Anti-aging and rheological behaviors, and supra-molecular structures in water for the composite of a biopolymer with nano-SiO2 applied in workover fluids of ultra-deep wells

A biological polymer S-657 (namely BLG) was compounded with nano-SiO 2 to obtain a workover fluid thickener applied in ultra-deep wells. This composite system is expected to improve the thermal stability of single BLG in brines containing high salinities and multi-valent salts at high temperatures. The thickening properties of the unaged BLG/nano-SiO 2 brines were slightly enhanced compared with the BLG brines. However, the anti-aging ability and the salt resistance of BLG could be reinforced significantly by the introduction of nano-SiO 2 at 140 °C in brines with high salinities, especially in mixed brines containing mono- and multi-valent salts. After aging at the high temperature for the BLG/nano-SiO 2 aqueous solution, stretched polymeric chain bundles and integrated aggregating structures complexed with nano-SiO 2 were still observed by TEM , resulting in a significant enhancement of the thickening property. The compound of nano-SiO 2 could keep the pseudoplastic and thixotropic performance of the BLG brines during the aged process at a high temperature. In contrast to the single BLG mixed brine, the viscoelastic property of the BLG/nano-SiO 2 mixed brine was still remarkable after aging for 16 h at 140 °C.