Effect of Water Salinity on Barium Sulfate Formation in the Context of Gas Wells in the Dutch Sector of the North Sea

Abstract

Abstract Operator has several gas wells which encountered barium sulfate scale. In one instance this had resulted in a complete blockage of the well and in another well not being able to perform a planned well intervention. This paper will present the investigation of the mechanism of formation of scales in combination with an overview of laboratory and field data on how to treat the barium scales. The production water of one of the wells is characterised by a very high salinity level close to saturation in combination with presence of barium, strontium, calcium, carbonate, and sulfate. The evaluation of the scaling risk has been carried out from bottomhole to wellhead with commercial software; 80 points modelling have been performed. This has helped to determine the scaling mechanisms for individual scales. Barium scale from a second well has been used to select an effective dissolvent via laboratory experiments. The evaluation of the scaling risk has been carried out with a help of a commercial software. The predicted scales are halite, barite, celestite, strontianite and calcite, while mainly barite was found in the recovered deposit. The difference between modelling results and field observation are discussed in the paper. As halite concentrations exceeded saturation levels, downhole water injection was applied in one of the wells to stay below the saturation levels, due to water evaporation caused by drawdown. However, the reduction of salinity decreases the solubility of barium sulfate which in combination with decreasing temperatures seriously increases the risk of precipitation of barium sulfate. This eventually resulted in a complete blockage of the well. For the second well, barium scale samples were tested in the laboratory to select the best dissolvent. This dissolvent had been batch injected in the upper part of the well to dissolve the scale. Collected production water at surface immediately after opening of the well, showed increased levels of barium, strontium, and sulfate. Continuous downhole injection of fresh water to compensate for evaporation losses due to drawdown should be carefully evaluated when barium sulfate scaling risk exists as the solubility decreases at lower salinity levels. One of the mitigation methods is therefore to add inhibitors to the injected water.