On the recycling and use of microsilica in the oil industry

: Cementing of casing strings is one of the most important processes of well construction for the exploration and exploitation of oil, gas and gas condensate fields. Despite more than half a century of research and development of grouting mortars, they are not able to provide high strength and good adhesion to the casing string. The article considers the possibility of using microsilica (waste of silicon production) in the oil industry, as an additive of a plugging solution to increase the strength, expansion valve, and reduce the permeability of the plugging stone.


Introduction
The formation, utilization and use of waste currently are one of the most pressing issues in the field of environmental safety and environmental protection both in Russia and worldwide [1,2,3]. This is due both to the rapid increase in the volume of production and consumption waste, and to the insufficiently high level of development of the waste management sector. One of such wastes is silicon production waste [4]. Disposal and use of dusty waste of silicon production (microsilica) should be considered as an important area of material resources saving [5]. The use of waste allows companies to provide additional resources and reduce the environmental load [6,7,8]. Modern requirements for the reliability and strength of the well construction dictate the need to develop technologies and materials for its construction. The cement ring located behind the casing string plays a significant role in the reliability of well construction.
During the final work on the construction of the well and its operation, the support receivessignificant loads that are constant, periodic, and short-term. The condition of the cement ring behind the casing string directly affects the tightness of the separation of productive and aquifer layers between them and the isolation of the casing strings from the negative influence of reservoir fluids.
Accordingly, the destruction of the plugging stone leads to the occurrence of intercolumn pressures, griffins, inter-layer overflows, and premature flooding of the formation. To improve the properties of the cementing slurry, a microsilica additive was proposed.
friendliness of the use of silicon production waste [3,9,10]; the experience of using microsilica additives from silicon production waste in concrete and construction technologies [11,12,13]. There are also works about possibilities and prospects for producing silumin with different silicon content using amorphous microsilica [14]; modification of fine-grained polymer concrete with microsilica [15]; high-performance concretes with modifying micro additives of microsilica and diopside [16]; development of cracking patterns in modified cement matrix with microsilica [17]; and the possibility of preparing alloys of the al-si system using amorphous microsilica [18]. The existing patents cover new ultra-high strength concrete [19], weighted grouting mortar [20], and highstrength fine-grained concrete [21].

Development and optimization of the composition of grouting material
Many articles have been written on increasing the resistance of the cement ring to dynamic loads. There are different approaches to solving this problem. Most scientists believe that Portland cement, despite all its advantages, has a significant drawbackas the strength of cement stone increases, its fragility also increases, and it is also characterized by low strength [12].
Therefore, a significant interest is directed to the improvement of expanding plugging systems for anchoring wells in the cryolithozone, which allows you to change such properties as strength, frost resistance, expansion, etc.
An additive based on silicon dioxide (microsilica) was proposed to increase the compressive and bending strength, the expansion factor of the grouting stone, frost resistance and reduce permeability.
Studies of the microsilica nanostructure show that the use of silicon dioxide nanoparticles leads to significant changes in the substancesignificant compaction of cement stone and a corresponding improvement in its mechanical properties (an increase in compressive strength by 3-6 times). Besides, the modification of the material with silicon dioxide nanoparticles stabilizes the most important Ca -Si -H valence interactions, which are responsible for concrete connectivity, reducing calcium leaching, and increasing its moisture resistance [22,23,24].
The development and optimization of the composition of the expanding plugging material took place in two stages. At the first stage, laboratory studies were conducted to study the effect of the amount of calcium oxide and microsilica on the density, setting time of the grouting solution, mechanical strength, the stone formed from it and its expansion at different water content.
The amount of calcium oxide added varied from 2,5 to 7.5% of the cement weight. During the preparation of the grouting solution, CaO was mixed with PCT -1 -50 Portland cement in dry form at different ratios. Fresh water and various concentrations of CaCl 2 solution were used as the closing liquid. Sealing of the slurry was carried out in accordance with GOST 1581-96. The tests were carried out at a temperature of 20 °C. The results of the study are shown in Tables 1,2. Analysis of the table shows that CaO additives from 2.5 to 7.5% have a slight effect on the setting time and strength of the cement, the largest expansion of the grouting stone was 0.19% (after three days of hardening). The best results are obtained by adding 4% calcium chloride to the grouting solution. However, taking into account the negative impact of the cryolithozone on the processes of hydration of the grouting material during cementing of wells in the Arctic, we recommended increasing the content of calcium oxide to 10-15%. Figure 1 and figure 2 show the dependence of the bending strength and expansion on the content of calcium oxide.   The analysis of the obtained results shows that the grouting mortar at the boundary values of silicon dioxide from 10 to 15%, calcium oxide from 10 to 15%, the water-hard ratio of 0.6-0.7 has a density of 1920-1990 kg / m 3 and is characterized by a significant expansion of the stone formed from it, as well as an increase in strength.
At the same time, it was found that with an increase in the addition of microsilica, the expansion of the cement stone slightly decreased during the first three days and stabilized on days 7-14 days of hardening. At this stage, the cement stone has an expansion of 0.24 to 0.34% at a hardening temperature of 20 °C and 0.13 to 0.15% at a hardening temperature of minus 2 °C.
An especially significant increase in the strength of cement stone is observed with the addition of microsilica in the amount of 10-15% of the weight of the binder. However, with an increase in the content of calcium oxide in the grouting mortar of more than 10%, the bending strength of the stone significantly decreases, and if the content of silicon dioxide is more than 10%, the expansion effect of the grouting stone decreases. The hardening temperature has a significant influence on the setting time, strength and expansion of the plugging stone.
The high strength helps maintain a constant density of the grouting solution over the entire height of the annulus space, unlike other systems. Cement slurry with additives microsilica is resistant to sedimentation. Cement stone with additives of silicon dioxide has low permeability.
The dependence of cement stone strength on microsilica contents at temperatures of 20 °C and -2 °C can be observed in Figures 3 and 4, respectively.

Conclusion
It was found that the strength of Portland cement solutions increased with an increase in the content of microsilica additives from 3 to 15%. However, it should be noted that with an increased amount of this additive (more than 15-20%), cracking of samples may occur, and the frost resistance of cement stone is significantly reduced. When adding silicon dioxide, there also is an increase in the density and porosity of the cement stone, which, as a result, leads to an improvement in its water and corrosion resistance. Experiments have shown that the content of microsilica in Portland cement solutions in the amount of 6-12% has no negative impact on their properties, and this amount of additives can be considered optimal for achieving maximum strength of solutions. The use of microsilica in the oil industry, namely in grouting solutions when cementing wells, improves the condition of the cement ring in the casing, the quality of cementing, increases the strength and tightness of the contact zones of the grouting stone, which allows to avoid various complications.