High-Efficiency Boron Crosslinkers for Low-Polymer Fracturing Fluids

Abstract

Guar-based fracturing fluids are the most commonly used fluids in reservoir stimulation. To provide high viscosity, boron crosslinked gels are preferred for their ability to heal after mechanical shearing and their favorable environmental properties. However, polymer loading determines amount of polymer residue, which can damage the formation and proppant pack. New, efficient boron crosslinking chemistry promises to reduce polymer loading in fracturing fluids and therefore minimize polymer residue and damage.As a first step toward improving crosslinking efficiency, several diboronic acids were tested as model compounds in a series of guar solutions against boric acid. The results demonstrated that such chemistry could effectively reduce C* (critical overlap concentration for polymer chains), C** (critical entangle concentration for polymer chains) and Ccc (critical crosslinking concentration) of guar solutions. Further observations show crosslinking efficiency is a function of the crosslinking sites and chemical structures of the crosslinkers.A new chemistry was developed to produce crosslinkers more cost-effective than the model compounds, whereby boron was incorporated via reactions between organometallic intermediates and boron reagents. Amines and polyimines were used as the base scaffolds and derivatized under mild conditions. The resulting compounds contain more crosslinking sites than the existing chemistry and can connect multiple guar chains to form a much more complex network for more efficient proppant transport.A series of crosslinkers based on the new amine derivatization were synthesized and evaluated. This paper will show the lab test results of model compounds and new crosslinkers versus the conventional boric acid crosslinker. Analysis and discussion of the chemistry, crosslinking performance and economics will be presented. BackgroundWells drilled in low-permeability reservoirs are often treated with hydraulic fracturing to increase their conductivity and thereby enhance recovery of hydrocarbons. The fracturing procedure increases flow by creating new fissures and facilitating the connectivity of the existing pores and natural channels contained in a reservoir rock that would otherwise not allow formation fluids to reach the wellbore in sufficient quantities for commercial value. This technology has been used for over 60 years in more than one million wells.Hydraulic fracturing cracks or "fractures" in the adjacent substrate or zone are created by forcing a fluid at a rate and pressure that exceeds the parting pressure of the rock. The continued injection of the fracturing fluid expands the fractures. As the pumping pressure at the surface is released, the "frac" fluid will retreat from the formation back to the well. Proppant incorporated in the fluids is left behind and acts to prevent the expanded fractures from closing, allowing the conductive channels to remain. Viscosity of fracturing fluids is important for transporting the proppant material into the fractures. Poor or low viscosity can l...

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