads is consistent, which can reduce the shrinkage of cement after solidification; there is a little gas inside, so it has a good heat preservation effect, which can speed up the hydration rate of cement, thus reducing the waiting time, and making the cement have a higher strength in a short time Degree.

In view of the narrow surface pressure window in deep water and ultra deep water cementing and the high water depth pressure on seabed cement slurry, ZOC-P62S with smaller particle size, lower density and higher compressive strength is selected as density reducing agent for deep water.