Abstract

The formation of asphaltene scale inside the tubing or in the reservoir is a common problem associated with crude oils in many parts of Italy and common to the industry as whole. In Italy regular treatments using coiled tubing or washing by bullheading are performed to re-establish production. While asphaltene inhibitors can be injected into the tubing string, asphaltenes can still create problems below the injection point and plug the perforations, formation pores, and/or natural fracture network systems.

There is a wide range of hydrocarbon-based solvents that have been used in the industry to remove asphaltenes. The more effective solvents have a low flash-point temperature, making them expensive and hazardous. In addition these hydrocarbon-based solvents leave the formation in an oil-wet state after asphaltene removal instead of re-establishing the water-wet condition that acts as barrier to slow down the deposition of the asphaltene on the formation. This effect accelerates the re-disposition of the asphaltene in the formation and increases the speed of the production decline, increasing the frequency of remedial treatments.

This paper describes the laboratory development and field application of a water/aromatic solvent emulsion system that has been successfully used to clean/dissolve asphaltene and leave the carbonate fractured formation in a water-wet state to delay the production decline. Other advantages when using this type of emulsion are cost reduction and improved effectiveness in removing asphaltene deposits, when compared to alternative solvents that have been employed. This is of particular significance to these wells where large volumes of a washing phase have to be pumped downhole. Hazards have also been reduced by using relatively high flash-point aromatics. Continuous mixing of the emulsion when pumping reduces waste and improves the logistics involved in pumping the large volumes needed to treat long, openhole sections and/or to treat the fractures deeper in the near-wellbore region.

Two successful field applications in south Italy will be discussed in detail, describing the placement technique employed and the results achieved with this new system. These treatments will be compared to previous treatments using a hydrocarbon-based solvent. In the first well, where previous treatments had failed to make significant improvements, following the application of this emulsion the production was practically fully restored and the production decline was significantly slower than previous treatments. The second well treated was a long, horizontal wellbore; again the emulsion and technique proved successful in returning the production to previous levels and sustaining the new level for an extended period of time.

Background

Asphaltene is well known in the industry for causing production loss through plugging the tubing, perforations, and formation. The term "asphaltene" is applied to the black, carbonaceous components of petroleum; these compounds occur in many crude oils in the form of colloidal, suspended, solid particles. They are characterized by their insolubility in light paraffin hydrocarbon solvents, such as pentane, petroleum ether, etc. Chemically, the asphaltene fraction of petroleum is composed of polycyclic, condensed, aromatic rings with several side chains. These compounds have relatively high molecular weights and are considered polar materials because molecules of sulphur, nitrogen, oxygen, and complex metals are present.

Asphaltene precipitation takes place when the crude oil loses its capability to disperse and stabilize the particles. The asphaltene stability depends on the composition of the crude oil, temperature, pressure and the nature of the reservoir rock surface. Under static reservoir conditions, asphaltenes are normally held in a stable suspension by resins, a family of polar molecules. Changes in fluid temperature and pressure that are associated with oil production from the reservoir may cause the asphaltene to flocculate and precipitate out of suspension and adsorb to the rock or pipe surfaces. Additionally, the asphaltenes may flocculate because of electrical charges created by the motion of flowing hydrocarbons. Asphaltenes may also flocculate by mixing of different oil types, for example, along a flowline collecting oil from different wells/reservoirs. To further compound the problem, emulsions can be stabilized by asphaltenes. Regardless of the mechanism causing the asphaltene to deposit the result is a plugging effect that inhibits or reduces oil production. Precipitation of asphaltene particles may also provide nuclei for paraffins to start precipitating e.g., in the case of the wells discussed in this paper where the deposits are frequently a combination of asphaltene and paraffin, often associated with inorganic material such as formation solids, salts, and iron oxides.

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