A Copolymer Water-Based Drilling Fluids Fluid-Loss Additive for High-Temperature and Salt-Calcium Condition

China's deep oil and gas resources have great potential, which is an important direction for oil and gas exploration and development in the future. However, the performance of water-based drilling fluid deteriorates under the conditions of high-temperature and high salinity. In this paper, we designed and synthesized a five-membered copolymer fluid loss reducer (AADDS) according to the performance requirements of a high-temperature and salt-resistant calcium drilling loss reducer. Acrylamide (AM), 2-acrylamide-2-ylmethylpropanesulfonic acid (AMPS), Dimethyldiallyl Ammonium Chloride (DMDAAC), N, N-dimethyl acrylamide (DMAM), and sodium allyl sulfonate (SAS) were selected as reaction monomers, and ammonium persulfate and sodium bisulfite were used as an oxidation-reduction system. We used Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (1H NMR), and gel chromatography (GPC) to characterize the rheological properties and filtration properties of AADDS in water-based drilling fluid. The results show that the molecular structure of the synthesized products is consistent with the designed target products, the distribution of subfractions is reasonable, and AADDS has good filtration performance at high-temperature and high salinity. In addition, the filtration reduction mechanism of AADDS in water-based drilling fluid is further revealed by SEM analysis, laser particle size analysis, and zeta potential analysis. AADDS has a temperature resistance of 180 ℃ under the condition of saturated salt and 1% CaCl2, which is expected to solve the problem of treatment agent failure under the condition of high-temperature and high salt during ultra-deep well drilling.