Summary
To ensure that a fracturing treatment will be successful, fracturing gels
are formulated and viscosity parameters are measured. Steady-shear viscosity
cannot measure the elastic component of viscoelastic fluids.
Dynamic-oscillatory shear can provide measurements of elastic properties, but
such measurements are made in the absence of any particles. A third technique,
a slurry viscometer (SV), can incorporate particles and measure
proppant-transport characteristics of the fluid. This paper compares all three
measurement techniques for verification of transport prediction.
Steady-shear viscosities of gelled fracturing fluids were measured using
couette viscometers. Viscoelastic characteristics of the same fluids were
measured with a dynamic-oscillatory viscometer to determine G' and
G" moduli and crossover frequency. Proppant particles were added to
these same fluids, and properties were measured with the SV to determine
elastic transport and viscous transport times.
Comparison of the three data sets shows that the elastic modulus, G'
, and crossover frequency have a high correlation with elastic transport times
measured with the SV. Steady-shear viscosities greater than 10 sec–1
do not correlate directly with transport times.
For years, dynamic-oscillatory measurements have been thought to predict
particle-transport phenomena, but any correlations generated could be
incomplete because particles are excluded from the measurements. The SV does
include particles in the fracturing gel and can be used to verify trends of
proppant transport indicated by G' and the crossover frequency.
Introduction
The intent of a fracturing treatment is to create a fracture, pump a slurry
of proppant and fluid into a subterranean formation, and transport proppant
into the fracture. The majority of fracturing treatments employ a viscoelastic
fluid to enable proppant transport. Viscous fluids with varying degrees of
elasticity include polymer solutions, crosslinked-polymer gels, foams,
emulsions, and surfactant gels. It is common practice to measure the viscosity
of such fluids with couette viscometers for both design and for
quality-assurance/quality-control (QA/QC) purposes (Cameron and Prud'homme
1989; RP 13M/ISO 13503-1 2004). Service companies may also include gel breakers
in test fluids for the purpose of estimating a time to reach minimum viscosity
for transporting proppants.
The elastic character of a fluid is an important component of viscoelastic
fluids and their ability to transport proppant (Harris and Walters 2000; Geol
and Shah 2001). Elastic character is not reflected in typical steady-shear
measurements. A dynamic-oscillatory rheometer can generate signals to
separately identify the elastic property. The elastic response is given by the
G' storage modulus, and the viscous response is reflected in the
G" loss modulus. The relative magnitudes of G' and
G" vs. oscillation frequency produce a crossover frequency that can
be used to infer an ability to support and transport particles, even though
particles are not a part of the measurement.
A third type measurement can be made with an SV (Harris et al. 2005; Walters
et al. 2004). The inner and outer cylinders of a typical couette viscometer
were replaced with a stator and outer cylinder, each having flags attached.
These flags bypassed each other during rotation of the outer cylinder, and the
cyclic shear induces a torque on the stator. Sufficient clearance between the
rotating and static flags allowed proppant particles in the test fluid.
Placement of the flags near the bottom of the SV make it sensitive to the
larger torque values as proppant settles during the experiment. The torque
value can be interpreted for both elastic and viscous properties. The SV gives
an indication of slurry viscosity in the wellbore. However, the primary
function of the device is to help determine the suspension properties of the
fluid when it is in the fracture.
This paper will examine these three methods of measuring fracturing-fluid
properties. Steady-shear viscosities measured at shear rates greater than 10
sec–1 provide no direct indication of proppant transport, and the
choice of minimum viscosity is arbitrary, on the basis of field experience with
the fluid. The dynamic-oscillatory technique provides a method of estimating
elasticity, and though proppant is not included in the test fluid, the
crossover frequency can correlate with settling measurements. The SV apparatus
enables a dynamic measurement of settling in fracturing-fluid slurry.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
26 June 2008
- Meeting paper published:
21 September 2008
- Revised manuscript received:
9 February 2009
- Manuscript approved:
31 March 2009
- Published online:
13 August 2009
- Version of record:
25 November 2009
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