Abstract
The failure of cement sheaths to perform as designed in oil wells can result in loss of zonal isolation. Bulk shrinkage of the cement sheath in the annulus is one of the main causes compromising zonal isolation. Conventional cement systems without compensation for bulk shrinkage have a high risk of failure during all phases of well operation. However, when the volume reduction is compensated, without compromising the mechanical properties, the risk of failure is significantly reduced.
In this study, nano-sized MgO with designed expansive properties has been introduced to the fresh cement slurry. The expansive properties of nano-MgO were achieved by controlling the preparation condition. A dilatometer with corrugated molds was used to measure the linear strain of samples cured at 40 °C.
The reactivity of nano-MgO played a main role in controlling the expansion performance at the required time. The efficiency of nano-MgO with different reactivities for shrinkage compensation of cement system, cured at 40 °C, was studied. Addition of only 2% nano-MgO with appropriate reactivity was sufficient to maintain expansion in the cement system. Controlling the expansion performance of the additive through designing its reactivity is a promising method to limit bulk shrinkage of cement sheaths in oil wells.
The results presented show that nano-MgO with controlled expansive properties can be used to design a cement system with short- and long-term zero-bulk shrinkage.
