Evaluation of the performance of different scale inhibitors to sulphate removal unit

Abstract

Abstract The Sulfate Removal Unit (SRU), a nanofiltration system, reduces seawater's Sulfate ions from approximately 2,800 mg/L for 100 mg/L or less. This technology minimizes the formation of Sulfate scales in the oil production. This process uses pressure as a driving force and it uses membranes with nanometric pores that are composed of polymeric material. Decrease in permeability is frequently observed in this nanofiltration process due to the deposition in the surface of the membrane (biofouling and inorganic deposits). The objective of this study is the evaluation of scale inhibitors for Calcium Sulfate that can be formed in the membrane as a consequence of the concentration of the seawater in the SRU. The feed, permeate and concentrate fluids of the SRU were characterized. The chemical composition of the seawater in the polarization region (fluid located in the membrane surface) was estimated considering mass transference between permeate and feed fluids of the SRU, and Fick Law. The chemical composition of the seawater in the polarization region presents higher concentration of Calcium, Magnesium and Sulfate than the one in the concentrate fluid (reject). So the polarized seawater is highly saturated regarding Calcium Sulfate. Thermodynamic model MultiScale was applied to assess the scaling tendency (saturation ratio and precipitate mass) in specific points of the SRU under temperature and pressure conditions. The performance of different chemical classes of scale inhibitors was evaluated. Chemical compatibility of these chemicals with seawater in the polarization region, as well as their inhibition efficiency to Calcium Sulfate precipitation in the conditions of the surface of the membrane were evaluated. Introduction The membrane separation process has grown and been developed in recent decades. The nanofiltration is a type of the membrane separation process that uses membranes with nanometrics pores (in this paper, whenever it mentions the word membrane it refers to the one with nanometric pores used in the Sulfate removal process). One of the applications of this process is the desulfation of the seawater which is treated to be injected in oilfield wells. Scale formation may occur as a consequence of incompatibility between the Barium and Strontium ions in the formation water and high concentration of Sulfate ions in the injected seawater. These precipitates form near the production well bore, perforations, tubing and subsea equipments impairing oil production (Bezerra et al 1996). One way to prevent this problem is to remove the Sulfate ions from seawater, before injection. The main aim of this process is to remove Sulfate ions selectively, allowing Sodium and Chloride ions to remain in the seawater. So that reservoir clays are not affected by low salinity water injection. This process was developed for Marathon Oil Company and was applied in 1988 on Brae field. The application in offshore fields was patented by Dow Chemical (manufacturer of the membranes) and Marathon Oil (responsible for the process) and since than the technology has been improved to make systems more efficient (Sinclair et al 1996). This process uses pressure as a driving force, it shows difference in selectivity of the ions, and it allows to get a permeate rate around 75 % (Davis et al 2002 and Davis et al 1996). Figure 1 shows a simplified scheme of nanofiltration system (SRU) for seawater desulfation. In this process seawater is fed on the permeation module and it generates two new fluids: permeate and concentrate fluid. The permeate fluid has a low concentration of Sulfate and the concentrate fluid as a consequence has a higher concentration of Sulfate (reject of the unit). The permeate fluid is injected in the reservoir to keep the pressure and improve the sweep efficiency of the oil, which is important to increase the oil recovery in the offshore field.