The defects of cement sheath in wells, like high penetration and shrinkage, are easily to cause the weight losing or sealing failure at the casing-cement and cement-formation interfaces. Our new liquid nano-silica, as an effective anti-fluid-channeling agent, is widely used in field. This paper focuses on the manufacturing method of liquid nano-silica and its advantages in increasing the sealing ability of cement sheath.

The silica particles range from 10 nm to 300 nm are needed to generate the liquid nano-silica. The high energy ball milling, steric repulsion, electrostatic repulsion, surface modification and emulsification are used to disperse these grained silica. The liquid nano-silica with a median particle diameter of 162 nm can stay in stable for more than one year and it is convenient for field application. This kind of liquid nano-silica contains multiple functions due to its small size, large specific surface and good property of filling into the pores. The cement slurry with density 1.90 g/cm3 is taken as an example. The initial and final gel strength of cement slurry without liquid nano-silica are 1 Pa and 3 Pa respectively. The free fluid is 1.1 mL and the density difference between top and bottom of slurry after two hours is 0.13 g/cm3. When the 5.0% liquid nano-silica is added into the cement slurry, the initial and final gel strength of cement slurry are changed to 2 Pa and 8 Pa respectively. No free fluid and no density difference. When the ratio of liquid nano-silica in cement slurry is 15.0%, the strength of cement stone increases by 51.4%. The hydration shrinkage is decreased by 46.1% and the permeability is reduced by 87.0%. The bonding strength of interfaces is increased by 766.7%. The applications of the liquid nano-silica in the oil and gas wells in Northwest, Southwest and East of China show its advantages.

1. Introduction

Cement sheaths are designed to protect the integrity of oil and gas wells by preventing the channeling and leaking of formation fluids (Ahmed et al, 2018). The cement sheath failure can lead to the loss of zonal isolation and result in sustained casing pressure. Cementing work encountered many difficulties due to shallow flows all around the world. Gas migration has been recognized as one of the most difficult problems in the oil and gas industry in a long time(Wang and Shen, 2020). The risk of environmental contamination due to oil and gas strongly depends on the frequency of integrity failure of wells (Lackey et al. 2017).

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