Paraffin Deposition in Oil Production

Abstract

Abstract Paraffin deposition problems in oil production, transportation and storage will be presented. An experimental methodology for deposits characterization and paraffin deposition control, oriented toward the identification of "tailor made" preventive or remedial treatments, will be shown. Research results including some correlations between oil composition and wax appearance temperature (WAT), as well as paraffin inhibitors efficiency, will be presented. The knowledge provided in this paper has direct application to paraffinic crude oils productivity enhancement in terms of costs reduction (cleaning and deposits removal), and oil production improvement. On the other hand, correct selection of a prevention treatment will avoid extremely expensive and inefficient "trial-and-error" procedures, and even severe formation damages due to eventually paraffin deposition in the reservoir. This last phenomenon can be a consequence of the precipitation of crude oil paraffin's heaviest components after improper operations, like "hot-oil" treatments. The designed experimental strategy starts with the deposit analysis, which includes a preliminary physical separation of the organic and inorganic components. The organic fraction characterization, in terms of its hydrocarbon components, allows to discriminate between wax and asphaltenes plugging. Once the paraffin deposition problem is identified, the original crude oil characterization is carried out. Specific compositional characteristics of the crude oil allow to select an adequate paraffin deposition treatment, based on previously established correlations. An example selected from field operations carried out in Venezuelan facilities, will illustrate the benefits derived from this methodology. Introduction Wax deposition during paraffinic crude oil production and transport is one of the most serious problems faced in downhole and surface operations. These deposits are mainly constituted by n-paraffins (linear alkanes) and small amounts of branched paraffins and aromatic compounds.1 Naphthenic (cyclic) and long chain paraffins also have a notorious contribution to microcrystalline waxes and have remarkable influences on macrocrystalline growing patterns.2 The carbon number of paraffinic molecules present in wax deposits is known to be higher than 15 atoms. Advanced analytical techniques had allowed to detect up to 160 carbon atoms in these deposits.3–12 It has been found that light paraffinic fractions along with the absence of a well defined maximum or a multimodal shape of the carbon number distribution curve, are responsible for keeping the crude oil pour point low enough even at high concentrations of heavier paraffins.13 A good correlation between oil composition and polymeric paraffin inhibitors activity was recently found.14–20 Paraffinic crude oils have been classified into two structural categories, considering their molecular weight distribution, C24+ paraffins concentration and (n-/cyclo+isoparaffins) ratio. Also, different responses to commercial paraffin inhibitors were obtained by these two crude oil types, assessing the inefficiency of some paraffin inhibitors for crudes rich in C24+ alkanes.15 It was demonstrated, through these previous studies, that there is a complex interaction between crude oil fractions (specially paraffin classes distribution) and the tendency of the crude oil to precipitate wax. There are numerous methods used to handle paraffin deposition. These can be divided into two categories: removal (mechanical, thermal, chemical) and prevention or inhibition (dispersants, crystal modifiers). Usage of an effective paraffin inhibitor has a potential for significant savings versus removal procedures. Since paraffin characteristics and contents vary drastically from reservoir to reservoir, production problems and solutions also vary. Methods that are effective in one system are not always successful in other reservoirs or even in various wells within the same reservoir.21