Prediction of sulphate scaling tendency and investigation of barium and strontium sulphate solid solution scale formation
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Sulphate scale occurrence is one of the major production problems encountered during waterflooding processes in oilfield developments. In particular, as sea water injection is a common practice in North Sea oil operations, severe production problems are caused by sulphate scale deposition in the production facilities, also concern is arising of the potential formation damage in the near producing well bore zone due to scale precipitation. Of all the scales, barium sulphate precipitation is the most dominant scaling problem in North Sea offshore fields and it is commonly accompanied by strontium sulphate to form barium and strontium sulphate solid solution scale, which has distinct features in terms of scaling crystal morphology, size and hardness. This study was devoted to predict the scaling tendencies of barium sulphate, strontium sulphate and calcium sulphate scales and to investigate the formation damage arising from (Ba,Sr)SO4 scale formation in the porous media. A theoretically consistent model was developed in this study for predicting the sulphate scaling tendencies in single brines or due to mixing incompatible brines, such as seawater and formation water, by calculating the supersaturations and amounts of precipitation of the sulphates at temperatures and pressures covering surface and reservoir conditions. The model is able to predict competitive simultaneous coprecipitation of BaSO4,SrSO4 and CaSO4 of which sulphate is the common ion, reflecting closely the precipitation of more than one sulphate mineral. The scaling tendencies predicted from this model agree well with field observations. The computer programme of the model is compact, optional and user-friendly. The scale prediction model is based on a solubility model which was also developed in this study from the Pitzer equation for electrolyte mean activity coefficient, an approach widely used for calculating properties of aqueous electrolyte solutions because of its sound theoretical basis and accurate representation of electrolyte properties. The predicted sulphate solubilities from the solubility model agree with the published data within the experimental measurement error. Experimental investigation of the (Ba,Sr)SO 4 scale formation was carried out in static bulk solutions and under flow influence in sandstone cores by mixing two incompatible waters. The brines used in the study were both simple artificial brines and full component synthetic North Sea water and formation waters. The rock cores were multi-pressure tapped and the pressure data recorded during the core flow tests were converted to permeability changes. The formation damage due to scaling was examined by studying the rock permeability decline as well as porosity reduction. The scaling crystals and scale distribution within a core were examined by scanning electron microscopy. The experimental results show substantial scale build-up in the cores and large permeability loss resulted from concurrently flowing North Sea water and field waters and from concurrently flowing two incompatible simple brines through cores. The scale nature and permeability damage were largely dependent on sulphate supersaturation and temperature and they were also affected by the change in the ratio between the scaling ion concentrations. The external morphology of the scaling crystals formed from mixing the sea water and formation waters differed significantly from the morphology of those crystals precipitated from the mixed simple brines, suggesting the influence of the presence of the foreign ions other than sodium and chloride ions on scale nature. It is concluded from the study that the scale formation was a rapid process initiated by heterogeneous nucleation and sustained by scaling crystal growth and deposition on the rock pore surface. The sulphate scaling tendency prediction model and the data acquired from the experimental study on formation damage due to barium and strontium sulphate solid solution formation have potential for use in a reservoir simulation model of scale formation.