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Introduction

Since 1983, the Gas Research Institute (GRI) has funded research to improve the knowledge and understanding of the viscous properties of fracturing fluids. One of the GRI research projects was the development and testing of the GRI Rheology Unit, which is a mobile rheology laboratory that can be taken to the field to measure the viscous properties of the actual fluids being pumped during the treatment.

The GRI Rheology Unit consists of a van containing two Fann Model 50 viscometers and a complete wet chemistry laboratory. In typical field operations before the treatment begins, the Rheology Unit is used to check the fracturing fluids for contaminants and to verify the fracturing fluid, mixed with the chemicals to be used during the treatment, meet the design specifications. During the treatment, the Rheology Unit is used to sample and test the fluids being pumped to ensure fracture fluid quality.

Field research with the GRI Rheology Unit has found that laboratory measured fracturing fluid properties are often not reproduced under field conditions. During the majority of treatments monitored with the GRI Rheology Unit, fluid problems were diagnosed by testing the gel using the Fann Model 50 viscometers. One of the most common fluid problems experienced in field operations is the scheduling and addition of the chemical breakers used to degrade the fracturing fluid after the treatment. Typical problems associated with the breakers were either (1) the wrong breaker type was being used or (2) the wrong amount of breaker was added for the given conditions.

Traditionally, very low breaker concentrations are added to degrade fracturing fluids. Recently, new fracture treatment techniques have been proposed that include aggressive" or very high breaker concentrations. Cooke demonstrated that the polymer concentration remaining in the fracture after closure is several times higher than the polymer concentration mixed at the surface because of leakoff. Breaker concentrations should, ideally, be sufficient to degrade the polymer concentration remaining in the propped fracture. However, the addition of enough breaker to sufficiently degrade the polymer concentration remaining in the fracture after closure is typically impractical because of rapid fluid degradation during the treatment.

To investigate the problems with scheduling breakers in HPG solutions and gels, a two part research project has been completed. In tile first part of the research, the variables affecting HPG solution and gel degradation by enzyme and oxidative breakers were examined ill a series of static break tests. ['be results of the first part of the research have been reported by Craig and Craig, Holditch, and Howard.

In the second part of the research, steady shear viscosity measurements were made with the GRI/Texas A and M University (TAMU) Rheology Flow Loop to evaluate the effects of breakers on the short term (less than 3 hours) viscous properties of HPG gels. The GRI/TAMU flow loop is a laboratory apparatus capable of measuring the viscous properties of hydraulic fracturing fluids with three different viscometers—a pipe viscometer, a Brookfield process viscometer, and a Fann Model 50 viscometer. The purpose of the flow loop at TAMU is to develop and evaluate techniques that can be used in the field with the GRI Rheology Unit to test fluid quality before and during actual fracture treatments. As such, we are also investigating the effects of Fann Model 50 bob and cup size on the measurement of viscosity for water-based crosslinked fracturing fluids. For this research, the GRI/TAMU Rheology Flow loop has been used to evaluate the effects of breakers on the viscous properties of HPG gels, and to compare the viscous property measurements from the Fann Model 50 using extended B2 and extended B5 bobs with the measurements from the flow loop pipe viscometer.

OBJECTIVES

The specific objectives of the work to be presented in this paper are described as follows:

  1. Compare the viscous property measurements from a flowloop pipe viscometer, Fann Model 50 viscometer with an extended B2 bob, and Fann Model 50 viscometer with an extended B5 bob to determine the appropriate bob to use in the field.

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