Evaluation of Microstructural and Mechanical Properties Development of Expansive Cement Slurry for P&A Applications

Abstract

Abstract The inclusion of expansive agents in cement slurries has been proposed as a method to avoid shrinkage and reducing the probability of generating a microannulus at the interface between cement sheath and casing or formation wall. This work focuses on the effect that an expansive agent has on the mechanical and microstructural properties of cement paste when compared to conventional class G cement, aiming to identify any potential detrimental effects that might be induced by its inclusion. Two cement slurries were evaluated: a class G cement used as a Reference and a second with the addition of 3 % of expansive agent. The slurries were fully characterized and cured under atmospheric or pressurized conditions for a duration of up to fifteen days. Cured cement specimens were analyzed to determine the influence of the expansive agent on early age mechanical properties development and microstructural properties. Development of mechanical behavior was measured using an ultrasonic cement analyzer (UCA). Microstructural properties were determined by scanning electron microscopy (SEM), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). Annular ring test at 27 °C for 15 days showed an expansion of 0.021 % in circumferential change for the expanded slurry, while the class G cement used as Reference achieved a value of 0.016 %. Both results are mainly due to silica effect. The same test performed at 70 °C for 7 days showed an expansion of 0.038 % in circumferential change for expanded slurry, which represents the effect of the additive dissolution and hydration, and a shrinkage of 0.012 % in circumferential change for Reference. UCA showed the tendency of acceleration of hydration kinetics caused by the expansion additive. An increase in compressive strength for expanded slurry can be related to its hydration and formation of a denser microstructure. SEM showed a uniform dispersion of magnesium in the cement matrix with some Mg(OH)2 crystals formed. Hydration products were also seen with a predominance of ettringite for samples cured at atmospheric pressure. XRD showed characteristic peaks of hydration products for all slurries, with an increased intensity for expanded slurries. TGA showed similar curves for all slurries, with the presence of thermal decomposition of magnesium hydroxide for expanded slurries. It was concluded that the presence of expansion additive in the cement slurry in an amount of 3 % was not found to be detrimental to the mechanical and microstructural properties. Literature has a lack of connection between microstructural analysis with mechanical properties of slurries developed with expanded additive in downhole conditions. This paper intends to fill this gap detailing the high potential of this additive through an in depth characterization and comparison with a Reference slurry concerning the mechanical and mineralogical properties.