Removal of Asphalt Deposits by Cosolvent Squeeze: Mechanisms and Screening
- Louis Minssieux (Inst. Français du Petrole)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- March 2001
- Document Type
- Journal Paper
- 39 - 46
- 2001. Society of Petroleum Engineers
- 5.3.1 Flow in Porous Media, 3.4.1 Inhibition and Remediation of Hydrates, Scale, Paraffin / Wax and Asphaltene, 4.1.2 Separation and Treating, 1.6.9 Coring, Fishing, 5.3.2 Multiphase Flow, 4.3.3 Aspaltenes, 1.8 Formation Damage
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Performance improvement of usual solvents for asphalt deposits was investigated in an effort to optimize squeeze treatments of formations damaged with asphaltenes. The approach adopted for increasing the solvent power for asphaltenes of such solvents as xylene was to design solvent mixtures with a polarity adjusted to specific chemical features of asphaltenes contained in the crude oil considered. For that purpose, Hansen's theory was used as a guide in defining the composition of cosolvents suitable for asphaltenes. In addition, to increase solvency and to disperse asphaltenes, proper additives were used as a means to create hydrogen bonding with the heteroatoms present in the asphaltene chemical structure or to modify the intermolecular interactions involved in asphaltene aggregation.
Laboratory screening of cosolvents involved four types of tests that were based on the physical/chemical mechanisms deemed predominant in a squeeze operation (i.e., the solvency of asphaltenes, the ability to desorb the asphaltenes adsorbed onto minerals making up the reservoir rock, and the capacity to maintain dispersion of the asphaltenes within the organic phase). Several cosolvents, with a few percent commercial chemicals containing an alcohol or amine functional group in their molecules, were found to exhibit the required properties vs. either solid extracted asphaltenes or well deposits.
The final cosolvent efficiency was checked at reservoir temperature in squeeze experiments carried out in core samples. After a permeability damage occurred during asphaltenic crude oil flow, the remedial effects of squeezes of different cosolvents were evaluated. Several examples illustrate the improvement gained with the designed cosolvents and additives in restoring part of the oil permeability and in reducing the in-situ asphaltene deposition accompanying crude oil flow through core samples.
Losses of well productivity in asphaltenic oil fields are often reported. They occur more or less rapidly during the depletion step of a reservoir. In several newer fields, such as Villafortuna Trecate in Italy1 and Boqueron Coral fields in South America,2 asphaltene deposition problems have even been mentioned in the early stages of field development.
Asphalt deposits, possibly mixed with salt, paraffins, or inorganic solids, generally appear first in surface facilities; then in tubings and chokes; downhole; and, progressively, in the near-wellbore formation itself. Asphaltic sludge formation is also known to be a risk inherent in acid stimulation treatments of oil wells.
A common practice for remediating or mitigating well impairment caused by asphaltene deposition consists of periodic treatments with a solvent (i.e., washing the tubing and squeezing into the near-wellbore formation). However, an economical limitation exists because of the transient effect of such cleanup operations. In addition, solvents in use in the field, such as xylene or naphtha, did not completely dissolve the asphalt deposits3 or completely extract asphaltenes fixed on clay minerals.4 Therefore, a need arises to improve the quality and efficiency of solvents to be used in asphaltene-removal operations. Field trials were conducted along this line.
Trbovich5 commented on a series of well treatments carried out with solvent blends in which a hydrophilic compound (only defined as moderate-carbon-chain-length alcohols) was added to the squeezed xylene. In most of the reported cases, a longer-lasting effect was observed compared with that obtained with straight xylene injection. The authors interpreted this to mean that these cosolvents perform better than xylene in water-wet formations even though their asphaltene dissolution capacity is not better.
Agip6 patented and uses special blends of aromatic hydrocarbons containing polyaromatic fractions for removing asphaltene deposits from surface facilities. Some other patents or studies recommend the addition of specific chemicals to solvent to improve the effects or duration of well chemical stimulation treatments (e.g., butylamine,7 p-alkyl phenols and alkylaryl sulfonic acid,8 or alkyl phenol mixtures from natural origin9). Some of these additives were field-tested in California and positive results were reported.3
In this work, we undertook a systematic study to rationalize the procedures used in designing the solvent medium appropriate for dissolution of well asphaltenic deposits in a given field application. The case of surface-active or polymeric additives that exhibit specific properties, such as rock wettability alteration, water/oil emulsification, or steric effects in solution, is not addressed here.
Our approach defines a series of tests corresponding to the relevant solvent properties. These tests include a final evaluation of selected cosolvents to be performed in reservoir core samples with the asphaltenic crude oil considered. These core-flood tests are thought to involve all the main mechanisms of asphaltene deposition that exist within the near-wellbore formation. They have been used to allow a laboratory comparison of cosolvent efficiency under conditions approaching those encountered during a solvent squeeze in the formation to be treated.
Asphaltene Dissolution and Dispersion of Aggregates in Organic Phase.
One goal of this work was to increase the solvent power for asphaltenes of common solvents used in the field (i.e., naphthas or aromatic hydrocarbons, such as toluene or xylene). The basic idea was to look for links between the solvent molecular structure and the main functional groups known to be present in the chemical species mixtures making up asphaltene fractions. To achieve this, the molecular structure of solvents and additives was selected to fulfill the following two types of interactions.
1. To replace or interfere in the hydrogen bonds involved in aggregation of asphaltenic entities in the oil and possibly also with those involved in intramolecular interactions. Even if hydrogen bonding is not the only cause of asphaltene aggregation, its existence was demonstrated in the buildup and stability of aggregates suspended within crude oil.10 This type of interaction implies the involvement of functional groups in asphaltene structures, especially those containing oxygen and nitrogen atoms. Indeed, asphaltene fractionation by liquid-chromatography techniques showed that asphaltenes generally contain acid, ketone, and phenolic fractions that correspond, respectively, to carboxyl, carbonyl, and hydroxyl groups, and base fractions related to the presence of nitrogen heteroatoms.11
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