Asphaltene precipitation and subsequent deposition is a potential problem in oil production because the significant costs for wellbore cleaning and the associated production loss. To better understand the mechanisms by which asphaltenes precipitate and deposit, in this work we present experimental evidence that supports the idea that precipitation and aggregation of asphaltenes is a multi-step process, where the former is driven primarily by thermodynamics whereas the latter is driven by kinetics. Under this multi-step mechanism, asphaltene precipitation is a fully reversible process. On the other hand, from the precipitated phase, subsequent aggregation and aging leads to the formation of more solid-like structures. Furthermore, we also present experimental results that suggest that the currently available commercial technologies to detect asphaltene precipitation (i.e. NIR spectroscopy and High Pressure Microscopy) might not be appropriate to detect the exact point of asphaltene precipitation, but instead they give a combined reading of precipitation plus aggregation. For this reason, the results obtained using these methods are very sensitive to the depressurization rates. The better understanding of the asphaltene behavior has enabled the development of an enhanced modeling approach based on the Perturbed Chain version of the Statistical Associating Fluid Theory equation of state (PC-SAFT EOS), which is used to predict the precipitation of asphaltenes at reservoir conditions and requires fewer simulation parameters than previous methods. A case study is presented in which our modeling technique was proven useful for the detection and correction of inconsistencies in experiments done using a bottom-hole sample at reservoir conditions.
It is well-known that asphaltenes are a polydisperse fraction present in crude oil with a broad distribution of sizes and molecular characteristics. However, in practice, asphaltenes are often treated both in the laboratory and in modeling work as a monodisperse fraction. In this work, asphaltene deposition tests with two model oils with the same total amount of asphaltenes but with different asphaltene polydispersity distributions were conducted using a high-pressure, high-temperature packed-bed deposition column. Although the asphaltenes used to prepare the two oils were extracted from the same source, a drastic difference in the amount of asphaltene deposited was observed, which is attributed to the difference in the asphaltene polydispersity between the two samples. Additionally, using UV−visible spectroscopy, it is possible to observe some similarities between the aromaticity of asphaltene subfractions obtained from different geographical locations. Modeling results provide additional insights into the impact of asphaltene polydispersity on the onset and the amount of asphaltene precipitation. The results presented in this work suggest that asphaltene polydispersity plays a major role in determining asphaltene precipitation and deposition tendencies observed in different oils.
Asphaltene deposition is a flow assurance problem that threatens the continuous production of crude oil. This problem is likely to get worse because of the current tendency to produce from deeper waters and also due to the implementation of enhanced oil recovery operations based on miscible injection of CO2 or hydrocarbon gas. In this article we present a review of the asphaltene precipitation and deposition problem in oil production, and we describe the commercial techniques used for its determination and quantification as well as their limitations. We also discuss the important progress achieved concerning modeling tools with enhanced capabilities that have been developed to forecast the occurrence and the magnitude of this problem. Prompt recognition of potential asphaltene deposition issues, either for new fields or as part of enhanced oil recovery projects, is necessary to implement successful prevention or mitigation strategies. Unfortunately, current commercial inhibitors have mixed results in the field and in some cases are responsible for worsening the deposition problem. After a thorough investigation we have elucidated a multi-step mechanism of asphaltene precipitation, aggregation and aging, which explains the poor performance of some chemicals and the limited capabilities of the current testing protocols. We provide some ideas for a new generation of asphaltene inhibitors and, for the cases in which asphaltene deposition has already occurred, we also provide a list of common techniques and best practices used for its remediation. With this work we aim to provide the reader with a broad understanding of this complex problem and to offer a starting point for the development of cost/effective strategies to manage asphaltene deposition in increasingly complex environments.
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