Résumé -Étude théorique et expérimentale de la précipitation de sulfate de calcium en milieu poreux sur micromodèle de verre -Le mélange de deux eaux incompatibles lors d'injection d'eau donne habituellement lieu à précipitation et formation de dépôts minéraux dans les milieux poreux. Ces dépôts qui réduisent la porosité et surtout la perméabilité de la roche ont une influence considérable sur les performances des scénarios d'injection d'eau. Dans cette étude, une série d'expériences a permis d'étudier l'effet de différents paramètres sur la précipitation progressive de sulfate de calcium. Il s'agit notamment de la température, de la concentration des saumures mélangées, de la pression et du débit. En raison de sa transparence, un micromodèle de réseau représentatif d'un grès mouillable à l'eau a été utilisé comme support poreux et a permis d'observer facilement la formation et la distribution des dépôts minéraux. De plus, le suivi du déplacement des particules solides est largement facilité par le nouveau dispositif expérimental proposé. Les photos de coupes obtenues en microscopie montrent que le dépôt est initié sur les parois et les seuils des pores, qu'il se propage vers l'intérieur de ces pores, et que les cristaux solides ont une forme en crête de coq. Pour une meilleure compréhension de l'effet de chacun des paramètres mentionnés ci-dessus, il a été tracé une courbe de la réduction de perméabilité en fonction du volume de solution de saumure injectée dans les pores. Les résultats ont indiqué que l'augmentation de la température, de la concentration de saumure et du débit conduisent à une augmentation de la tendance au dépôt de minéraux. La pression n'a qu'un rôle mineur sur le développement du processus. Le dépôt de CaSO 4 se modélise comme une fonction de réduction de la perméabilité dépendante des différents paramètres. C'est pourquoi il a été proposé une fonction exponentielle (de corrélation) qui intègre sous une forme adimensionnelle tous les paramètres physiques ayant une influence sur le comportement du système. Le nombre de Reynolds, l'index de précipitation et l'écart par rapport aux conditions d'équilibre constituent le fondement de cette corrélation. Les exposants ajustables de l'équation ont été déterminés et optimisés au moyen d'un algorithme génétique. Cette corrélation significative peut également prédire, avec une précision raisonnable, les résultats issus d'expériences sur carottes. Abstract -Experimental and Theoretical Study of Calcium Sulphate Precipitation in Porous Media
Water injection is usually associated with various types of mineral scale formation and deposition which significantly undermine the performance of the injection by changing some reservoir properties such as permeability and porosity. Mineral scale deposition in surface and subsurface petroleum streams is one of the main concerns affecting the economy and safety of any production scenarios. Among various types of mineral scales, calcium sulfate is one of major scales in petroleum industry that can cause sever flow assurance and formation damage issues. Many parameters are affecting this problem. Temperature, pressure, fluid concentration, ratio of brine to hydrocarbon, fluid dynamic and type of porous media are among these parameters. Interpretation and prediction of permeability reduction in such complicated system is very complex. In this work an experimental and theoretical study has been conducted to investigate the permeability reduction due to CaSO4scale formation. For this purpose, brine solutions containing calcium and sulfate ions were injected in a packed glass beads porous media at elevated temperatures and various flow rate and brine concentrations. The pressure decline and permeability changes were measured in all the conducted tests. It was found that the permeability reduction by calcium sulfate scale formation follows a systematic trend considering various above mentioned important parameters. Hence, a novel empirical equation has been suggested, which can predict permeability reduction rate with high accuracy to experimental data. Introduction The formation of mineral scale due to flowing of fluid containing saline solutions in porous media is a common problem in many industries, such as oil and gas production, geothermal and membrane separation 1–6. Reliable prediction of calcium sulphate scale in production from oil and gas fields is a challenging and complex issue. This issue has been investigated and addressed by numbers of researchers in open literature. Among them, Vetter and Phillips2 described a thermodynamic model to predict the equilibrium condition of CaSO4 scale formation under down hole condition. Shen and Crosby4 investigated the mechanisms of CaSO4 scale deposition in oil producing wells. Villafafia Garica et all.5 presented a sulphate scale prediction method based on the extended UNIQUAC model. Hennessy and Graham7 reported the effect of additives on co-crystallisation of calcium with barium sulphate scale deposition. Almost in all available works in open literature, the complexity of prediction of various scale formation has been described and addressed. From the reservoir and downhole to surface conditions there are wide ranges of temperature, pressure and fluid compositions. The formation of various calcium sulphate crystalline forms including the anhydrite (CaSO4), dehydrate or gypsum (CaSO4, 2H2O), hemihydrates (a-CaSO4,½H2O or ß-CaSO4,.½H2O) are affected by changes in temperature and pressure and ionic strength, however, it is more sensitive to temperature rather than pressure 2,8. The solubility and formation of various crystalline forms has been schematically shown in Fig. 1. Yuan and Tood6 investigated the effect of brine ion concentrations and temperature and pressure on sulphate based scales precipitation. The formation of scales can cause many problems such as, plugging of equipment and producing formation pores. The prediction of the scale formation in well production path and the associated permeability and porosity reduction rate, therefore, are necessary for the success of the operation. For better understanding the permeability reduction mechanism due to calcium sulphate mineral scale formation, various important parameters need to be taken into account. These parameters can include the kinetic of precipitation reaction, morphology of crystalline form and transformation from one to another, the rate of calcium sulphate crystal growth, surface properties of porous media, the effect of pressure and temperature, the role of flow dynamic to entrainment of fine particles and the interactions of ions, fluid-fluid and fluid-rock interactions1,5,8,10. Interpretation and prediction of permeability reduction in such complicated system is very complex, as, the role of these important parameters needs to be considered. In this work, some of these parameters have been experimentally and/or theoretically studied. In the first part of the work, the carried out experimental tests have been described. The development of a novel empirical correlation for prediction of permeability reduction due to calcium sulfate scale formation based on the experimental work is then explained.
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