Evaluating and Inhibiting Gum Formation Tendencies in Gas Condensates

Abstract A report is presented on the evaluation of gum formation tendencies of natural gas condensates from two gas fields in the Niger Delta region of Nigeria. Following reports that certain condensates from the region have high gum formation tendencies and thus their market prices devalued, a study was embarked upon to evaluate and inhibit the formation tendencies of these condensates, if any. The reports indicated that there was general consternation around high unwashed gum contents and this parameter was sometimes considered in conjunction with the washed gum contents when devaluing the condensates in question. To investigate the suitability of current evaluation methods, a review of ASTM D381, the gum test method developed for gasoline and jet fuels which is now being applied to gas condensates, was carried out. Further tests such as fingerprinting, boiling ranges and asphaltene content were also carried out. To investigate the effectiveness of anti-oxidant injection in inhibiting gum formation, two products were selected and injected into a condensate sample at varying concentrations and the gum content of resulting mixture was tested using ASTM D381. The results show that while ASTM D381 was developed for gasoline and jet fuels, its use for these products is not firmly established and thus, its extension to gas condensates is even more uncertain. This is because heavy hydrocarbon fractions which are not present in processed fuels are present in condensates. Thus, the typical values of unwashed gum content in refined liquids cannot be expected of condensates where they are bound to be significantly higher. For the samples under-investigation, confirmatory tests using boiling ranges as a complement to the n-heptane washing step of ASTM D381 supported the observation that >60% of the unwashed residue was not gum, but instead naturally-occurring heavy hydrocarbon fractions which would not normally evaporate under the test conditions of ASTM D381. Furthermore, the use of antioxidants at high concentrations showed a decrease in the washed gum content. However, the decrease was only achieved at very high concentrations. Insights made from this study presents the industry with evidence that the existing method for evaluating gum content in only applicable to gas condensates as far as the washed gum content in concerned. Condensate quality concerns based on unwashed gum content are unfounded and thus, should not be considered by suppliers and buyers of condensates. Furthermore, it is recommended that the further work be carried out to develop and/or evaluate antioxidant chemistries that are cost-effective and can perform at lower concentrations.