Borate-crosslinked gels are preferred for hydraulic fracturing treatment of wells over metal-crosslinked gel because of their ability to reheal after shear and their reduced proppant pack damage. Conventional monoborate-crosslinked fluid is mostly limited to wells with low to medium bottomhole temperatures (120 to 200°F), while polyborate-crosslinked fluid is applicable at moderate to high temperatures (250 to 350°F). Recent work has found that the viscosity of conventional borate-crosslinked fluid underwent sequential viscosity loss at room temperature with stepwise pressure increases from 500 to 10,000 psi. This compromise in viscosity at high pressure can undermine fracture geometry as well as proppant transport and distribution in created hydraulic fractures. Also, it will be interesting to carry out this type of study under high-temperature conditions.

We reported in SPE 140817, 164118, and 168186 a high-temperature-tolerant polyamino-boronate crosslinker (PAB) containing multiple boron sites capable of interacting with multiple polysaccharide strands to form more complex crosslinking networks at lower polymer loadings than conventional fluids. Two crosslinkers constituted with different polyamine backbones (tetraethylenepentamine and tris-(2-aminoethyl)amine) were synthesized and crosslinked with hydrated guar. The optimized rheology was investigated at 250°F under a pressure ramp of 500 to 10,000 psi. The results indicate that viscosity of a 30 pound per thousand gallon (ppt) PAB-crosslinked guar is sustained up to 7000 psi at 250°F, whereas viscosity of conventional 30 ppt guar crosslinked by conventional borate crosslinkers fluid declines on ramping pressure from 500 to below 5000 psi.

The detailed results of the rheology tests at high temperature and pressure, 11B NMR of synthesized PAB crosslinkers and intermediates, as well as fluid chemistry will be discussed in this paper.

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