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Basic Knowledge
6.1 Micro-silica
  • Detailed description

In order to adjust the slurry performance to meet the cementing requirements, lots of admixtures are be used in slurry system such as suspension agent, it includes the micro-silicon powder, bentonite, etc.; lighting agent, such as cenophere, flyash,puzzolan cement, etc.; weighting agent, such as barite, heamatite etc; The high temperature degradation agent, such as silica powder, etc
The following is the performance introduces of several admixtures
Micro-silicon powder
The silicon powder also called silicon ash is the escaping dust along with the waste gas in the process of melting industrial silicon and ferrosilicon under high temperature. the content of SiO2 in smoke is about 85% of total smoke, color is gray white, particle degree is very small, the average particle size is about 0.1 ~ 0.15 microns, the minimum size is about 0.01 microns.
The physical and chemical properties of micro-silicon powder
Chemical composition

index

85

92

SiO2

≥85%

≥92%

Al2O3

≤1.0%

≤1.0%

Fe2O3

≤1.0%

≤1.0%

MgO

≤2.72%

≤2.72%

CaO

≤2.72%

≤2.72%

Ignition loss

≤3.0%

≤3.0%

Water content

≤3.0%

≤3.0%










When the micro-silicon powder is used as the oil well cement additive, the index of SiO2 content and the water content is important. When SiO2 content is higher than 85%, it has good effect, the content more than 90% is the best, and the water content is the less the better. The micro-silicon product can be classified into: the original ash, half increased density product and increased density product, the half increased density product is used more in cementing, and the increased density product is used in building industry.
1.2 Micro-silicon powder is very fine. Generally speaking, the fineness less than 1 μ m is more than 80%. Compare to the other powder, the silicon powder’s specific surface area can reach  to 200000-280000 g/cm2, is about 50 times bigger than fly ash, and bigger than cement about 80 ~ 100 times.
1.3 other properties
Density: 2.1~3.1g/cm3,generally 2.3~2.6g/cm3
Bulk density 0.2~0.25g/cm3
2 The application of micro-silicon
The condensing process is very rapidly when the micro-silicon powder is formed, so SiO2 have not form a crystal, and the mineral belongs to amorphous mineral, it is a kind of high active surface ash material, is a kind of high-quality cement admixture. Due to the bentonite can affect the cement strength; the micro-silicon powder has basically comprehensive replace the bentonite.
The micro-silicon powder can fill the porosity of the cement particles, and at the same time can generate gel with hydration products. Adding appropriate amount of micro-silicon powder in the cement can play a role as follows: 1) improve the strength of set cement and the permeability of slurry 2) has the role of water conservation, preventing segregation and reducing pumping resistance 3) significantly extend the service life of the oil wells, especially in chlorine salt pollution erosion, sulfate erosion and high humidity and so on harsh environment 4) react with Ca (OH) 2  released by cement hydration and generate high temperature resistant CSH gel at high temperature, improving the high temperature strength.