Researching the complexing conditions of residual boron in produced water from oil & gas fields

•Recycling produced water as subsequent preparation of fracturing gels after shielding residual boron.•The produced water can be directly prepare subsequent fracturing gels without extra filter of boron & complexes.•Directly shielding technology shows good costs and efficiency compared with conventional boron treatments.•The HPG system containing boron & complexes does not affect subsequent rheological property of fracturing gel.•Mannitol is a cost-effective and readily biodegradable material. Large volumes of fracturing flowback fluids are generated as produced water (PW) during oil & gas extraction. There is a method that uses such produced water as a suitable source of fracturing water, recycling it into subsequent fracturing gels. Not only does it reduce processing costs and environmental pollution, but it also effectively alleviates the problem of oilfield water shortage. However, the existence of residual boron in PW is a crucial obstacle to reutilization because it prematurely crosslinks hydroxypropyl guar (HPG) that affects pumping and fracking. So it is necessary to eliminate the adverse effect of residual boron in terms of recycling PW. Fortunately, boric acid easily reacts with polyols by bonding with other active cis-hydroxyl groups to form complexes, preventing residual boron from prematurely crosslinking with HPG. This study demonstrated the feasibility of shielding residual boron crosslinker in PW by a series of simulated crosslink systems. Results indicate that mannitol has a high complexation capacity for boron since it forms stable 5-member ring complexes by bonding with borate ion. When the mole ratio of ligand to boric acid exceeds 2.5 and the pH value is around 10.5, the shielding efficiency of mannitol to boron reaches 95%. The excessive amounts of ligands completely shield the boron and prevent its continuous crosslinking with HPG. The fracturing gels prepared by PW no longer show the premature crosslinking phenomenon after the shielding agent is added, and its rheological properties loading the new crosslinker are similar to the system which is prepared with deionized water. These results indicate that mannitol can effectively shield the residual boron crosslinker, removing the adverse effects of premature crosslinking with HPG base fluid. This has formed stable complexes, which still exist in the HPG base fluid system and do not affect the rheological properties of the subsequent fracturing gels. Compared with conventional boron tr