Contamination of cement slurries with diesel-based drilling fluids in a shale gas well

Mixing cement slurry with OBMs during shale gas well drilling can lead to contamination; mixing subsequently creates significant problems, such as reducing the abnormal thickening time and the compressive strength, which affect the quality of the cementing of the shale gas well. Thus, this study investigates the mechanism of cement slurry contact with OBMs to help to solve the problem of contamination. The research methods used include the combination of rheological properties, compressive strength and microcosmic structure. This study combines Freeze-drying/Environmental Scanning Electron Microscope (ESEM), X-Ray Diffraction (XRD), Thermogravimetry (TG), Scanning Electron Microscope (SEM) and Energy Dispersive Spectrometer (EDS) to explore the mechanism of OBMs contamination in slurry. The results show that the changes in the rheological properties of the mixture are caused by the effects of both osmotic pressure and demulsification, which lead to changes of the free water in the mixed slurry. The presence of OBMs in the cement stone can hinder hydration, forming the honeycomb porous structure without forming a new phase. After identifying the mechanism, the remedies adding surfactants in the cement slurry can relieve the contamination by OBMs.