Experimental study and analysis on the microstructure of hydration products on the well cementation second interface and interface strengthening strategies

Abstract

The cement-formation adhesion on the second interface in cementing engineering is a crucial factor that affects the long-term sealing capacity of the cement sheath. The bonding strength of the cement on the second interface would deteriorate easily under complex geological conditions, resulting in annular fluid channeling and sustained casing pressure, which severely affects oil and gas development. Accurate understanding on the microstructure and evolution characteristics of hydration products on the second interface is fundamental to improve the sealing capacity of the cement sheath. In this study, an experimental device for testing the sealing capacity of the cement sheath is developed. The testing device has the ability to simulate various conditions including high temperature, high pressure, dynamic formation water, and real-core interface. Using the testing device, a testing procedure is established to determine the sealing capacity of the cement sheath on the second interface. Through analysis on the microstructure and components of interface products under curing conditions, and comparison of interface breakthrough pressure for different slurry systems, the following conclusions can be obtained The main reason for the failure of cementing on the second interface is the crystal growth of calcium hydroxide (Ca(OH)2 or CH) and ettringite (3CaO·Al2O3·3CaSO4·32H2O or AFt). With the further progress of the hydration reaction, crystals in the transition zone near the interface continue to expand, inhibiting the formation of calcium silicate hydrate (C–S–H) gel and causing insufficient bonding strength on the interface; The non-polar spacer liquid, the foam cement slurry, and a system of nano-liquid silicon cement slurry are recommended to be utilized to introduce non-polar groups and a silicon-oxygen (Si–O) bond for changing the surface energy of the interface, inhibiting the formation of crystals, enhancing the bonding between the C–S–H gel and the interface, increasing the cementing strength of C–S–H gel on the interface, and improving the sealing capacity of the cement sheath. In the paper, a new testing device is introduced and a testing method for the microstructure on the second interface is developed, and a new idea is provided for research and development on bonding strength reinforced functional materials, cement slurry and spacer fluid systems. Under experimental conditions in this work, the interfacial bonding strength is increased by 20% compared with the conventional cement slurry, which is expected to provide guidance and reference for improving the long-term sealing capacity on the well cementation second interface.