Characterization of Residual Hydrocarbons in Produced Water Discharged From Gas Production Platforms

Abstract

Abstract An investigation is described to select the best practical technique for reducing the residual practical technique for reducing the residual hydrocarbon in produced water discharged from gas production platforms to less than 40 mg/l. The aliphatic and aromatic components of the dissolved and dispersed hydrocarbon have been separately measured on a number of platforms, together with the droplet size distribution for the dispersed phase. In addition to previously recognized problems caused by high pressure drops together with corrosion inhibitor, disposal of condensed vapours from glycol regeneration has been identified as a major contributor. Significant reduction in the level of total residual hydrocarbon can be achieved in future by process modifications instead of 'end of pipe' process modifications instead of 'end of pipe' solutions. Introduction The Nederlandse Aardolie Maatschappij (NAM) produces gas from a number of offshore fields produces gas from a number of offshore fields in the Dutch sector of the North Sea. Free liquids (condensate and water) are separated from the gas and the water dew point of the latter is controlled with a glycol absorption dehydration process. Following pressure reduction of the liquids to typically 5 bar, the condensate is separated by gravity and is transported to shore with the gas in a two phase pipeline. Prior to discharging the water to the pipeline. Prior to discharging the water to the sea, it is further de-oiled. Simplified flowschemes of typical installations are shown in figures 1 and 2. Dutch legislation requires that from 1/1/1988, water discharged into the sea from offshore production platforms may not contain more than production platforms may not contain more than 40 mg/l residual hydrocarbon. Recognising the difficulty which has been experienced by the operators of many gas/condensate platforms in meeting the specified limit, an exemption is allowed for the produced water stream from these platforms, provided that the best practical technique for cleaning the water is utilized. In comparison to typical oil production platforms in the North Sea, the quantity of produced water discharged is very small, ranging from ca. 40 to 150 m/d per platforms. On 1/1/1988, NAM was discharging produced water into the sea from 9 gas platforms and the specified limit of 40 mg/l residual hydrocarbon was exceeded, to varying degrees, from all platforms. Most water clean-up techniques which platforms. Most water clean-up techniques which have been considered practical for offshore installation, remove only dispersed oil. A better understanding of the problem was required, in order to define modifications necessary to reduce the residual hydrocarbon and to demonstrate that the best practical technique has been employed. The investigation focused on the separate quantification, at a number of key points, of the aliphatic and aromatic content of points, of the aliphatic and aromatic content of the dispersed and the dissolved components of the residual hydrocarbon in the produced water from all of the platforms. Additionally, the droplet size distribution of the dispersed oil was measured on a number of platforms. The data obtained has enabled treatment equipment to be retro-fitted to a number of platforms, with high confidence that it will meet platforms, with high confidence that it will meet the required target. It has also indicated where upstream modifications to the process can be made to achieve further reductions in discharged hydrocarbon. Areas requiring further development work are identified and a number of important process concepts have been identified for inclusion into future designs, with the aim of reducing the hydrocarbon content of the water at source, rather than with 'end of pipe' solutions. pipe' solutions. P. 1