Cementing a string in one stage is a challenging task, especially in the presence of weak formations. Cement slurry losses during placement is highly possible in S-1 (sandstone) and S-2 (carbonate) formations, if the equivalent circulating density (ECD) exceeds 11 lb/gal during placement. A conventional method to overcome this challenge is to use multi-stage cementing by setting the stage tool above the loss circulation zone. However, the success of this tool is limited. A second method is to use low density cement.
In this study, we present extensive lab work evaluation of a low density cement system based on the use of hollow microspheres at field conditions. The tests included shrinkage, compressive strength development, porosity, gas and brine permeability. Data generated during three months supported the use the cement examined.
The hollow microspheres cement was applied in several gas wells. The technical objective of the cement job was to achieve zonal isolation across the weak S-1 & S-2 formations. The operation objective was to cement the casing to the surface in one stage without using stage tools.
This paper will discuss case histories that include job design, execution, and evaluation of the hollow microspheres cement. Field treatments were conducted without encountering any operational problems. The treatment was successful and maintained isolation for more than three years which confirmed the effectiveness of this cement system.
Low density cement slurries are used to reduce the hydrostatic pressure on weak formations and to cement lost circulation zones. Examples of low density cements are water extender cements, foam cements and hollow microsphere cements. Water extender cements are limited in density nearly to 11.5 lb/gal.1 Cement fallback often occurs and top of set cement can be hundred feet below the ground level because the formations cannot withstand hydrostatic load by water extender cements even if full circulation is maintained to surface and cement returns were noticed.2 Sulfide containing water can then corrode the uncemented casing.3
Water extender cements can be used in multistage operations, however multistage cementing is limited in their success.2 Stage tools can fail resulting in remedial operations such as perforation and squeeze jobs.4–6 In addition, stage tools are considered weak point and not good for long term seal.4
The only two types of low density cements that can be used to avoid using multistage tools failure are foam cement and hollow microsphere cement.2 We will focus on hollow microspheres cements in this paper.
The idea of mixing hollow glass or ceramic microsphere with cement was developed.7–9 Hollow microsphere cement was used for the first time in the oil industry in 1980 to prepare 9.2 lb/gal cement.10 Gas is contained in the microspheres to reduce cement density down to 8 lb/gal.11
Patents review showed there are several methods other than just mixing hollow microspheres with cementto prepare low density cement.One way is to prepare a mixture of coarse and fine cement particles, fly ash, fumed silica, hollow microspheres and water.12 Another method to utilize hollow microsphere is to mix them with plasticizer, cement and a strengthening agent such as aluminum metal powder and sodium sulfate.13
In this study we will investigate low density cement (LDC) that was prepared by class G cement, aluminum silicate, crystalline silica, hollow microspheres and water. The volume ratio of the water to the total volume of the whole composition varies from 0.25 to 0.50.14Fig. 1 shows cement slurry densities vs. water/solid ratio.
To the best of the authors' knowledge no work was done before to investigate porosity, permeability, bulk shrinkage and elemental analysis of LDC cured at 150 ºF, 1,800 psi for 3 months. The work mentioned in this paper was performed in Saudi ARAMCO research and development center facilities in an attempt to improve cementing S-1 and S-2 formations. Table 1 gives a slurry composition LDC.