IntroductionTo address the difficulty of simultaneously achieving high density, high-temperature stability, low-friction transport, effective gel breaking, low formation damage, and operational reliability in ultra-deep well fracturing, a sodium-formate-weighted non-crosslinked fracturing fluid was developed using a hydrophobically associating polymer containing C16AMSE-Na. Unlike previously reported sodium-formate-weighted polymer systems that rely on chemical crosslinking, the developed fluid establishes its viscosity-bearing structure through physical association without the addition of a crosslinker.MethodsA sodium formate base fluid with a density of 1.32 g/cm³ was selected as a practical balance between hydrostatic-pressure contribution and rheological retention, and the optimum thickener concentration was determined to be 0.50 wt%. The polymer structure and morphology were characterized by FTIR, ¹H NMR, and SEM. Rheological performance, drag reduction, gel-breaking performance, mixed-fluid compatibility, and core damage were systematically evaluated, followed by field application in Well CT-1.ResultsAfter shearing at 160 °C and 170 s⁻¹ for 2 h, the optimized fluid maintained a residual viscosity of 55 mPa·s. FTIR and ¹H NMR analyses confirmed the characteristic functional groups and the incorporation of C16-containing hydrophobic structural units, while quantitative SEM analysis showed that the freeze-dried polymer framework retained an interconnected porous morphology in concentrated sodium formate brine. The maximum laboratory drag-reduction rate reached 76%. At 160 °C, 0.50 wt% NaBrO₃ produced a broken-fluid viscosity of 3.0 ± 1.0 mPa·s and a residue content of 0.53 ± 0.15 mg/L. Mixed-fluid compatibility tests showed no visible precipitation, phase separation, or abnormal rheological response, and all mixed systems exhibited broken-fluid viscosities of no more than 4.0 mPa·s under the investigated conditions. Core testing gave a permeability damage rate of 26.9%. Field application demonstrated a theoretical hydrostatic-pressure increment of 17.8–18.1 MPa. Surface-pressure reductions of 9.8, 7.8, and 6.9 MPa were observed at pumping rates of 1.95, 2.90, and 3.89 m³/min, respectively, while field-derived drag-reduction rates of 60%–61% were retained under high-rate pumping conditions. The cumulative flowback ratio ultimately reached 83.88%.DiscussionThese results demonstrate that the developed non-crosslinked weighted fracturing fluid provides an effective combination of high-density hydraulic assistance, high-temperature rheological stability, low friction, efficient cleanup, low formation damage, and compatibility with the sequential fluid strategy used in ultra-deep well stimulation.