Borate chemistry has a number of applications in oil and gas recovery. Significant among these is polymer crosslinking to form rheological fluids used to carry proppants in hydraulic fracturing operations. This is based on the reversible formation of borate ester linkages between chains of polyhydroxylated polymers, such as guar gum. Current practice utilizes “instant” and “delayed” crosslinkers. Borax pentahydrate and decahydrate, Na2B4O7●xH2O (x = 5, 10), are common instant crosslinkers. Delayed crosslinking allows time to pump fluids into deep formations before developing high viscosities. The time to develop required viscosity depends primarily on pH and the concentration of borate in solution. Ulexite mineral, impure NaCaB5O6(OH)6●5H2O, added in nonaqueous suspensions, is used to provide a few minutes delay in crosslinking time. Certain borate esters having relatively slow hydrolysis rates also provide delays. Evolving industrial practice presents a need for longer delay times. We recently developed metal and non-metal magnesium dodecaborates as delayed crosslinking agents to provide substantially longer delay times than conventional borate crosslinkers. These compounds, of general formula M2MgB12O20●xH2O (M2Mg[B6O7(OH)6]2●xH2O) where M is a monovalent cation such as NH4+ or K+, are prepared under non-hydrothermal conditions from simple reagents using efficient self-assembly reactions developed in our lab. Synthetic and structural chemistry of these compounds as well as dissolution kinetics are presented.

Development of new borate delayed crosslinkers for oilfield applications

251st National Meeting (2016)

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