Crosslinking of Guar and Guar Derivatives
- Cuiyue Lei (The University of Alabama) | P.E. Clark (The University of Alabama)
- Document ID
- Society of Petroleum Engineers
- SPE Annual Technical Conference and Exhibition, 26-29 September, Houston, Texas
- Publication Date
- Document Type
- Conference Paper
- 2004. Society of Petroleum Engineers
- 4.3.1 Hydrates, 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.8 Formation Damage, 2.5.2 Fracturing Materials (Fluids, Proppant)
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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 information about 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 will be examined.
The polymer literature is filled with methods of characterizing polymer solutions almost none of which find wide use in the development of crosslinked fracturing fluids. Dawson, et. al.,1 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 a guar or guar derivative solutions. This finding was controversial, to say the least. To test this assertion, we have conducted a series of experiments that not only show that the transition concentration is an important indicator, but also present a framework for exploring the potential of other crosslinked systems. Interestingly, polymers that crosslink well at low concentrations do not always produce the best crosslinked gel at higher concentrations. Based on our experiments, we will present an explanation for this finding. This emphasizes the importance of understanding the crosslinking process in order to optimize the selection of a polymer for a particular application.
This paper examines guar and guar derivative systems used in the industry and reports on a new guar that can be successfully crosslinked at low concentrations. It also provides a outline for studying crosslinking at low concentrations and presents insights into the crosslinking process.
Water-soluble polymers have been used for a number of years as thickening agents for stimulation fluids. In order to improve the performance of these materials without increasing the concentration, crosslinking was developed. Over the years, a number of different crosslinking agents have been used with success. There are several good discussions of polymer and crosslinking systems available2,3, 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 versus concentration curve appears to be exponental. This phenomenon has been studied for a wide range of polymer - solvent systems and appears to be universal. Early work divided the viscosity - concentration curve into two regions4,5 that were separated at a critical concentration labeled c*. As the understanding of polymer solution behavior developed and more sensitive instruments and experimental techniques were developed, the solution behavior was determined to be more complex.
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