Summary
Crosslinking of guar and guar derivatives has played a major role in
improving stimulation of oil and gas wells. While crosslinking has been used
for a number of years, many facets of crosslinked systems are still not well
understood. Part of the problem is that the traditional methods of determining
the properties of crosslinked fluids work well for obtaining the data necessary
for treatment design, but yield little insight into the nature of the
crosslinked system. A good example of this is found in the development of low
polymer concentration crosslinked gels. These gels are important because they
lower costs and help to minimize formation damage. In this paper, methods for
predicting crosslinkability at low concentrations are reported.
The polymer literature is filled with methods for characterizing polymer
solutions almost none of which find wide use in the development of crosslinked
fracturing fluids. Dawson et al. (2000) first reported that the concentration
at which a polymer solution transitions from dilute to semidilute could be used
as a method for determining the potential for low concentration crosslinking in
guar or guar-derivative solutions. To test this assertion, we have conducted a
series of experiments that not only shows that the dilute-semidilute transition
concentration is an important indicator for the polymers used in this study,
but also presents a framework for exploring the potential of other polymer
systems. These experiments show conclusively that low-polymer concentration
crosslinking is strongly related to the value of the critical overlap
concentration, c*. Both the critical overlap concentration and the critical
crosslinking concentration increase in the order guar-3 < CMG < CMHPG
< guar < HPG. In addition, we show that the critical crosslinking
concentration for the range of polymer-crosslinking systems studied is
correlated to the critical overlap concentration ( ). A strong case is
presented for the ability to crosslink at low concentrations is a strong
function of the polymer type and a weak function of the crosslinker type.
Introduction
Water-soluble polymers have been used for a number of years as thickening
agents for stimulation fluids. Crosslinking was developed to improve the
performance of these materials without increasing polymer concentration. Over
the years, a number of different crosslinking agents have been used with
success. There are several good discussions of polymer and crosslinking systems
available (Economides and Nolte 1989; Gidley et al. 1989), and it is not the
purpose of this paper to expand these discussions. Rather, we will approach the
problem of crosslinking from the prospective of the molecular or solution
properties that control or influence crosslinking.
When a water-soluble polymer is hydrated, the viscosity of the resulting
solution increases as a function of concentration. Starting at low
concentrations and building to high concentrations, the viscosity appears to
exhibit an exponential increase. This phenomenon has been studied for a wide
range of polymer—solvent systems and seems to be universal. Early work divided
the viscosity—concentration curve into two regions (Menjivar 1986; Robinson et
al. 1982) that were separated at a critical concentration labeled c*. As the
understanding of polymer solution behavior progressed and more sensitive
instruments and experimental techniques were developed, the solution behavior
was determined to be more complex (Rubinstein and Colby 2003).
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
7 June 2004
- Meeting paper published:
26 September 2004
- Revised manuscript received:
27 February 2007
- Manuscript approved:
31 May 2007
- Version of record:
20 September 2007
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