Nano-CaCO3/AA-AM-AMPS cross-linked polymer core-shell structural nanocomposite as high temperature and high salt resistant filtration reducer in water-based drilling fluid

Effective filtration loss control under high temperature and high salinity environments is a great challenge in oil and gas drilling engineering. In this study, a core-shell structural nano-composite (CSNC) using precipitated nano-CaCO3 as the core material and cross-linked polymer between acrylamide (AM), 2-acrylamido-2-methylpropanesulfonic acid (AMPS), acrylic acid (AA) and methylene-bis-acrylamide (MBA) as the shell was prepared by reverse emulsion polymerization. The structure of CSNC was characterized with Fourier infrared spectrum (FT-IR), thermogravimetric analysis (TGA), scanning electron microscope (SEM), transmission electron microscope (TEM) and particle size distribution analysis. The filtration control properties of CSNC particles were investigated in both aqueous solution and bentonite suspension with various contents of NaCl and CaCl2 respectively. The high temperature and high pressure (HTHP) filtration properties between CSNC and typical high temperature resistant filtration reducers were compared. The filtration controlling mechanism of CSNC particles were probed through zeta potential analysis, X-ray diffraction (XRD) analysis and filter cake morphologies observation. The results indicated that for the 4% w/v bentonite suspension treated with 3% w/v CSNC, the low temperature and low pressure (LTLP) filtration loss and filter cake thickness decreased by 84.2% and 76.1% respectively after thermal aging at 220 °C. When the bentonite suspension was contaminated with 30% w/v NaCl and 0.5% w/v CaCl2 respectively, incorporation of 1% w/v CSNC reduced the filtration loss by 72.5% and 86.9% after thermal aging at 200 °C. The core-shell structure, cross-linked structure, and salt tolerant sulfonic acid groups on the outer surface of CSNC in combination contributed to extraordinary filtration loss control under stringent high temperature and high salinity conditions.