Document Type : Research Paper
1 Ph.D. Petroleum, Department of Petroleum Engineering, School of Chemical and Petroleum Engineering, Shiraz University, Shiraz, Iran
2 Assistant Professor,Department of Petroleum Engineering, School of Chemical and Petroleum Engineering, Shiraz University, Shiraz, Iran
Sarvestan and Saadatabad oilfields produce more than 140 bbl/day of wastewater due to oil processing. Due to environmental issues, the produced water is injected into a disposal well through a pipeline with a diameter of 8 inch and a length of 5 km. Formation of inorganic scale may accelerate the need for frequent reservoir acid stimulation, restrict flow path, and generally add unpredicted costs for water injection operations. This study predicts scaling tendency and examines scale precipitation at different pressures, temperature, and mixing ratios of injection wastewater with formation water in Sarvestan and Saadatabad oilfields. The experimentally measured chemical analysis of the injection water and formation water was used to estimate the amount, type, and composition of scale due to mixing and changes in thermodynamic conditions. Scaling tendency values for eight types of scale, namely CaCO3 (calcite), CaSO4 (anhydrite), CaSO4.2H2O (gypsum), FeCO3 (siderite), Fe(OH)2 (amorphous), NaCl (halite), Mg(OH)2 (pyrochroite), and KCl (sylvite), were investigated by commercial software packages OLI ScaleChem and StimCADE. The results show that the significant scales are CaCO3 and FeCO3 formed in Sarvestan and Saadatabad oilfields. The formation of these scales can lead to severe problems, such as disrupting equipment and decreasing production; thus, it is necessary to predict all types of scales before forming. It allows design and planning for chemical inhibitor treatment and prediction of injectivity problems and acid stimulation.
- Temperature change has the primary effect on scale formation. At higher temperatures, calcium carbonate deposition increases because the solubility of CaCO3 decreases with increasing the temperature;
- The scale deposition of calcium carbonate and iron carbonate increases by pressure drop. Pressure drop increases CO2 partial pressure and raises the scale deposition of calcium and iron carbonate;
- The incompatibility between formation water and desalting unit water results in the formation of CaCO3 and FeCO3 as the dominant scales in disposal wells.