The Unexpected Rheological Behavior of Borate-Crosslinked Gels

Abstract

Abstract Fracturing fluid testing methodology and equipment has evolved, as have industry-accepted standards for testing. These standards have undergone a number of revisions over the years and were published first as a recommended practice under the American Petroleum Institute (API RP-39) and subsequently under the International Standards Organization as ISO-13503-1. These standards define the rotor/bob geometry (for a concentric-cylinder viscometer), shear-rate, temperature control and other parameters selected to determine the fluid performance under in situ conditions. Little focus was given to the pressure applied to the fluid, other than to ensure that it would not boil under test conditions. This lack of emphasis on pressure is completely understandable as condensed matter (solids and liquids) typically have a limited response to the pressures experienced during hydraulic fracturing operations. It has been discovered that pumping pressures encountered during hydraulic fracturing treatments can materially reduce the viscosity of a borate-crosslinked fluid. The loss in viscosity, depending on fluid formulation, temperature, and pressure can range from negligible to complete resulting in a fluid with the viscosity of an uncrosslinked polymer solution (linear gel). This phenomenon has been studied using high-pressure rheometry and high-pressure nuclear magnetic resonance (NMR). This paper presents the results of recent testing of a variety of aqueous-based fracturing fluid systems under a range of pressures typical of hydraulic fracturing treatments. Detailed results for various borate-crosslinked fluids commonly commercially available will be presented as well as non-typical systems.