Hybrid effects of carbon nanotubes and nano-rubber on the mechanical properties and microstructure of oil well cement paste cured at different temperatures: Experimental studies and a micromechanical model

A ductile oil well cement paste (OWCP) with lower autogenous shrinkage is of paramount importance to the integrity of cement sheath under downhole condition. Taking advantage of multiple experimental tests and a micromechanical model, the hybrid effects of nano-rubber (NR) and carbon nanotubes (CNTs) on the mechanical properties, autogenous shrinkage, hydration behavior and microstructure of OWCP were investigated in this study. Results show that the hybrid addition of NR and CNTs effectively enhances the mechanical properties and ductility of OWCP, 7-day tensile strengths of OWCP incorporated with 4 wt% NR and 0.04 wt% (N4C4) cured at 30 °C, 60 °C and 90 °C exhibit 1.4%, 13.7%, 13.0% strength gain with respect to those of plain OWCP (P) at the similar curing temperature; T/E ratios (tensile strength/Young’s modulus) of 7-day N4C4 cured at 30 °C, 60 °C and 90 °C exhibit 82.3%, 39.3% and 40.8% increase as compared to that of P at the similar curing temperature; the hybrid addition of NR and CNTs suppresses the autogenous shrinkage of OWCP, leading to about 31.9%, −12.8%, and 20.8% reduction of 72-hour autogenous shrinkage of OWCP at 30 °C, 60 °C and 90 °C, respectively. The developed micromechanical model is capable of well quantifying the hybrid effects of NR and CNTs on the mechanical/poroelastic properties of OWCPs. The present work is anticipated to shed light on the development of a ductile OWCP with lower autogenous shrinkage under harsh downhole conditions.