Title: Borate-Crosslinked Fluid Rheology under
Various pH, Temperature, and Shear History Conditions
Author(s): S.N. Shah and B.N. Rao, The University of
Oklahoma and D.L. Lord, Halliburton Energy Services
Presentation: SPE Production Operations Symposium
Location: Oklahoma City, Oklahoma USA
Date: March 9-11, 1997
Abstract: 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 filed-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
testes 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 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 heat-up 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
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 degrees F.