The significance of nanosilica on degradation of oil well cement in carbonated brine environments

Emergent interest in geological carbon dioxide (CO2) sequestration has brought a need for understanding the mechanism of potential degradation of oil well cement (OWC) in carbonated brine. In this paper, Type G OWC paste specimens - produced with water-to-binder (w/b) ratio of 0.45 and incorporating 0, 1 and 3% nanosilica by weight of cement - are hydrated for 28 days under two conditions, room condition (20 C with 0.1 MPa pressure) and a simulated oil well condition (80 C with 10 MPa pressure). Conditions of geosequestration in a sandstone formation at a depth of 1 km, were simulated by bubbling CO2 into a heated vessel containing 0.5 M NaCl (brine). Fresh cured OWC cylindrical specimens were exposed to this environment. Slices of the cement specimens were taken periodically, during and after exposure, to quantify degradation progression. The modulus of elasticity of the specimens was examined prior to and after exposure, and a damage metric was computed. The results show that the addition of 1% nanosilica can significantly limit OWC degradation in carbonated brine environments. Furthermore, microstructural characterization using BET-N2, X-ray diffraction (XRDA) and ²⁹Si nuclear magnetic resonance (NMR) was performed to explain the macroscale observations.