Thermal stability of amine methyl phosphonate scale inhibitors

Abstract

The performances of five different amine methyl phosphonate-based scale inhibitors have been tested against sulphate and carbonate scale after thermal ageing at 160 °C, to determine whether they can be applied in HP/HT reservoirs. The successful application of phosphonate scale inhibitors in these reservoirs would result in significant cost savings when compared with some of the polymeric species that are currently used. These tests have indicated that after thermal ageing at an initial pH of pH 5 and pH 2 all five inhibitors were still able to prevent carbonate scale in dynamic tests. However, the performances of some of the phosphonate species against sulphate scale were reduced by thermal ageing. The performances of HMDP, HMTPTP and NTP against sulphate scale were also reduced after thermal ageing at 190 °C and pH 5. Ion exchange chromatography has shown that the reduction in inhibitor performance against sulphate scale corresponded to the structural breakdown of the phosphonate molecules. Potential breakdown products of the phosphonate species have been determined theoretically. The performances of commercial samples of these potential breakdown products have been tested against both carbonate and sulphate scale to determine experimentally whether they could be the products formed during thermal ageing. It is suggested that the thermal stabilities of the phosphonate species are determined by their molecular structure, with the species containing hexyl linkages between the amine groups having less steric strain and hence greater thermal stability than the species which only have shorter ethyl linkages between amine groups.