Asphaltene-stabilising molecules having a tetrapyrrolic ring

This is a National Phase Application filed under 35 USC 371 of International Application No. PCT/FR2007/052619, filed on Dec. 24, 2007, the entire content of which is hereby incorporated by reference in its entirety.

The invention relates to novel asphaltene dispersants which precipitate in crude oils or crude oil derivates such as, in particular, heavy fuel oils and asphalts. It also relates to a method for stabilising the asphaltenes present in the abovementioned crude oil and crude oil derivates.

Asphaltenes are a family of compounds soluble in aromatic solvents and insoluble in aliphatic hydrocarbons (N-pentane, N-heptane, etc.). Their structure and composition vary from one crude oil to another, but some atoms and groups of the said structure are always present in variable proportions. Among these atoms, mention can be made of oxygen, sulphur, nitrogen, heavy metals such as nickel and vanadium. The presence of numerous polycyclic groups give the asphaltene molecules a highly aromatic character. Due to their insolubility in aliphatic hydrocarbons and according to the more or less aromatic grade of the crude oil or petroleum cuts (also called derivates), the asphaltenes may precipitate. This mechanism causes the formation of deposits in the lines, production equipment, and on the source rock, which disturbs the production of the reservoirs and may, in certain cases, cause the shutdown of the installation.

The problem of asphaltene instability has been known for many years. Several solutions have been proposed. Thus “mechanical” solutions are known, such as desasphaltening, solvent washing, mechanical scraping, and also “chemical” solutions using stabilizers in the form of polymers or surfactants.

Document FR-A-2 679 151 thus describes a mixture of alkyl aryl sulphonic acid and a cationic surfactant of the imidazoline type. Similarly, document U.S. Pat. No. 6,180,683 discloses formaldehyde/alkyl phenol resins used in synergy with ethoxylated amines.

Even if these products have satisfactory stabilising capacities, they do not always offer the desired ecotoxic and toxicological profile. In fact, the oil industry as a whole is experiencing problems today associated with the handling of this type of product, as well as the harmful effects observed on the environment. This is especially true of the production of offshore oil fields (offshore platform, particularly in the North Sea, Gulf of Mexico, Coast of West Africa). At the same time, the products available on the market are seldom compatible with the OSPAR recommendations aimed to limit the environmental impact of the treatment products. In particular, they are not biodegradable.

The problem is therefore to propose an alternative to the synthetic chemical asphaltene stabilizers by developing products which are compatible with the OSPAR recommendations and which are in particular biodegradable.

As part of his researches, the Applicant has found that molecules comprising a tetrapyrrolic ring, such as chlorophyll for example, are particularly effective for stabilising the asphaltenes present in crude oils and petroleum cuts.

Document U.S. Pat. No. 5,826,369 describes a combustion catalyst for use as an additive in a boiler. In practice, the catalyst injected in the burner is in the form of a mixture of β-carotene, chlorophyll and jojoba oil. No reference is made to the stabilising properties of chlorophyll with regard to asphaltenes.

The invention therefore relates to the use of a biodegradable molecule or a mixture of biodegradable molecules having tetrapyrrolic rings, as stabilizers for the asphaltenes present in crude oil and crude oil derivatives.

In the rest of the description, the expression “molecules having tetrapyrrolic rings”, means any molecule comprising at least one tetrapyrrolic ring in its structure, the tetrapyrrolic ring being substituted or not, the molecule being biodegradable.

Among the usable molecules, mention can be made in particular, but in a non-limiting manner, of chlorophyll, porphyrins and chlorins.

In an advantageous embodiment, the biodegradable molecule used as a stabilizer is chlorophyll.

Chlorophyll can be obtained by chemical synthesis or used in the form of a plant extract. Any chlorophyll-rich plant can be used as a source of chlorophyll. It may for example be an extract of alfalfa, Graminaceae, algae, etc. These extracts are obtained from the parts of chlorophyll rich plants. The extracts used are advantageously alcohol extracts, in particular using ethanol.

Several chlorophylls exist, differing from one another in the type of substitutes present on the nucleus, the chlorophylls being capable of complexing metals. Magnesium is usually the sequestered metal; it is easily replaced by other metals including copper. The magnesium form is unstable at temperatures above 50° C. The chlorophyll used in this case is cupric chlorophyll because of its stability. The ions can be substituted directly on the plant extract.

The invention also relates to a method for stabilising asphaltenes in crude oil or crude oil derivatives, consisting in injecting into the oil a biodegradable molecule or a mixture of biodegradable molecules having tetrapyrrolic rings.

In practice, the molecules of the invention are objected at the rate of 1 to 500 ppm and advantageously from 1 to 50 ppm of the oil.

When the molecule comprising tetrapyrrolic rings is chlorophyll in the form of a plant extract, it is previously diluted before use to a mass concentration of between 2 and 10%. Obviously, a person skilled in the art will appropriately select the useable diluent, which is selected in particular from alcohols, ketones, aromatic solvents and, ideally, a mixture of esters of vegetable oils or the vegetable oils themselves.

The invention and the advantages thereof will appear more clearly from the following embodiment.

Preparation of the Stabilizer of the Invention

An alfalfa leaf extract taken up in a hexane/acetone mixture is mixed with copper nitrate. The concentrated cupric chlorophyll extract obtained by ion exchange is diluted in a methyl ester of oleic acid to a mass concentration of 10%.

The effectiveness of the additive as asphaltene stabilizer and dispersant is evaluated by the procedure described below.

The asphaltenic components of the hydrocarbon medium, crude oil or petroleum cut are separated from the medium by selective precipitation with N-heptane.

The components thus separated are dissolved in toluene (asphaltene solubilizer) at the rate of 20 g/l. The additive is prepared with 0.05% of active matter in toluene.

In 20 ml test tubes, 200 μl of asphaltene solution is mixed with 20 to 400 μl of additives.

The tubes are shaken. An aliphatic solvent (N-heptane) is rapidly added to each tube. The tubes are closed, then overturned five times. The effectiveness of the additive is determined by observation of the precipitation and settling of the asphaltenes over time, after 24 h. A clear and dark solution free of solid precipitates characterizes a satisfactory stabilization of asphaltenes in the precipitating aliphatic solvent, while a clear and pale solution having a dark deposit is an indicator of a lack of stabilization.

The tests were performed on asphaltenes extracted from various crude oils.

The table below shows the volume of cupric chlorophyll solution used to stabilize these various asphaltenes in comparison with a common additive sold under the trade name ANTICOR DSA 801.

As the table shows, the quantity of cupric chlorophyll required to stabilize the various asphaltenes is very low and comparable to the batches used with the product ANTICOR DSA 801.

The invention and the advantages thereof clearly appear from the above description. Noteworthy is the use of a biodegradable stabilizer which can be obtained particularly simply and has effects on asphaltenes of different origins and compositions.