Effect of Alcohols on Asphaltene-Particle Size and Hydrate Non-Plugging Behavior of Crude Oils
Authors: Delgado-Linares Jose G. , Costa Salmin Davi , Stoner Hannah , Wu David T. , E. Zerpa Luis , A. Koh Carolyn , Mateen Khalid , Bodnar Scot , Prince Philippe , Teixeira Adriana 
Published: April 2020

Gas hydrate formation in oil and gas flowlines can represent a safety concern and a cause for hindered production, resulting in economic losses. Hydrate risk mitigation can be attained through hydrate avoidance or management strategies. Hydrate avoidance methods aim to keep the flowline outside of the hydrate stability region through, for example, the use of thermodynamic hydrate inhibitors. Thermodynamic hydrate inhibitors (THIs) increasingly shift the hydrate boundary towards higher pressures and lower temperatures as a function of THI concentration in solution. Hydrate management strategies allow the flowline to operate inside the hydrate stability zone, without the risk of forming a plug by using kinetic hydrate inhibitors or commercial hydrate anti-agglomerants. Some crude oils, denoted non-plugging crude oils, have naturally occurring surfactants (e.g., asphaltenes) that can behave as a hydrate anti-agglomerant and allow the formation of a transportable non-agglomerating hydrate slurry. Recent work has suggested that the asphaltene-aggregation state is a parameter that may dictate the natural hydrate anti-agglomeration behavior of non-plugging crude oils.

The ability of naturally occurring anti-agglomerants to prevent hydrate plugs is limited by the intrinsic amount found in the non-plugging oils and thus depends on the amount of hydrates formed in the flowline. Therefore, there is an opportunity to use thermodynamic hydrate under-inhibition (i.e., partial thermodynamic inhibition) in non-plugging crude oil systems that can no longer safely avoid a hydrate plug while relying on natural surfactants alone. If THI under-inhibition is used to partially reduce the amount of hydrates formed, the natural anti-agglomerants can prevent hydrate agglomeration of the remaining hydrates formed. The THI volume required for under-inhibition would be lower than that of complete THI inhibition, thereby reducing operational costs.

In this work, alcohols with different hydrocarbon chains and mono-ethylene glycol were shown to have a key impact on the asphaltene-aggregation state, inferred through size measurements of solvent-extracted asphaltene particles, correlating with changes in emulsion stability, and the non-plugging potential of a crude oil as assessed by rocking cell tests. The effect of alcohols on the asphaltene-particle size was also shown to be highly sensitive to the presence of a free-water phase, likely due to observed alcohol partitioning into the water phase. Alcohols with intermediate-hydrocarbon chains prevented asphaltene aggregation more effectively and reduced their emulsification tendency compared to alcohols with shorter carbon chain lengths. Furthermore, short-chain alcohols or MEG showed no antagonism when used with the non-plugging oil tested, resulting in a partially inhibited system able to avoid hydrate agglomeration at a higher water cut compared to a non-inhibited system. On the other hand, alcohols with intermediate-chain length were found to be detrimental to the non-plugging potential of the specific crude oil tested, potentially due to its effect of reducing the asphaltene-particle size in solution. More experimental work is required to better understand these phenomena and determine if other non-plugging crude oils show a similar behavior.