Quantitative Analysis Method for Residue Flushing Fluid Residue on Cement Sheaths and Its Effect on Set Cement

The cleaning of oil-based drilling fluids is a long-term problem faced with the exploration and development of unconventional resources such as shale oil and gas. Flushing fluid is the primary means to address this problem; however, few studies are conducted currently on the residual characteristics of flushing fluids at the interfaces and their impacts on set cement. A method of quantitatively measuring the amount of organic solvent-contained residual flushing fluids on the casing wall and borehole wall was established based on the fluorescent dye staining technique, and the pattern of how residual flushing fluids influence the mechanical properties of a set cement was analyzed. The research results show that after displacing the oil-based drilling fluid, the amount of the residual oil-based drilling fluid is lower, while the amount of the organic components in the residual flushing fluid is relatively higher, and the amount of these residual organic components is remarkably higher on rougher formation walls. The organic components of the flushing fluid can reduce the compressive strength and increase the permeability of the set cement at the interface of the cement sheath; however, the oil droplets in a nanoemulsion are small and have a uniform particle size distribution, which help mitigate the impact of the nanoemulsion on the microstructure of set cement. The good stability of a nanoemulsion in a cement slurry is a key prerequisite for the oil droplets to have small sizes and uniform distribution. Nonionic surfactants exhibit low adsorption capacity on the surface of cement particles, which reduces the impact of the cement particles on the stability of the interface film of the surfactants, and helps maintain the stability of the organic droplets of the nanoemulsion in a cement slurry. The quantitative method established for residual flushing fluid analysis and the understanding of the pattern how set cement is influenced obtained in this study provide a scientific basis for performance evaluation and optimization design of flushing fluids, as well as support for efficient exploration and development of deep and unconventional oil and gas resources.