(181c) Effect of Emulsified Water on Asphaltene Instability in Crude Oils

Authors: 
Chen, A., Rice University
Asphaltene precipitation and subsequent deposition during oil production are of great importance for the oil industry nowadays because of the potential risk associated to this heavy fraction in plugging wellbores and production equipment. Although water is commonly present in the produced fluids, because of instrument limitations and inadequate techniques, it is usually separated from the oil prior to any experimental analysis. Therefore, the effect of water on asphaltene stability and deposition tendency is not completely understood and the information available in the open literature is scarce. The focus of this work is on the determination of the effect of emulsified water on asphaltene instability in crude oil systems. Three crude oil samples, ranging from 26 to 40 °API, and 1.2 to 13 wt% asphaltene content, from Canada, Middle East and Gulf of Mexico, and one bitumen sample from the Athabasca region in Canada were used in this study. A total of nine systems were investigated with and without the presence of emulsified water. First the three crude oils were studied, and then model oil blends were prepared with extracted asphaltenes from the different oils using n-pentane as the asphaltene precipitant. Furthermore, three different asphaltene fractions were extracted from the Athabasca bitumen: the n-pentane insoluble (also known as n-C5 asphaltenes), the n-heptane insoluble (or n-C7 asphaltenes) and the n-pentane insoluble but n-heptane soluble fraction (n-C5-7 asphaltenes). The stability of asphaltenes in the presence and absence of emulsified water was investigated by the indirect method. This is a multi-step method that includes the formation of an emulsion (when water is added), the precipitation of asphaltenes upon addition of a normal alkane for a specified aging period (24 hours in this case), centrifugation of the samples at 10,000 rpm for 25 min, and determination of the absorbance of the supernatant fluids at a wavelength of 1,100 nm. The centrifugation step is necessary to remove the water from the supernatant fluid and settle the asphaltene aggregates that are 100 nm and larger. The indirect method is used to determine the onset of asphaltene precipitation and how this is affected by the presence of emulsified water by analyzing the absorbance of the supernatant fluids as a function of the volume fraction of the normal alkane added to induce the precipitation. It was found that for the crude oils from the Middle East and Canada and their corresponding model oils, the addition of water neither did have a significant effect on the onset of asphaltene precipitation nor the amount of precipitated asphaltenes. However, the crude oil from Gulf of Mexico and the model oils from the Athabasca bitumen (containing n-C5 and n-C7 asphaltenes) were significantly affected by the presence of water. In the latter case, there is even a significant shift on the onset of asphaltene precipitation and the total amount of asphaltene precipitated. Interestingly, the model oil that was prepared with n-C5-7 Athabasca asphaltenes did not show a significant change upon addition of water. The experimental evidence suggests that some asphaltenes are more prone to interact with water at the oil − water interface, which are in turn settled by the effect of centrifugation− than others. The difference between the affinity of asphaltenes from different sources for the oil − water interface and even within the polydisperse distribution of asphaltenes in a given oil sample is evident in this work. The effect of temperature on the water / asphaltene interactions in the crude oil from Gulf of Mexico was also investigated. It was found that at increasing temperatures, although the solubility of water in the oil phase does not change considerably, there is a significant effect on the drop of absorbance of the supernatant fluids after centrifugation, which we believe indicates an increased interaction between asphaltenes and water. With this study we aim to contribute to a better understanding of the interaction of water and asphaltenes in crude oil systems, which will eventually lead to the development of cost-effective strategies for the mitigation of this flow assurance problem.