Solid-free brine completion fluids, characterized by their exceptional reservoir protection capabilities and optimal rheological behavior, are highly desirable for applications in oil and gas reservoirs and have attracted significant attention in recent decades. However, as the core component of completion fluids, the viscosifier was prone to curling or even precipitating in high-temperature, high-density inorganic salt (divalent calcium) environments, leading to failure in thickening performance. In this study, a micro-crosslinked amphoteric viscosifier (i.e., A-DDAS) resistant to high temperature and calcium ions was synthesized via free radical copolymerization of N,N-dimethylacrylamide (DMAA), diallyl dimethyl ammonium chloride solution (DMDAAC), 2-acrylamido-2-methylpropane sulfonic acid (AMPS), [2-(methacryloyloxy) ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide (SBMA), and pentaerythritol triallyl ether (APE). The molecular structure and physicochemical properties of the copolymer were systematically studied by NMR, FTIR, XPS, TGA and XRD. Rheological experiments demonstrated that calcium bromide brine containing A-DDAS copolymers exhibited outstanding shear-thinning behavior and rapid thixotropic recovery, essential for efficient wellbore cleaning and fluid displacement during completion operations. As the density of calcium bromide brine increased, more calcium ions shield electrostatic attractions between the cationic and anionic moieties along the copolymer backbone, thereby promoting full extension of the polymer chains and enhancing the binding energy with water molecules. After adding 1.0 wt% A-DDAS copolymer to a 1.75 g/cm³ calcium bromide brine and aging the mixture at 180 ℃ for 16 h, the completion fluids exhibited an apparent viscosity of 71 mPa·s, plastic viscosity of 64 mPa·s, and yield point of 7 Pa, which were significantly better than common viscosifiers (HE300 and Dristemp). Therefore, A-DDAS copolymers demonstrated exceptional thickening capacity and dynamic shear enhancement in high-temperature, high-density calcium bromide brine, notably rendering it ideally suited for deployment in completion fluids for deep and ultra-deep wells.

Shale oil and gas, as typical unconventional resources, have gradually altered the global energy supply and demand landscape, attracting significant attention over recent decades. However, challenges such as wellbore instability and reservoir damage caused by drilling fluids invasion during shale drilling remain unresolved. In this study, we reported the synthesis and preparation of biomimetic inspired superhydrophobic nanofluids (SHN) with multiple functions by utilizing nano-silica, low surface energy fluorinated compounds, and cationic compounds with adsorption capabilities. Firstly, SHN with nano effects could plug micro-nano pores in shale, thereby reducing the filtration loss of drilling fluids (from 24 to 11 mL). Furthermore, SHN could adhere to shale surfaces through electrostatic interactions to increase its roughness from 1.121 to 3.567 μm, thereby transforming the shale surface from hydrophilic (26.4°) to superhydrophobic (152.8°). This not only reduced self-priming by 83.7% and decreased the capillary rise height to 5 mm below the liquid surface but also suppressed hydration expansion and improved the rolling recovery rate by 84.74%. Overall, this study provided new insights into the design and manufacturing of high-performance drilling fluids materials that could support wellbore stability and reservoir protection during shale oil and gas drilling processes.

During ultradeep oil and gas drilling, fluid loss reducers are highly important for water-based drilling fluids, while preparing high temperature- and salt-resistance fluid loss reducers with excellent rheology and filtration performance remains a challenge. Herein, a micro-crosslinked amphoteric hydrophobic association copolymer (i.e., DADC) was synthesized using N,N-dimethyl acrylamide, diallyl dimethyl ammonium chloride, 2-acrylamido-2-methylpropane sulfonic acid, hydrophobic monomer, and pentaerythritol triallyl ether crosslinker. Due to the synergistic effects of hydrogen bonds, electrostatic interaction, hydrophobic association, and micro-crosslinking, the DADC copolymer exhibited outstanding temperature- and salt-resistance. The rheological experiments have shown that the DADC copolymer had excellent shear dilution performance and a certain degree of salt-responsive viscosity-increasing performance. The DADC copolymer could effectively adsorb on the surface of bentonite particles through electrostatic interaction and hydrogen bonds, which bring more negative charge to the bentonite, thus improving the hydration and dispersion of bentonite particles as well as the colloidal stability of the drilling fluids. Moreover, the drilling fluids constructed based on the DADC copolymer exhibited satisfactory rheological and filtration properties (FL_HTHP = 12 mL) after aging at high temperatures (up to 200 ℃) and high salinity (saturated salt) environments. Therefore, this work provided new insights into designing and fabricating high-performance drilling fluid treatment agents, demonstrating good potential applications in deep and ultradeep drilling engineering.