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Abstract
The low-salinity waterflooding is an attractive eco-friendly producing method, recently, for carbonate reservoirs. When ferrous ion is present in the formation water, that is, acidic water, the injection of low-salinity water generally with neutral pH can yield precipitation or dissolution of Fe-minerals by pH mixing effect. FeSO4 and pyrite can be precipitated and re-dissolved, or vice versa, while siderite and Fe(OH)2 are insoluble which are precipitated, causing permeability reduction. Particularly, pyrite chemically reacts with low-salinity water and release sulfate ion, altering the wettability, favorably, to water-wet. In this aspect, we analyzed oil production focusing on dissolution of Fe-minerals and Fe-precipitation using a commercial compositional reservoir simulator. From the simulation results, the quantities of precipitation and dissolution were enormously large regardless of the type of Fe-minerals and there was almost no difference in terms of total volume in this system. However, among Fe-minerals, Fe(OH)2 precipitation and pyrite dissolution were noticeably large compared to troilite, FeSO4, and siderite. Therefore, it is essential to analyze precipitation or dissolution for each Fe-mineral, individually. Meanwhile, in dissolving process of pyrite, sulfate ions were released differently depending on the content of pyrite. Here, the magnitude of the generated sulfate ion was limited at certain level of pyrite content. Thus, it is necessary to pay attention for determining the concentration of sulfate ion in designing the composition of injection water. Ultimately, in the investigation of the efficiency of oil production, it was found that the oil production was enhanced due to an additional sulfate ion generated from FeS2 dissolution.
Highlights
Low-salinity waterflooding (LSWF) is a simple and environmentally friendly enhanced oil recovery (EOR) method, which can improve the oil recovery by lowering the salinity of injected brine or by adjusting the ion concentration
When low-salinity water (LSW) was injected into the carbonate reservoir, Fe-mineral dissolution and precipitation occurred as the pH increased, which was the main factor influencing the chemical reactions
When LSW was injected into the carbonate reservoir containing Fe-minerals, the precipitation or dissolution that occurred due to chemical reactions caused a permeability change (Figure 9)
Summary
Low-salinity waterflooding (LSWF) is a simple and environmentally friendly enhanced oil recovery (EOR) method, which can improve the oil recovery by lowering the salinity of injected brine or by adjusting the ion concentration. When LSW was injected into the carbonate reservoir, Fe-mineral dissolution and precipitation occurred as the pH increased, which was the main factor influencing the chemical reactions.
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