Abstract Wellbores drilled through low-pressure formations encountered offshore or in depleted formations, require use of light-weight cement slurries (less than 13 pounds per gallon (ppg)). These densities in cements can be achieved through foaming, increasing the water content, or using silica-based microspheres. Water-extended cements have a threshold down to weights of approximately 13 ppg and to achieve densities lower than this require the use of foaming and/or silica based microspheres. Each of these methods has limitations that can severely impact hydraulic properties of cement. The foamed cements have the potential to become unstable at high pressures, while silica-based microspheres have chemical instability in the high alkalinity environment of wellbore.
This chemical instability of silica-based microspheres used in cements, creates a hydrophilic gel that is expansive and creates fractures within cement matrix as it expands. This is more formally referred to as alkali-silica reactivity (ASR). Prevention of ASR involves the application of additives to the cement that act as a sink for the alkalinity during hydration for long-term prevention of the ASR. Lithium nitrate is one of these prevention methods that is theorized to allow for other beneficial reactions.
This study investigates the effects of a highly alkaline cement pore-water on the chemical stability of microspheres. Microstructural characterization involves identification of reaction products in alkali- reacted glass beads within 28 days hydrated wellbore cement at wellbore temperatures, as well as the impact of lithium nitrate as a prevention method. The scanning electron microscopy of polished and fractured surfaces reveal two different reaction processes, with the ASR clearly absent in the slurry containing lithium nitrate. The micro-mechanical properties of these changes were also tested using microindentation tool. Lastly, porosity values were tested using helium gas Porosimetry. Lithium nitrate shows an effect on mechanical properties but not on porosity values as compared to cements solely containing microspheres.