Research into application of polyaluminum chlorides in coagulation of pollutants in reservoir waters with a wide range of pH

Abstract

Reservoir waters extracted during exploitation of crude oil and natural gas are a serious threat to the environment. The reason is high content of dissolved salts, mainly chlorides, as well as toxic components, such as: petroleum substances with aromatic hydrocarbons, and various chemicals applied during hydrocarbons production, e.g. corrosion, hydrates and paraffin inhibitors, surfactants, methanol etc. For this reason, the extracted reservoir water should be managed in a way that is least harmful to the environment. One of the methods of management that meets this requirement is injection of reservoir water into the absorptive horizon. In order to provide failure-free injection, with maximum use of the capabilities of an absorptive horizon, it is necessary to prepare the water properly by removing the components (solids, deposits and suspensions) that cause damage of the injection well. One of the most effective methods of water treatment is coagulation. Aluminum(III) sulfate(VI), commonly used in coagulation, requires a neutral pH of water. Exceeding this level results in rapid reduction in coagulation efficiency, and even in increase in the content of suspended solids in purified water. Extracted reservoir waters drastically change their properties from strongly reducing to oxidizing, due to methane removal after pressure reduction and oxygenation in contact with air. Moreover, while changing the electrochemical potential to positive, presence of iron and manganese ions in the water causes precipitation of oxides and hydroxides in the form of suspensions and the pH decreases due to their hydrolysis. Due to the dynamic course of these processes, as well as the additional acidification of water by addition of the coagulant (aluminum(III) sulfate(VI)), adjustment of the pH to the level appropriate for effective coagulation is often a serious problem. The solution could be application of properly selected modern coagulants of much greater efficiency, enabling a decrease of the agent dose as well as reduction of volume and improvement of the structure of post-coagulation sediments. Additionally, they are highly efficient in a wide range of water reactions. The article describes laboratory tests on a selected coagulant (polyaluminium chloride), which allow for determination of the range of deposit water reaction in which this agent can be used. For the tests, Flokor 1ASW was selected due to its high degree of hydrolysis and slight acidity (pH ~3.5) compared with other polyaluminium chlorides, which means low consumption of total alkalinity of purified water. The results obtained show a wide range of water reaction (pH from 8.0 to 4.0), in which the effectiveness of coagulant activity ensures the correct level of purification. In addition, the use of Flokor 1ASW with effective Stabpol K flocculant allows for production of big and quickly settled flocs. Post-coagulation sediments have much smaller volume than when using aluminum(III) sulfate(VI). The results of industrial tests, carried out in order to prepare reservoir water for injection, without pH correction, have been presented in the article. They confirmed the high efficiency of the selected chemicals.