6.+Causes+of+Deposition

Causes of Paraffin & Asphaltene Deposition
Paraffin and asphaltene deposition is a great detriment to any oil company trying to produce large quantities of crude oil. These paraffins and asphaltenes deposit on the insides of pipes and reservoir pores, reducing the flow rate of the desired product, as shown in Figure 1.

 Figure 1: Pipe which has undergone a fouling process (Integrity Engineering 2009)

There are three steps that lead to the precipitation of large alkanes (range of 18 carbon to 70 carbon alkanes), thereby clogging oil reservoirs, well bores, and transportation pipelines (Ring et al. 1994):
 * 1) Solubility/Precipitation
 * 2) Transportation and deposition of precipitate
 * 3) Reduction in permeability due to precipitate deposition

When drilling into an oil reservoir, large pressure drops occur in the reservoir. Due to this rapid change in pressure, larger n-alkanes can precipitate out of the oil and flocculate, not only in the oil well, but in pipelines which transport these hydrocarbons. Both decreases in temperature and pressure cause these large alkanes to precipitate out of the oil (Soulgani et al. 2008). In Figure 2, it can be seen that as pressure decreases, melting point temperatures also decrease.



Figure 2: Typical Pressure-Temperature Diagram (Wikipedia: Phase Diagram... [updated 2010])

Once the temperature increases, it can be shown that solubility of these large alkanes also increases, (Ring et al. 1994) thereby reducing the amount of paraffin deposited on the walls of well bores, allowing oil flow rate to increase. Figure 3 displays this solubility trend as well. In most cases, solubility of a solute in a specific solution increases with increasing temperature (Kallos MS et al. 2010).



Figure 3: Solubility Charts for various solutes (Saskatchewan Schools 2006)

Once the temperature decreases, the paraffins arrange themselves into orthorhombic shapes; thereby creating stable, three dimensional wax crystals. Before the temperature decreases to which the paraffins precipitate out of solution (also called the "cloud point"), the paraffins tend to trap oil, creating a gel. More paraffin diffuses into this gel matrix, thereby increasing the paraffin build up around the annulus of the pipe (Hemant et al. 2008).

Summary of How Deposition Occurs
The mechanism of paraffin and aspahltene fouling is not a particularly complex one. However, the knowledge of said mechanism can greatly help one understand the possible chemical treatments/solutions to such a persistent and time consuming problem.

1) Solubility/Precipitation
High pressures are associated with an untapped oil reservoir. Once the reservoir is drilled into and perforated, a pressure gradient is created, thereby allowing oil to flow out of the reservoir (Wikipedia Article: Oil Well). As a result, this drop in pressure decreases melting point temperature (See Figure 2) and allows some of the larger hydrocarbons to precipitate out of the oil solution ( Soulgani et al. 2008). Decreasing the temperature also allows some of these paraffins to create orthorhombic shaped wax crystals which are carried through the oil (Hemant et al. 2008).

2) Transportation and Deposition of Precipitate
The aforementioned wax crystals create networks and trap oil molecules (Hemant et al, 2008). This [|amalgam]of wax crystals and oil then creates a gel matrix (Hemant et al. 2008). This gel matrix ends up trapping other wax crystals present, and the amount of wax in the pipes increases over time (Hemant et al. 2008). With a further decrease in temperature, this waxy, gel-like substance hardens and deposits on the walls of the pipe in which the hydrocarbons are being transported (Hemant et al. 2008).

3) Reduction in Permeability due to Precipitate Deposition
As these suspended crystals and gels deposit on the surface of pipes, the permeability (smallest area available for flow) of the pipe decreases, thereby decreasing flow rate ( Ring et al. 1994 ). A decrease in flow rate means a decrease in oil produced and/or transported and, in turn, a decrease in revenue for the oil company.

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