Summary
Asphaltene study is now becoming a regular menu as a part of gas-injection
studies (Kokal et al. 2003, 2004; Yin et al. 2000; Srivastava et al. 1999; Yin
and Yen 2000; Parra-Ramirez et al. 2001; Sarma 2003; Jamaluddin et al. 2000;
Negahban et al. 2005; Okwen 2006; Moghadasi et al. 2006). The asphaltene onset
pressure (AOP) is one of the most important factors in understanding asphaltene
precipitating behavior. The solid detection system (SDS) based on
light-scattering technique has been quite popular and widely used in all over
the world (Kokal et al. 2003, 2004; Jamaluddin et al. 2000; Negahban et al.
2005; Gholoum et al. 2003; Garcia et al. 2001; Oskui et al. 2006; Gonzalez et
al. 2007) to measure AOP. The simple experiments to measure AOP are usually
conducted using a mixture of reservoir fluid and injection gas, and various
gas-mixing volumes are assumed to be investigated. These various experimental
specifications of gas-mixing volume are useful in understanding asphaltene
risks during gas-injection projects. However, this type of investigation can
show only a static asphaltene behavior, and sometimes might overlook true
asphaltene risks.
In the gas-injection pilot (GIP) project in an offshore carbonate oil field
in the Arabian Gulf, the static asphaltene behavior was studied by the SDS
using near-infrared (NIR) light-scattering technique. For this study, a
single-phase bottomhole sample was collected from the same producing zone, but
the sampling location was 90 ft shallower than the GIP area. Various
combinations of mixtures were examined to measure AOP (i.e., reservoir fluid
mixed with 0, 25, 37.5, 43.5, and 50 mol% injection gas). Furthermore, the
numerical models were generated and calibrated with the experimental findings.
To evaluate the asphaltene risks at the GIP area, the models were adjusted to
the target oil composition by considering existing oil compositional gradient
in the field. However, the modeling analyses showed that the operating
conditions of producing wells are outside the estimated
asphaltene-precipitation envelope (APE). This result was inconsistent with the
field fact in which actual asphaltene deposits were observed and collected from
the bottomhole of some wells in the GIP area. Thus we were obliged to recognize
that our current experimental results of static asphaltene behavior overlooked
the actual asphaltene risks. What is insufficient for realistic modeling? Our
hypothesis is the dynamic asphaltene behavior.
During a gas-injection process, the injected-gas composition is changed
because of a vaporizing-gas-drive (VGD) mechanism, in which gas was enriched
with the intermediate-molecular-weight hydrocarbons from reservoir oil. Our
latest experiments investigated a static asphaltene behavior only; that is, it
did not include this process. Therefore, the sensitivity analyses were
motivated to realistically evaluate the actual APE, counting the VGD effects
with the calibrated model. Various enriched-gas compositions were investigated
in terms of how these enriched gases would affect APE. Consequently, it was
found that the enrichment of intermediate components expanded the APE, and the
operating conditions of asphaltene-problematic wells could be placed inside the
APE. Therefore, we concluded that the dynamic asphaltene behavior must be
understood for a realistic risk evaluation in the gas-injection project.
© 2011. Society of Petroleum Engineers
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History
- Original manuscript received:
17 February 2010
- Meeting paper published:
8 December 2009
- Revised manuscript received:
28 September 2010
- Manuscript approved:
20 January 2011
- Published online:
3 August 2011
- Version of record:
15 August 2011
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