Using a Model 50 Fann viscometer, the viscous behavior of a delayed-crosslink fracturing fluid was evaluated after the fluid was subjected to various combinations of thermal and shear history. Fluid thermal and shear histories were created by pumping a mixture of polymer solution and crosslinking additive through tubing coils situated in a series of thermally-controlled baths. Specifications of the preconditioning device are provided in the paper. By varying bath temperatures and flow rates through the preconditioning coils, it was possible to create a wide variety of fluid thermal and shear histories. An organo-metallic crosslinking agent was continuously mixed into the polymer solution as it was being injected into the preconditioning device. At the point where the chemically reacting mixture exited the preshear device, it was routed, without stopping, into a rotating viscometer cup by means of a quick-connect. In this manner, the fluid sample captured within the viscometer cup experienced a continuous shearing action similar to that experienced in the field environment. Sample rheology was then monitored over a 2-hour period at elevated temperature using a R1B5X measurement configuration. The merits of using the R1B5X rather than the wide gap R1B2X measurement configuration are discussed. Rheology results from nine uniquely different thermal-shear history combinations were compared to determine the relative influence of each parameter on the final fluid viscosity.
Crosslinking chemicals, such as transition metal ions, are used to enhance the viscosity of polymer solutions used in the hydraulic fracturing process. Crosslinked polymer solutions (gels) are typically used in fracturing applications to provide high-viscosity fluids at elevated bottomhole temperature. In general, a crosslinking additive is continuously added at a very low concentration to the polymer solution as it is being injected downhole. Depending upon the particular crosslinking additive used, gel formation can occur immediately or be delayed by thermal and/or chemical means until the fluid reaches the perforations or is within the fracture system.
Without question, the fracture flow rheology of rapid crosslinking gels is known to be controlled by the particular thermal and shear conditions present in each application. Very high heating rates and high shear rates are experienced by these fluids as they move down the wellbore.