Application of Low-Toxicity Crosslinking Systems in Production of Thermally Stable Gels

Abstract

Abstract Earlier reports from our laboratory described a number of thermally-stable water-soluble polymers which tolerate extensive aging times in brines with high salinity and hardness levels at temperatures up to 149° C (300°F). These polymers can be crosslinked with phenol and formaldehyde crosslinking system to produce stable gels under hostile environment conditions typically defined as temperatures higher than 167° F (75° C) and brine hardness levels above 2000 ppm. The resulting gels can be used for profile modification and water shutoff treatments under hostile environment conditions. In one paper we proposed a mechanism for the gelation of acrylamide containing polymers. This mechanism postulates the formation of salicyl alcohol as an intermediate through the condensation of phenol and formaldehyde. Formation of a thermally stable gel by direct reaction of salicyl alcohol and formaldehyde, with a thermally stable acrylamide containing polymer, is consistent with this mechanism. Concerns about the toxicity of phenol and especially the carcinogenic character of formaldehyde motivated us to search for alternative crosslinkers with lower toxicity, which can be used in place of phenol and formaldehyde to produce stable gels. Through an extensive study, we have identified and tested several compounds which can be used in place of phenol. These compounds include o- or p-aminobenzoic acid, m-aminophenol, aspirin, furfuryl alcohol, methyl phydroxybenzoate, phenyl acetate, phenyl salicylate, salicylamide and salicylic acid. The only suitable compound identified in our study, which can be used in place of formaldehyde and produce stable gels, is hexamethylenetetramine (HMTA), a precursor of formaldehyde. HMTA thermally hydrolyzes to formaldehyde and ammonia at elevated temperatures which will go on to crosslink with acrylamide containing polymers and phenol or its replacements. However, not all formaldehyde precursors can produce thermally stable gels when used in combination with phenol or with an alternative compound to phenol. No suitable gels were produced with glyoxal or 1,3,5-trioxane as formaldehyde precursors. This paper will describe a number of low toxicity thermally stable gels suitable for applications under hostile environment conditions. The gels described here exhibit a lot of flexibility in terms of gelation time. With the choice of proper crosslinking system, one can produce gels which can set from a few hours to several months. The slower gelation systems could be used for in-depth treatments of hot reservoirs such as those in the North Sea. The bulk gels produced in synthetic seawater or brines containing up to 20% sodium chloride and 2% calcium chloride, have remained stable for several years of aging at 250° F (121° C). This study has only been directed at evaluating thermal stability of bulk gels. The performance of these gels in porous media will be the subject of future studies.