Summary
A large set of field data have been collected during the last decade related
to various facets of asphaltene instability problems at our production
facilities. These problems include compatibility of heavy oil with hydrocarbon
diluents in a Venezuelan operation, commingling of live oil and condensate in a
North Sea production facility, compatibility of drilling mud-base oil, and
miscible injectants with reservoir fluids in Alaskan operations. In each of the
field cases, significant lab data were generated by titrating the neat dead
crude oil, and oil-solvent blends with n‑alkanes. The solvents used to study
the asphaltene issue were toluene, condensate, and base-oil. We have applied
available asphaltene prediction techniques (Heithaus 1960, 1962; Wiehe and
Kennedy 2000a, b; Wiehe et al. 2001; Andersen 1999; Wang and Buckley 2001; Wang
et al. 2003, 2004; Leontaritis 1998) to explain the field data. None of the
models has been found comprehensive enough to explain flocculation at all the
conditions, including the flocculation that occurs at ambient conditions in the
presence of paraffinic diluents, stability enhancement that occurs upon
addition of aromatic solvents, and the instability that occurs in a live fluid
because of changes in composition, pressure, and temperature. To handle a
complex crude oil system, these models made some simplifying assumptions that
enabled them to make the problem manageable. In doing so, they lose some
predictive capability. We found there are two forces that need to be accurately
captured—dynamics of the alteration of solubility parameter of the hydrocarbon
matrix, and change in entropy of mixing—to model the asphaltene behavior. The
latter has been either empirically estimated by extrapolating the ambient
titration data or neglected in many of the previous models. The basic
parameters for our model can be calculated from lab data generated by titrating
the dead crude oil, or oil solvent blends with n-alkanes, at different
temperatures. So far, this model has been applied to various field conditions
in production facilities and has been found successful in matching the field
data.
Introduction
Asphaltenes are generally defined as the fraction that is soluble in
aromatic solvents such as benzene or toluene; and insoluble in
normal alkanes, such as n-pentane or n-heptane (Speight 1999). Asphaltenes are
typically stable in a live fluid at reservoir condition. Once the drilling and
production starts, the change in pressure, composition, and temperature can
cause asphaltenes to destabilize. Asphaltene precipitation and deposition
in production and operation are undesirable situations that are quite
expensive to remediate.
The solubility parameter concept first described by J.H. Hildebrand and R.L.
Scott (1964) was used as a compatibility indicator, such that if the solubility
parameter of any mixture was greater than the solubility parameter at the onset
point of precipitation, that mixture was stable—otherwise asphaltene would
flocculate..
This paper focuses on the development of a comprehensive asphaltene
precipitation model based on the solubility parameter concept. A successful
model should be able to predict onset of asphaltene flocculation under any
field condition. Several field projects are also included in this paper to show
the wide application of the asphaltene precipitation predictor in several
different circumstances. For example, asphaltenes can precipitate if an
incompatible blend of two or more hydrocarbon liquids are mixed in sales
pipeline or in a refinery. Asphaltenes can cause sludge formation and emulsion
issues in surface facilities (flow line or separators). Asphaltenes can drop
out during depressurization of live oil near wellbore during production or
commingling with miscible injectant (MI) in the reservoir during enhanced oil
recovery.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
6 February 2006
- Meeting paper published:
1 May 2006
- Revised manuscript received:
2 August 2006
- Manuscript approved:
5 August 2006
- Version of record:
20 August 2007
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