The oil and gas industry has now long been aware of the fact that the rheological characterization of crosslinked fluids under field simulated conditions is very difficult. Particularly, the testing of borate-crosslinked fluids under realistic conditions for better understanding of their rheological behavior, is very complex. For this purpose, the Gas Research Institute (GRI), the U. S. Department of Energy (DOE), and the University of Oklahoma jointly established the Fracturing Fluid Characterization Facility (FFCF) which includes a pilot-plant scale, high temperature, high-pressure simulator (HPS) coupled with a fluid pre-conditioning system. Employing the field-size equipment of the FFCF and field-simulated procedure, various borate-crosslinked fluid formulations are tested and evaluated for their rheological responses.
This paper presents the results of pre-conditioned fluid rheology tests conducted with borate-crosslinked Guar and HPG gels employing the HPS over a range of pH from 9 through 11, subjected to varying levels of shear history and temperature. The network structure of these crosslinked fluids at any time is dependent on the current shear state, past shear history, and borate ion concentration which in turn depends on pH and temperature. Even though the effect of pH and temperature on the borate ion concentration is relatively well understood, the characterization of the shear state of the gel at field conditions is still in its infancy. In this investigation, test fluids are subjected to all conditions, to the maximum degree practical, experienced by a fracturing fluid in the field, including its formulation in surface equipment, its injection through tubular goods and perforations, and its flow and heatup in the fracture. Besides the effects of polymer type, temperature, and fluid pH, the effects of shear history on the rheology of borate-crosslinked fluids are also investigated. The presented results show dramatic effects of shear history on the rheology of borate-crosslinked gels which have been ignored in previous investigations. Certain gel formulations corresponding to specific pH and temperature conditions, were found to be shear history insensitive. Furthermore, these shear history insensitive formulations were also found to exhibit an optimum viscosity which was independent of temperature over the range of ambient to 185 °F.