Remediation of oil-based mud contaminated cement with talcum in shale gas well primary cementing: Mechanical properties, microstructure, and hydration

Abstract

Mineral oil (MO) is the main component of a type of oil-based mud (OBM) used in shale gas wells. The contamination of oil-well cement (OWC) by MO negatively affects the mechanical properties of cement and the cement–casing or interfacial adhesion during cement formation. Although spacer fluids are widely used in primary cementing jobs, contamination usually occurs due to poor displacement. In this work, talcum was incorporated into cement to decrease the negative effects of contamination. The effect of MO contamination on the compressive strength, Young’s modulus, Poisson’s ratio, and shear bonding strength of OWC and talcum-modified OWC (TMOWC) was investigated by evaluating the mechanical properties. The pore structure, micromorphology, and hydration of cement were investigated by computerized tomography (CT), low-field nuclear paramagnetic resonance (LF-NMR), scanning electron microscopy (SEM), X-ray diffraction (XRD), and conductivity and thermogravimetric analyses. The mechanical compressive strength, ultrasonic compressive strength, and Young’s modulus of the OWC with 5 wt% MO decreased by 67.5, 35.2, 11.7, and 41.8%. Talcum clearly slowed retrogression of the mechanical properties. The mechanical compressive strength, ultrasonic compressive strength, and Young’s modulus declined by only 13.3, 14.6, 0.12, and 21.5% for contaminated TMOWC, respectively. Porosity and hydration analyses revealed that the mechanical properties declined due to the large pores in the OWC matrix and the reduced degree of hydration due to MO contamination. However, the porosity and pore size of contaminated TMOWC decreased and the hydration degree evidently improved with respect to those of contaminated OWC. The results suggest that talcum is promising for OWC modification and remediating MO contamination in shale gas well primary cementing.