Synthesis and Performance Evaluation of SF-4 Spacer Fluid for Open-Hole Completion with Oil-Based Drilling Fluid in the East China Sea

As oil and gas exploration and development in the East China Sea advance toward deeper and hotter formations, direct contact between oil-based drilling fluid and water-based completion fluid can cause severe rheological incompatibility during open-hole completion. This study developed an amphiphilic SF-4 spacer based on AM/AMPS/ODAAC copolymerization and evaluated its structure, interfacial behavior, wettability alteration, rheology, thermal stability, and field performance. Characterization of the synthesized copolymer gave a weight-average molecular weight of 1.28 ± 0.06 × 106 g/mol, a monomer conversion of 96.8 ± 0.7%, and total residual monomer content of 0.071 ± 0.008 wt%. Under the test oil/brine conditions, SF-4 reduced interfacial tension from 34.8 ± 1.6 to 3.2 ± 0.2 mN/m at 25 °C and from 31.4 ± 1.4 to 3.7 ± 0.3 mN/m at 150 °C, corresponding to reductions of 90.8% and 88.2%. The sandstone water-contact angle decreased from 132.4 ± 3.8° to 48.7 ± 2.9°, while oil-film surface coverage decreased from 92.1 ± 2.7% to 18.4 ± 3.0% after treatment. Compatibility tests showed that SF-4-containing mixed fluids remained free of harmful thickening after 16 h at 130–170 °C and after 10 d at 150 °C. In the field trial, 7 m3 of SF-4 was placed at 2000 L/min and approximately 1350 psi; the well achieved complete liquid recovery within 4.2 h and subsequently reached a maximum gas rate of 4.8 × 105 m3/d. These results support SF-4 as a high-temperature spacer for isolating incompatible oil- and water-based working fluids during open-hole completion.