One of the major unsolved complex systems confronting the petroleum and natural gas industries at present is the untimely deposition of heavy organic compounds present in the oil. The production, transportation and processing of petroleum, bitumen, and other heavy-organic-containing hydrocarbons could be significantly affected by flocculation and deposition of asphaltene, resin, paraffin / wax, Diamondoids, organo-metallics, etc. in the reservoir rock tubulars, oil well, pumps, storage vessels, transfer pipelines, and refinery and upgrading equipment with devastating economic consequences.
One question of interest in the oil industry is "when" and "how much" heavy organics will flocculate out under certain conditions. Since a petroleum crude generally consists of a mixture of hydrocarbons and heavy organics it has become necessary to look at each of its constituents as a polydisperse or discrete mixture interacting with one another.
Four different effects (mechanisms) are recognized for such deposition s. One or more of these mechanisms would describe the organic depositions that may occur during oil production, transportation or processing.
The degree of dispersion of heavy organics in petroleum fluids depends upon the chemical composition of the petroleum. The ratio of polar/non-polar and light/heavy molecules and particles in petroleum (Figure 1) are the factors primarily responsible for maintaining the stability of the polydisperse oil mixture.
Deposition of heavy organics can be explained by
an upset in the polydisperse balance of oil composition. Any change in
(ii) pressure , or
(iii) composition (such as addition of a miscible solvent to oil as demonstrated by Figure 2) may destabilize the polydisperse oil. Then the heavy and/or polar fractions may separate from the oil mixture into steric colloids, micelles, another liquid phase or into a solid precipitate.
Segments of the separated fractions which contain sulfur, nitrogen, and/or hydrogen bonds could start to flocculate and as a result produce the irreversible heavy organic deposits which may be insoluble in solvents.
Some of the heavy organics (specially asphaltenes) will separate from the oil phase into an aggregate (large particles) and then will remain suspended in oil by some peptizing agents, like resins, which will be adsorbed on their surface and keeping them afloat as demonstrated by Figure 3.
Stability of such steric colloids is considered to be a function of concentration of the peptizing agent in the solution, the fraction of heavy organic particle surface sites occupied by the peptizing agent, and the equilibrium conditions between the peptizing agent in solution and on surface of heavy organic particles. The amount of peptizing agent adsorbed is primarily a function of its concentration in the oil. A concentration variation of a peptizing agent (such as resins) in oil will cause its adsorbed amount on surface of heavy organic particles to change. Migration of peptizing molecules (shown by arrows) from the surface of heavy organic particles could take place due to the change in their chemical-potential-balance between the bulk oil phase and the surface phase as shown in Figure 3.
The peptizing agent concentration in oil may drop to a point at which its adsorbed amount would not be high enough to cover the entire surface of heavy organic particles. This causes the potential for aggregation of heavy organic particles due to
development of free active sites on their surfaces, and their eventual flocculation as shown by Figure 4. This may then
permit the heavy organic particles to come together (irreversible aggregation), grow in size, and flocculate. The nature and shape of the
resulting aggregates will determine their effect on the behavior of the petroleum fluids.
Visit The Laboratory Procedure to Identify and Measure Heavy Organic
Various aggregating macromolecules follow different aggregation pattenrs. For example, the irreversible aggregates of asphaltene are considered to follow an aggregation growth pattern shown by Figure 5 which is proven to be
When a crude oil is flowing in a conduit (porous media, well, pipeline, etc.) there is an additional effect (electrokinetic effect) to be considered in the behavior of its heavy organic constituents. This is because of the development of electrical potential difference along the conduit due to the motion of charged particles. This electrical potential difference could then cause a change in charges of the colloidal particles further down in the pipe, the ultimate result of which is their untimely deposition and plugging of the conduit as shown by Figure 7. The factors influencing this effect are the electrical and thermal characteristics of the conduit, flow regime, flowing oil properties, characteristics of the polar heavy organics and colloidal particles, and blending of the oil.
Depending on the kinds of operation and the heavy organics present in a crude oil one or more of the effects described above will have to be considered in prediction and modeling of any deposition problem.
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