Effect of Molecular Chain Flexibility on Adsorption Behavior and Filtration Control of Salt-Responsive Polymers for Water-Based Drilling Fluids Under High-Temperature and High-Salinity Conditions

Summary

With the depletion of shallow oil and gas resources, the development of deep reservoirs has become a dominant trend. These reservoirs are typically characterized by high-temperature and high-salinity conditions, which place more stringent demands on water-based drilling fluids, particularly their key additives—filtrate reducers. Existing strategies mainly enhance thermal and salt resistance by increasing polymer chain rigidity or introducing crosslinked architectures; however, the quantitative relationship between molecular chain flexibility and overall filtrate-reducer performance remains unclear, hindering rational structural design and limiting further improvement in material performance. In this study, three comb-type and three crosslinked salt-responsive filtrate reducers were synthesized from conventional monomers. Molecular chain flexibility was quantified using intrinsic viscosity and the Mark-Houwink equation, and its correlations with bentonite adsorption capacity, American Petroleum Institute (API) fluid loss, and rheological parameters were systematically analyzed. Quadratic relationships were observed between chain flexibility and drilling-fluid performance. Among all samples, crosslinked Reducer D exhibited optimal flexibility (α = 0.75), achieving an API fluid loss of only 4.0 mL after aging at 200°C, outperforming comparable products while maintaining a high adsorption capacity (0.97 g/g after aging 96 hours at 200°C) and favorable shear-thinning behavior (n = 0.69) under saturated sodium chloride (NaCl), along with a 207.72% increase in bentonite surface negativity. These findings establish a quantitative flexibility-performance relationship and identify molecular chain flexibility as a key structural parameter for designing filtrate reducers suitable for extreme formation conditions.