Asphaltenes are high molecular weight, polar and paramagnetic hydrocarbons responsible for significant blockages and downtime. Often, chemical programs are used to manage fouling but it may prove difficult to determine the effectiveness and that makes optimization challenging. Our newly introduced surveillance technique, the Quantum RF Asphaltene Analyzer ("QRF") measures the total amount of asphaltene by quantifying its paramagnetic signature and this technology has been used by operators in Canada, Middle East, US Land, and now, most recently, in offshore Gulf of Mexico.

While asphaltenes are technically described as a solubility class (polar molecules that dissolve in toluene but precipitate in n-alkanes), the general structure of each asphaltene molecule is known to consist of a number of polyaramatic rings from which long alkane chains emanate. Each molecule is flat and some of the aromatic rings contain hetero-atoms such as sulphur or nitrogen. Some of the rings may also be combined with trace amounts of heavy metals, particularly chelated vanadium and nickel. Atomic force microscopy images of two asphaltene molecules are shown below (from [1])

The non-uniformity in the electron cloud triggered by the hetero-atoms encourages some electrons to delocalize within their aromatic ring structures, which means that the electrons are not confined to a single bond but can move freely across the entire aromatic system. This contributes not just to the molecule's overall polarity but also its paramagnetism because these individual electrons can all have the same spin direction. (Recall that a molecule is polar if it has an uneven distribution of electrons and paramagnetic if it has one or more unpaired electron in an outer orbital).

Asphaltene molecules have a strong tendency to self-aggregate, first like pancakes stacking on top of one another due to π–π interactions, and then via Van der Waals forces into yet larger structures, e.g., see [2]. An important point about this stacking though, is that it does not destroy the paramagnetism of each individual molecule. So the number of paramagnetic electrons will stay the same whether the asphalene is completely dissolved in toluene, floating in a colloidal suspension within a crude, or flocculating aggressively in heptane.

The number of paramagnetic electrons can be quantified by a known laboratory procedure called Electron Paramagnetic Resonance (EPR) which uses a combination of DC magnetic field and GHZ excitation to maps paramagnetic electrons into a measurable voltage. This is the basis for our QRF Asphaltene Analyzer. We industrialized an EPR measurement so that it could be incoporated into oilfield production facilities. Then by measuring the EPR signature of crude oil in real-time, cloud-based software determines the number of paramagnetic electrons and that is directly indicative of the mass of asphaltene in the crude.

By keeping microwave excitation at a fixed frequency and sweeping the strength of a DC magnet, our measurement apparatus applied to an asphaltene sample in crude oil will convert resonating electrons into a signal similar to that below. There is a dominant “doublet” peak at a particular value of magnetic field that corresponds to the ratio of 357 Gauss/GHz and there may be other smaller resonance peaks in the asphaltene spectrum. Those relate to quantum interactions between the free radicals and nearby nuclei such as iron or vanadium nuclei that may be embedded as porphyrin structures within the asphaltene molecule. The height of that dominant feature is measurable in milliVolts by taking the peak-to-peak distance which we refer to as Vpp.

Note that the peaks for iron, vanadium, etc, are at a different Gauss/GHz ratio, so software can automatically extract the central peak. A review of over a hundred oils has demonstrated that paramagnetic signature consistently correlated with asphaltene content. The graph below (from [3]) demonstrates linearity between asphaltene measured by ASTM D5650 and the Vpp signal from our EPR.

Paramagnetism in presence of solvents

Key to the utility of the paramagnetism to manage asphaltene-related flow assurance challenges is the observation that the asphaltene paramagnetism is largely unchained as the molecule agglomerates. This has been verified by multiple clients, independently of MicroSilicon. An example is shown below where hexane has been successively added. The Vpp drops linearly with the amount of oil remaining, which means that the Vpp per gram of asphaltene is not changing.

References

1. Schuler, B.; Meyer, G.; Peña, D.; Mullins, O.C.; Gross, L. "Unraveling the Molecular Structures of Asphaltenes by Atomic Force Microscopy". J. Am. Chem. Soc. 2015, 137, 9870–9876. https://doi.org/10.1021/jacs.5b04056
2. Mullins, O.C.; "The Modified Yen Model", Energy & Fuels, 24, 2179 –2207, (2010) https://doi.org/10.1021/ef900975e
3. Lovell, J., Kulbrandstad O., Madem S., Meza, D., 2021, “Real-Time Digital Chemistry Offshore Transforms Flow Assurance Management”, Offshore Technology Conference, Houston, OTC-31121-MS. https://doi.org/10.4043/31121-MS