Low temperatures affect the development of cement slurry strength in cementing operations, thereby increasing the risks of shallow water/gas channelling and operation costs. Accelerators such as calcium chloride are often used to rapidly improve the mechanical properties of cement. However, accelerators often have negative effects, including decreased cement stone volume, increased permeability and decreased protection against sulfate corrosion. The effects of nanosilica (NS) on the properties of cement slurries were studied at low temperatures. It was found that the early strength of cement slurries was significantly enhanced with the addition of NS. Furthermore, sedimentary stability and the ability to control fluid loss were improved at low temperatures. Mechanistic analysis showed that the early strength improvement was primarily due to the pozzolanic effect of the NS and the bridging and pull-out effects of the AFt (Al2O3-Fe2O3-tri) phase. The use of NS as an accelerator for low-temperature cementing may have broad applications.

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Purpose This paper aims to study a multi-level reinjection current source converter (MLR-CSC) that adds attracting properties such as the self-commutation and pulse multiplication to the thyristor converter, which is of great significance for increasing the device capacity and reducing current harmonics on the grid side. Particularly, designing advantageous driving methods of the reinjection circuit is a critical issue that impacts the harmonic reduction and operation reliability of the MLR-CSC. Design/methodology/approach To deal with the mentioned issue, this paper takes the five-level reinjection current source converter (FLR-CSC), which is a type of the MLR-CSC, as the research object. Then, a method that can fully use combinations of five-level reinjection switching functions based on the concept of decomposition and recombination is proposed. It is worthy to mention that the proposed method can be easily extended to other multi-level reinjection circuits. Moreover, the working principle of the three-phase bridge circuit based on semi-controlled thyristors in the FLR-CSC that can achieve the four-quadrant power conversion is analyzed in detail. Findings Finally, the simulation and experimental results of FLR-CSC verify the effectiveness of the proposed reinjection circuit driving method and the operating principle of four-quadrant power conversion in this paper. Originality/value The outstanding features of the proposed driving method for FLR-CSC in this paper include combinations of reinjection switching functions that are fully exploited through three simple steps and can be conveniently extended to other multi-level reinjection circuits.

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