In preparation of a gelant solution for making crosslinked polymer gels for water shutoff applications, unpublished experiments and chemical intuition suggest that, unless hydrolyzed polyacrylamide (HPAM) polymer is fully hydrated before addition of crosslinker, the final gel will have lower than optimum mechanical strength, presumably because polymer chains need to be fully unfolded before proper crosslinking can occur. When using dry polymer, which is usually the lowest cost form on a delivered basis, this may require more equipment and a large tankage footprint. However, if conditions exist where crosslinker can be added to wetted but not fully hydrated polymer, then dry polymer and crosslinker can be blended in a small continuous flow unit, with full hydration occurring as the gelant flows downhole prior to gelation.

We have evaluated gel strengths of "flowing" gels for water shut off in natural fractures and other non-matrix features as a function of time of addition of crosslinker relative to time of hydration of polymer. Gels were prepared from moderately high molecular weight HPAM crosslinked with chromium(III) acetate (CrAc) or polyethyleneimine (PEI). Crosslinker was added after either (1) initial wetting of solid polymer particles or (2) complete dissolution of the polymer.

Gel strengths were determined using a common qualitative coding system. Comparisons were made for gels prepared in an identical manner, except for the timing of crosslinker addition. Samples were prepared either in fresh water or 4% NaCl brine and then hydrated either at an ambient temperature or 122 °F. Gelant viscosity and crosslink time were also characterized with a viscometer.

Results of this work demonstrate that for most field applications using CrAc as crosslinker, optimum quality gel can be obtained using dry polymer and a small continuous mixing system for initial wetting of the polymer, after which the crosslinker can be added to the polymer solution on-the-fly. This practice can decrease the footprint and cost of large volume flowing gel treatments.

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