In the South Atlantic area, it is possible to found huge reserves of Pre-Salt fields which are common on the coast of Brazil and West Africa. According to available data, the huge oil and natural gas reserves are located below about 2000 m thick layer of salt which covers the sedimentary basins of Campos, Santos and Espírito Santo (Beltrão, 2009). Below it there is a large oil reserve estimated in approximately 150.000 km2 and 300 km from the coast. This area is known as “pre-salt polygon” (Reuters, 2020).
The discovery of the pre-salt represented self-sufficiency in Oil for Brazil, since historically the country has always needed to import it. According to the IBP (Brazilian Petroleum Institute), by 2035, Brazil will stop importing and will become an energy exporter. The first Pre-Salt Oilfield was discovered by Petrobras in the Santos Basin in 2006 and, since then, this type of application has grown fast and become very important (Beltrão et al. 2009). This boosted the demand of oilfield production chemicals, such as scale and corrosion inhibitors, emulsion breakers, biocides, among others, in order to ensure the treatment of several operational issues in these types of scenarios (Husseini, 2018). Among the main problems on operations are the appearance of inorganic salts scale, which can be difficult to remove and cause significant production losses (Haarberg et al. 1992; Kamal et al. 2018; Li et al. 2017).
There are different types of scale deposits in oilfields, the most common being calcium carbonate and sulfate scales (barium, strontium or calcium) mainly due to its low to moderate solubility in water, as can be seen in Table 1. The scale formation is influenced by many thermodynamics and kinects factors (Bai and Bai 2019; Crabtree et al. 1999; Hoang 2015; Kamal et al, 2018). Carbonate deposits, for example, are formed mainly due to the decrease in pressure and the increase in temperature. The solubility of CaCO3 decreases with increasing temperature (Dyer and Graham, 2002; Li et al. 2017). Through the release of carbon dioxide from oil during its production process, an increase in the pH of the medium is observed, which can promote, for example, the occurrence of carbonate incrustations (Atkinson and Mecik, 1997; Crabtree et al. 1999; Kamal et a. 2018). On the other hand, sulfate scale is present whenever there is a mixture of incompatible waters – waters that can chemically react with each other. This mixture occurs, for example, when injection of sea water is used in production wells on offshore fields (Abib et al. 2018).
Table 1
Solubility of the constituent salts of main types of scale (Lide, 2005)
Salt | Solubility (mg/100 g H2O) T(ºC) |
Barite (BaSO4) | 0,3120 |
Calcite (CaCO3) | 0,6620 |
Celestite (SrSO4) | 13,525 |
Anhydrite (CaSO4) | 20525 |
Considering the severity of Pre-Salt fields conditions, to prevent problems related to scale formation, an ideal inhibitor needs to have broad capacity to prevent several scales, tolerance to high salinity brines, temperature resistance and suitable pH range of action (Bai and Bai 2019). The study of scale inhibitors with strong chelating properties, high thermal stability and huge calcium tolerance had fundamental importance in this work. The development of a high-performance solution was achieved after dynamic performance evaluation, saturation index and mass of precipitate prediction on Pre-Salt fields conditions, as well as the chemistry type impact on brine compatibility, especially when submitted to high-stress scenarios. A promising PAPEMP based scale inhibitor was developed after achievement of satisfactory test results.