Rheological measurements of a typical delayed titanium crosslinked hydroxypropyl guar (HPG) fluid were made with crosslinking reactions occurring under controlled shear rates (40, 120, and 170/sec) and heating rates (4 and 8 deg. F/min and preheated bath). Five different bobs were used, showing the preheated bath). Five different bobs were used, showing the effect of gap width and percentage of total fluid within the gap. Some bob-cup combinations allowed the gel plugs formed in low shear rate conditions to easily circulate back Into the gap and influence subsequent fluid viscosity. other combinations allowed little circulation or had minimal plugs. As a result of this study more accurate fluid rheology measurements can be made with the Fann Model 50 viscometer if a realistic thermal and shear history are used, along with a bobsleeve combination which maximizes the fluid contained within the gap and minimizes circulation of gel plugs back into the gap. Application of the findings in this work will help reduce the large differences typically observed between laboratories testing crosslinked fluids.
It has been known for some time that the heat-up rate and continuous shear rate during the crosslinking reaction can profoundly influence the resulting rheology of a delayed profoundly influence the resulting rheology of a delayed crosslinked fracturing fluid. At the same time, various petroleum industry users of the Fann Model 50 viscometer have petroleum industry users of the Fann Model 50 viscometer have devised their own standard methods for preparing, preconditioning, and measuring the fluid properties of preconditioning, and measuring the fluid properties of crosslinked fracturing fluids. It was desirable to determine these effects independently using a typical delayed titanium crosslinked HPG fluid. The intention was to be able to determine the magnitude of varying the operating conditions of the viscometer on measured fluid properties. From this information it was expected that an optimum bob and/or set of experimental conditions could be chosen to best represent the conditions fluids experienced in a fracturing treatment. In this study the various methods of preparing and preconditioning a fluid were not varied. Instead, a preexisting standard set of procedures was followed closely and only the effects of procedures was followed closely and only the effects of viscometer experimental conditions were varied.
There have been a number of approaches used to describe the shear field in the gap of a couette viscometer. However, generally accepted equations are known and were used in this study. These equations provided the nominal Newtonian shear rate in the gap, non-Newtonian factors for converting the power law model K y for the viscometer to either a pipe K p or a fracture K v, calibration of the viscometer by dead weight testing, and maintenance of calibration even upon changing of bobs.
Equation 1 converts the cup rpm to a nominal Newtonian shear rate in 1/sec.
After the n' and K' determination from a least squares fit of in(stress) vs. ln(y), the resulting viscometer K'v was usually converted to fracture K'f for use In fracturing simulators. To do this conversion one must have the non-Newtonian correction factors for the two geometries. Equations 2, 3, and 4 give the correction factors for the couette, fracture, and pipe geometries, respectively.