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).
