Donald Edward Putzig

Chemical Structures of Group 4 Metal Crosslinkers for Polygalactomannans

In SPE 37280 and SPE 39531, Harry et al., and Moorhouse, et al., respectively, demonstrated that seemingly minor variations in aqueous based crosslinker chemistry can produce significant differences in the rheological responses of polygalactomannan-based fracturing fluids crosslinked with these agents. While the work contained some references to boron crosslinking and the principle conclusion can be applied to both boron and zirconium fracturing fluid systems, the emphasis was on zirconium crosslinked systems. Not surprisingly, some of the observed differences in performance could be attributed to variations in the crosslinker chemistry. Crosslinker differences that were isolated and evaluated included differences in the ligand chemistry, the ligand-to-metal-ratio, the nature of the zirconium starting material, etc. Moorhouse, et al., suggested several mechanisms by which these phenomena might be explained. The authors stopped short, however, of making any attempt to correlate the chemical structure of the crosslinker with its rheological performance. With this paper the authors advance their studies to present a more comprehensive examination of the proposed structures of titanium and zirconium alkoxides and aqueous-based chelates, including a general discussion of the methods employed to manufacture these crosslinkers and factors present in Group 4 metal crosslinkers that may affect their crosslinking response with polygalactomannans. There is also some discussion on other methods that have been employed to control the gelation rate. Finally, there is some discussion on the effects of polymer attributes to fracturing fluid performance, particularly as it relates to crosslinker efficacy. The findings presented in the paper are supported by laboratory analyses that includes the examination of crosslinking agents by nuclear magnetic resonance (NMR) imaging, X-ray crystallography, and the rheological performance evaluation of systematically altered crosslinking agents. The authors suggest chemical structures of titanium and zirconium compounds used to crosslink polygalactomannans as supported by the findings of the analytical evaluations already mentioned.