A New Correlation Predicting the Formation of Iron Sulfide Precipitation in Wastewater Disposal Wells
- Alireza Bahadori (National Iranian Oil Co.) | Khalil Zeidani (U. of Alberta)
- Document ID
- Society of Petroleum Engineers
- SPE International Oilfield Scale Symposium, 31 May-1 June, Aberdeen, UK
- Publication Date
- Document Type
- Conference Paper
- 2006. Society of Petroleum Engineers
- 5.2 Reservoir Fluid Dynamics, 4.2.3 Materials and Corrosion, 4.1.4 Gas Processing, 4.2 Pipelines, Flowlines and Risers, 4.1.5 Processing Equipment, 3 Production and Well Operations, 4.1.2 Separation and Treating, 2.2.2 Perforating, 4.3.4 Scale, 1.8 Formation Damage
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Scale formation and well plugging due to the incompatibility of injected wastewaters is a critical field problem in wastewater disposal wells. When different wastewaters are mixed it is necessary to evaluate their compatibility prior to the injection in disposal wells. The individual wastewaters may be quite stable at all system conditions and present no scale problems. However, once they are mixed, reaction between ions dissolved in the individual wastewaters may form insoluble products that cause permeability damage in the vicinity of the wellbore.
In this paper, the composition of different wastewaters that were collected from southwest Iranian desalting plants disposal wells were analyzed critically. Laboratory studies as well as field experience has shown that formation damage in wastewater disposal wells may occur mainly due to the conception of Iron Sulfide in the case of mixing a wastewater which contains Iron ions with a wastewater containing H2S. A new correlation is developed estimating the critical concentration of Iron ions, Fe(2+) (ferrous ion), which will stay in solution at various pH values and a wide range of H2S concentration in crude oil desalting plants disposal wastewaters. This correlation eliminates the need for compatibility assessment, which is usually assessed either by solubility calculations or by experimental testing, for water mixtures that contains Iron ions and dissolved H2S. Finally, a real case field problem was analyzed and based on the correlation's results three different potential solutions were recommended for further field trial implementation.
Production of salty wet crude had affected the quality of Iranian crudes and a number of wells had to shut in for lack of treating facilities. The produced water with crude in Iranian oil fields contains salts in the concentration of 150,000 to 220,000 ppm. In almost all cases, the salt is found dissolved in the water that is dispersed in the crude oil. This salt water is present in the crude in the form of emulsion (water-in-oil) and its separation is not an easy task. Application of right technology and installation of proper desalting facilities were required to solve this problem. Therefore, it was decided to install electrostatic desalting plants progressively in Iranian oil fields. By end of 2004 more than 20 plants with a total capacity of 207 Mm3/Day (1.3 MMSTB/Day) of treated crude has been installed. It is expected that production of wet crude raise to 400 Mm3/Day (2.5 MMSTB/Day) in Iran by the year 2007. The performance of the majority of current desalting plants have been tested and found satisfactory. Figure 1 depicts the schematic of typical desalting plants in Iranian oil fields. In the desalting process considerable amount of salt will be removed by addition of comparatively fresh water to the crude; this addition of fresh water dilutes the original brine so that the salt content of the water that remains after treatment is within acceptable limits [2, 4].
The desalting process is undoubtedly associated with generation of considerable amount of wastewater that needs to be disposed properly. Its volume is typically about 15% of the crude oil desalting plant capacity, consists of 10% associated formation water and 5% of added wash water. The wastewater processed through wastewater treating system before it is injected into disposal wells. The wastewater treating system consists of a skimmer tank, API gravity separator, filter, and disposal tank.
Different crude oils bearing different formation waters are desalted in the Iranian desalting plants resulting in production of different wastewaters compositions. When different wastewaters are mixed it is necessary to evaluate their compatibility prior to the injection in the disposal wells. One of the primary causes of scale formation and injection well plugging is mixing two or more wastewaters which are incompatible. The individual wastewaters may be quite stable at all system conditions and present no scale problems. However, once they are mixed, reaction between ions dissolved in the individual wastewaters may form insoluble products that cause permeability damage in the vicinity of the wellbore. Depending on the amounts of each constituent present, the pH, temperature, and the ratio in which the two waters are mixed, you might expect any or all of the following precipitates to result: Calcium carbonate, calcium sulfate, barium sulfate or Iron Sulfide.
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