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asphaltene treatment. Asphaltene inhibitors prevent the asphaltenes fromdepositing into the system. The inhibitors work by stabilizing the
asphaltenes, preventing their flocculation and/or deposition.
This paper describes the evaluation of a Gulf of Mexico (GOM) deepwater
crude oil to determine its asphaltene deposition potential. This paper
further describes a systematic evaluation of asphaltene inhibitors in thecrude oil at reservoir and at stock tank conditions.
Discussion:
Pressure compensated DST fluid samples were taken from a deepwater
well in the Gulf of Mexico. This well is sub-sea tie back located in 5400
feet of water with a 10-mile tie back to the host platform. During the
course of system design it was necessary to evaluate this fluid forasphaltene deposition potential. The steps used to do this evaluation are
summarized as follows:
(1) Assess asphaltene stability in reservoir fluids at realistic production
conditions of temperature, pressure and/or composition using PVTanalysis, High Temperature Gas Chromatography, SARA Analysis,
Isothermal Depressurization Study and Isobaric Depressurization
Study.
(2) Screen various asphaltene inhibitors and optimize respective treatmentconcentrations using representative dead oil samples at ambient
conditions (i.e. minimize cost) with the Flocculation Point ApparatusTest and Asphaltene Dispersant Test.
(3) Confirm/validate (independently) the effectiveness of the selected
candidate asphaltene inhibitors and respective dosage for live fluids at
realistic production conditions of temperature, pressure and/orcomposition using the Isothermal Depressurization Study.
For asphaltene stability evaluation it is important to take a representativeDST sample in a pressure compensated cell in order to maintain reservoir
conditions. The results of the PVT analysis and analyses on dead oil are
depicted below.
TABLE 1: Downhole Conditions
Reservoir
Pressure
Temperature B.P. Depth
7063 psia/69MPa 190F/88C 3390 psia/33MPa 14,8000ft/3031M
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Table 2: Measured Fluid Properties
COMPONENT MOL%
CO2 0.15
N2 1.15
C1 40.91
C2 5.67
C3 5.37
I-C4 0.79
N-C4 2.39
I-C5 1.19
N-C5 1.3
C6 2.29C7+ 38.78
Molecular Weight 101.7
In situ density (g/cm3) 0.8445
Table 3: SARA Analysis on Dead Oil
Saturates Aromatic NSO Asphaltenes C15+
47.1 32.1 16.8 4.0 53.2
Figure 1: Isothermal Depressurization Graph and Asphaltene Onset Point
0 10 20 30 40 50 60 70
Pressure (MPa)
LaserPower(ArbitraryUnits)
OIL 1 @ 80 .6 C)
OIL 2 @ 68 .3 C)
Bubble Point
Asphaltene Onset
~ 50.4 MPa
Redissolution onset
~ 24.4 MPa
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Figure 2: FPA of Dead Oil
SDS Isothermal Depressurization Test (Figure 1) indicates that theasphaltene onset pressure point is approximately 4000 psia, which is about
650 psia above the bubble point. The asphaltene onset point is the
pressure at which asphaltenes phase out of the oil and start to agglomerate,forming particles large enough to prevent transmittance of the light
through the SDS cell. Generally, the higher that onset point pressure is
above the bubble point, the more unstable the asphaltenes are in the crude.
The SDS Isothermal Depressurization Test also measures the amount of
asphaltene particles that can be filtered out. At the end of the Isothermal
Depressurization Test the oil was filtered at 3000 psia through a 0.2mfilter. The filtrate was washed with warm n-pentane. The extracted solidsweighed 360 mg. The solids were then analyzed and they were
determined to be 90% asphaltenes. The amount of asphaltenes extracted
from the oil was 0.62% of the mass of the oil tested. This amount wasconsidered to be significant.
The FPA (Figure 2) test also indicated that the asphaltenes in the oil wereunstable. The FPA gave a flocculation point value of 16 mL. Oils with a
flocculation point value of 20mL or less are generally considered unstable.
The SARA analysis (Table 3), which also indicates asphaltene stability,was inconclusive.
Overall, the tests indicated that the oil contained unstable asphaltenes andasphaltene deposition problems were likely. Because of the possibility of
asphaltene deposition in this system, asphaltene inhibitors were evaluated
F P A T e s t o f G O M O i l
1 6 . 5 9 6
0
0 .2
0 .4
0 .6
0 .8
1
1 .2
1 .4
0 5 1 0 1 5 2 0 2 5
V o l u m e o f I s o -o c t a n e ( m L )
Voltageoftransmittance(V)
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on this oil. The ADT test was used to screen the chemical inhibitors. Thebest inhibitor was ratio tested using the ADT test. The results of the ratio
test are listed below in table 4.
Table 4: ADT Asphaltene Inhibitor Ratio Test
Asphaltene Inhibitor (ppm) ppt mls %T
0 ppm (untreated) 1 31
100 0.9 31
250 0.7 24
500 0.2 17
750 0.05 13
1000 0.05 13
The ADT test showed that it would take between 250 to 500 ppm of theasphaltene inhibitor to stabilize the asphaltenes enough to prevent
deposition. These results were further validated on the FPA test (Figure3).
Figure 3: FPA Test With Asphaltene Inhibitor
The FPA results showed that with 400 ppm of the asphaltene inhibitor,
there was no flocculation point indicating that the asphaltenes in the oil
were completely stabilized.
Isothermal Depressurization testing was repeated using 500 ppm of
asphaltene inhibitor. The onset point was not significantly affected by theaddition of the inhibitor. The amount of asphaltene recovered through
filtration however was reduced from 360 mg to 24 mg. The filtered
asphaltenes were considered negligible.
F P A T e s t G O M O i l T r e a t e d w i th 4 0 0 p p m i n h ib i to r
0
0. 1
0. 2
0. 3
0. 4
0. 5
0. 6
0. 7
0. 8
0. 9
1
0 5 10 1 5 2 0 25 3 0
V o l u m e o f is o - o c t a n e (m L )
VoltageofTransm
ittance
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Conclusion:
A systematic experimental program was developed to critically assess andcontrol the potential for asphaltene deposition from reservoir fluids. Byfollowing the methodology, the GOM crude oil was successfully evaluated
for its tendency to precipitate and deposit asphaltenes under typical
production conditions. An asphaltene inhibitor that stabilizes theasphaltenes was also successfully evaluated.
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Completions, Workover, and Stimulation (second edition), Oil & Gas
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