Summary
Anticipating when and where asphaltenes may flocculate during oil production
is a key step in successfully preventing or mitigating asphaltene problems in
the field. Because there will be no deposition without precipitation, mapping
of asphaltene stability over a wide range of temperature, pressure, and
composition is required. The asphaltene-instability-trend (ASIST) allows the
determination of the onset of asphaltene instability to be established with a
series of liquid n-alkanes. These data are used to predict asphaltene
stability of live fluids by extrapolating the onset condition from the base
data to reservoir conditions by use of a linear extrapolation of the onset
solubility parameter vs. square root of the partial molar volume of the
precipitant. This extrapolation has been demonstrated previously to be accurate
for methane and a model oil. The present work verifies that such an
extrapolation is valid for predicting the asphaltene instability for mixtures
of methane, ethane, and propane with a representative stock-tank oil (STO). The
STO was combined with known amounts of methane, ethane, or propane. The
asphaltene onset pressure was determined by a combination of near-infrared
(NIR) light scattering and microscopic observation. The onset conditions at
ambient pressures were examined for flocculation periods ranging from 20
minutes to 24 hours. Onset pressures calculated with the 5-hour ASIST trends
compared well with measured onset pressures.
Introduction
Asphaltenes are materials in a crude oil that are among the highest
molecular weight and/or those with the highest degree of polarity (Long 1981)
and aromaticity (Cimino et al. 1995). These large molecules have limited
solubility in saturated hydrocarbons, tend to form aggregates that are
dispersed in crude oils, and form separate heavy phases in response to changes
in pressure, temperature, or composition of the oil mixture. The amount of
material that separates (asphaltene amount) and the solubility conditions at
which separation occurs (asphaltene-flocculation onset) vary not only with the
composition of the asphaltenes themselves, but also with the size of the
paraffinic precipitants, whether these are the native crude-oil light ends or
added nonsolvents (typically, n-pentane or n-heptane).
Wang (2000) demonstrated that asphaltene onset conditions--defined in terms of
the mixture refractive index (RI), which for nonpolar species can be related to
the mixture solubility parameter (δ) (Buckley et al. 1998)--changed
predictably with molar volume of n-alkane precipitating agents.
Extrapolation of the relationship established by measurements with liquid
n-alkanes to predict asphaltene instability during depressurization was
proposed (Wang and Buckley 2001a). Numerous successful applications of ASIST
(Buckley et al. 2007) have since been published (e.g., Wang et al. 2004)
suggesting that extrapolation of the ASIST relationship is reasonable.
Nevertheless, direct verification is needed of the link between asphaltene
instability, upon addition of normal alkanes, and the asphaltenes that appear
during depressurization of a crude oil.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
4 February 2008
- Meeting paper published:
5 May 2008
- Revised manuscript received:
16 September 2008
- Manuscript approved:
3 October 2008
- Published online:
1 May 2009
- Version of record:
1 May 2009
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