Abstract

This paper describes the laboratory development and field evaluation of crosslinker systems which delay crosslinking until after the fluid has passed the high shear region of the treatment. One system is temperature-activated at 45 °C, while the second system provides guaranteed, immediate, partial crosslink.ing plus high-temperature viscosity. This ensures adequate cool viscosity to carry high proppant concentrations through the perforations plus temperature-stable viscosity. Field testing confirms the advantages of these systems.

Introduction

Cross linked fracturing fluids were introduced to provide increased viscosities without increasing gel loadings, to extend the working temperature range of the fluids and to increase working tire of the fluids. The advent of these new f1uids necessitated the development of new methods for studying the rheological behavior of fracturing fluids. The viscoelastic nature of crosslinked fluids made them difficult to study by the traditional methods of parallel plate. cone and plate, and concentric cylinder rheometers.1,2 Viscosities produced using the API RP39 procedure (modified), 3 for example, were often either too high to measure or too difficult to reproduce. In addition, field experience with the new fluids indicated there was a difference between the crosslinked fluids made during an actual treatment and those which were prepared in a 1aboratory. Friction pressures were significantly lower than would be predicted from laboratory testing.4 Therefore, new methods were developed to simulate the way a fracturing fluid is prepared on location and the shear conditions it is exposed to during a treatment. Craigie,4 Gardener and Eikerts5 and Conway2 et al. reported methods in which a batch-mixed fluid is crosslinked while being pumped through tubing and then subjected to varying periods and rates of shear before its rheological properties are measured. In each case, significant differences were found between the initial viscosities attained by fluids crosslinked under high shear and those crosslinked under low shear or static conditions. In addition, the viscosity building which does take place is highly sensitive to heating and to further shearing. Time at shear and rate of shear were both found to be important in determining fluid properties. Borate crosslinked fluids have the ability to recover much of their viscosity when the shear rate is decreased. However, these fluids have a temperature limit of about 95 °C. The organometallic type crosslinkers which are used above that temperature do not give the fluids the ability to recover.

This new understanding of crosslinked fluid behavior under shear changed ideas about what could be done to improve fracturing fluids. One idea was to slow down (delay) the crosslinking reaction sp that most of the crosslinking would take place after the fluid has passed the high shear of the pumps, treating lines and wellbore. Then the fluid under high shear would be nearly like the uncrosslinked linear gel which is relatively insensitive to the shear rates normally encountered during a fracturing treatment. The fluid would develop most of its viscosity when it slowed down in the fracture and should be much more stable at high temperatures than fluid which crosslinks under high shears.

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