Abstract
Asphaltene deposit accumulations, either in surface facilities or within the producing interval of the formation, can seriously decrease productivity up to the point where remedial work is required to remove the deposit and restore production. Focusing on the problem of asphaltene accumulation within the formation, the most common treatment is based on washing the reservoir in the near well-bore region with aromatic solvents either pure or doped with solvent-enhancing additives. These remedial treatments are often unsuccessful owing to incomplete removal of the deposit. In our opinion, underestimation of the specific asphaltene/formation rock interactions within the problem formation may be an important factor in determining the failure rate. This paper describes a new experimental approach for selecting or defining the best suited solvent or solvent/additive system for a given asphaltene/formation rock combination, so that more effective removal of the asphaltene deposits can be achieved.
INTRODUCTION
Asphaltene deposits within the producing formation are constituted by two different types: the asphaltenes adsorbed onto the formation rock and bulk asphaltenes. Bulk asphaltenes are defined as asphaltenes aggregated from the oil medium and deposited onto the adsorbed asphaltenes. Because these two types of asphaltenes are characterized by interaction forces that are substantially different both in quality and intensity, they might be expected to present quite a different degree of removability with respect to a chosen solvent or chemical. In fact, considering those asphaltenes obtained by n-heptane precipitation from stock tank oil (i.e., the "softest" form of such material), toluene, as a typical solvent, shows a very high up-take (several tens of a %, w/w) when the asphaltenes are in the bulk state; on the contrary, the asphaltene up-take by toluene is very low (10-20 %, w/w) when the same material is adsorbed on a rock surface (clays, dolomia, quartz, etc.). In spite of this evidence, the most common remedial approach, based on solvent washing, refers only to bulk asphaltenes. Moreover, the activity of solvent enhancing additives, which range from aliphatic amines to alkyl benzene sulphonic acid (1,2), are still defined with respect to bulk asphaltenes. In our opinion, underestimation of the specific asphaltene/formation rock interactions within the problem formation may be an important factor in determining the failure rate. In fact, reports indicate only limited success with such solvent treatments, owing to incomplete removal of the deposits (3).
In order to carry out a positive remedial treatment, the washing system (solvent or solvent/additive) has to be optimized on both bulk and adsorbed asphaltenes. Focusing on adsorbed asphaltenes, in this paper we describe a new experimental approach for selecting or defining the best-suited solvent or solvent/additive system for a given asphaltene/formation rock combination, so that more effective removal of the asphaltene deposit can be achieved. For the particular asphaltene/formation rock combination under examination, first one determines the fraction of the adsorbed asphaltenes that a reference solvent, such as toluene, is capable of dissolving. Next, the efficacy of other solvents or solvent/additive pairs is defined in terms of their ability to dissolve an additional fraction of the residual asphaltene deposit. In this way it is possible to establish a ranking of different asphaltene-removal chemicals with regard to their asphaltene dissolving power and their rate of removal; the optimum quantity of additive to achieve the maximum asphaltene removal is also readily determined by this method.
EXPERIMENTAL
The asphaltenic materials were isolated from a crude oil (42°API) by precipitation with n-heptane. Powdered dolomite (average particle size 60μm; BET surface area/N2= 10m2/g) was prepared by crushing and sieving big lumps of pure dolomia. Reagent grade toluene was employed as solvent. The additives (additive A: based on alkyl benzene sulphonic acid; additive B and C: based on complex polymers) were obtained from commercial sources. Asphaltene solution concentrations were determined spectroscopically with a Perkin-Elmer Λ2 spectrophotometer.
RESULTS AND DISCUSSION
The reference samples, for additive tests, were prepared by the following procedure: first, maximum asphaltene adsorption (0.5mg asph./g dolomia) was obtained by the procedure of reference (4); next, the asphaltene-treated rocks were isolated and extracted with fresh toluene in a Soxhlet apparatus. By this procedure we have established that, on average, 73% of asphaltenes were irreversibly adsorbed on dolomia with respect to toluene solvent. The dissolution efficacy and removal rate of the commercial additives were determined by suspending a portion of the reference sample in a toluene solution containing different additive amounts and spectrometrically measuring, with time, the solution concentration of the deadsorbed asphaltenes. The experimental results, reported in Table 1, show that all the tested additives improve the toluene removal efficacy. Amongst them, the one based on sulphonic acid has shown to be the most active both in terms of dissolving power and removal time. This behavior well agrees with core-flooding experiments performed with the same kind of asphaltenes and powdered dolomia (5).
| Additive (%,w/w) . | Time (hours) . | Removal Efficacy (%, w/w of adsorbed asphaltenes) . | ||
|---|---|---|---|---|
| 0.1 | 1 | 40.5 (A) | 14.0 (B) | 2.7 (C) |
| 0.1 | 18 | 48.9 (A) | 21.4 (B) | 6.0 (C) |
| 2.0 | 1 | 49.9 (A) | 32.9 (B) | 8.9 (C) |
| 2.0 | 18 | 51.5 (A) | 46.0 (B) | 10.1 (C) |
| Additive (%,w/w) . | Time (hours) . | Removal Efficacy (%, w/w of adsorbed asphaltenes) . | ||
|---|---|---|---|---|
| 0.1 | 1 | 40.5 (A) | 14.0 (B) | 2.7 (C) |
| 0.1 | 18 | 48.9 (A) | 21.4 (B) | 6.0 (C) |
| 2.0 | 1 | 49.9 (A) | 32.9 (B) | 8.9 (C) |
| 2.0 | 18 | 51.5 (A) | 46.0 (B) | 10.1 (C) |
CONCLUSION
This new testing method presents the following advantages: 1) it allows straight-forward determination of the optimum quantity of additive and the needed removal time to achieve the maximum asphaltene deposit removal; 2) it can be applied to any asphaltene/formation rock combination; 3) very simple and basic experimental set-up; 4) facile up-grading to perform additive efficacy ranking at T and P of interest.
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