Summary
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 is common
to the industry as whole. In Italy, regular treatments with 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 redeposition of the asphaltene in the formation and
increases the rate 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 used successfully 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 used.
This is of particular significance to those wells where large volumes of a
washing phase have to be pumped downhole. Hazards also have 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 southern Italy will be discussed,
describing the placement technique used 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 almost 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 or petroleum ether. 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 atoms of sulfur, 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 normally
are 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 (e.g., along a flowline collecting oil from different
wells/reservoirs). To compound the problem further, 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, as 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.
The variable nature of the asphaltene problems is caused by reservoir
conditions and chemistry of the oil. Intervention-treatment design and timing
is based generally on local practices that are put in place to manage the
problem. In the field described in this paper, there had been long-established
practices to determine the timing and the method of the intervention. The same
practices, however, were no longer achieving the success of the past, and the
severity of the problem was increasing with the age of the field. To improve
the performance of the treatments with the changing reservoir conditions, a
review of the local practices was implemented. It was during this review that
the emulsion system described in this paper was developed. The remainder of the
paper describes the methods used to develop and optimize the solvent, leading
to the development of the emulsion system. This emulsion system was then
applied in the field. Two case histories in two different well configurations
(perforated casing and horizontal open hole) in the same field are described to
illustrate the application.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
28 February 2006
- Meeting paper published:
24 September 2006
- Revised manuscript received:
30 April 2007
- Manuscript approved:
7 November 2007
- Version of record:
15 August 2008
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