Materials and Methods
2.2.5 Characterization of Crude Oil
The laboratory analysis of crude oil is necessary for characterizing a particular reservoir fluid sample and also to make a preliminary assessment of the effectiveness of EOR schemes. In this study, the crude oil was analyzed for API gravity, viscosity, asphaltene-resin content, wax content, pour point, and acid number.
Density is the mass of liquid per unit volume, while specific gravity is the ratio of the mass of a given volume of liquid at 15.50C to the mass of an equal volume of pure water at the same temperature. The ASTM D1298-67 [IP 150/68] hydrometer method was followed for the determination of the density of crude oil samples. The hydrometer is made of glass, graduated in units of density. The API gravity was calculated from the specific gravity by using the following equation:
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0 0
0 141.5
- 131.5 Sp.gr. at15.6 C/15.6 C
API gravity = (2.5) The dead oil viscosity of the oil samples was measured following the ASTM D445- 65 method using U-tube reverse flow viscometer (type: BS/IP/RF). The selected viscometer was charged with crude oil and allowed to remain in the bath long enough to reach the test temperature. Suction pressure was used to adjust the head level of the oil sample to the first timing mark of the capillary arm of the viscometer. Allowing the sample to flow freely, the time (in sec) required for the meniscus to pass from the first timing mark to the second was recorded. The kinematic viscosity was calculated as follows:
Kinematic viscosity (cSt) = Ct (2.6) where C = Calibration constant of the viscometer (cSt/sec),
t = Flow time (sec).
From the kinematic viscosity, the dynamic viscosity was calculated as follows:
Dynamic viscosity (cP) = ργ (2.7) where ρ = Density at the same temperature as kinematic viscosity,
γ = Kinematic viscosity (cSt).
The experiments were repeated for three times and the average values are considered.
As per standard test method IP 15/67, the pour point is the lowest temperature, expressed as a multiple of 3, at which crude oil is observed to flow when cooled and examined under prescribed conditions. The oil sample was poured into a cylindrical test jar (25 mm OD and 60 mm in height). The oil was initially slightly heated in a hot water bath to ensure it flowed into test jar. The temperature of the cooling bath was maintained at 0 to 2 oC. The specimen was first examined at 30 oC. The test jar was tilted just enough to ascertain if there was movement of the specimen. If movement was noticed, the test jar was replaced in the jacket and the test repeated again at 30C lower. This was continued
Chapter 2 Materials and Methods
until the point when the specimen showed no movement when the test jar was held in a horizontal position for 5 sec. The reading observed in the test thermometer was recorded.
As per the method, 3 oC was to be added to the temperature recorded, which gives the pour point of the crude oil [145].
The asphaltene content of oil samples was measured by asphaltene precipitation with n-heptane (ASTM D2007-80) as reported by Bon [146]. 5 gm of the crude oil was mixed with 150 ml of n-heptane. The mixture was agitated with a stirrer for 1 hour and filtered through preweighted 125 mm No.42 Whatmann filter paper. The filtrate was used for the determination of resin content. The filter paper with solids was dried in an oven at a temperature of 60 0C until a constant weight was obtained. The solids from the filter were re-dissolved with toluene and filtered again through a new filter paper. The filter paper with solids was again dried in the oven at a temperature of 60 oC until constant weight obtained. From the measured change in weight of the filter paper, asphaltene content of the crude oil was determined accordingly. To estimate the resin content, the filtrate was mixed with 200 g of silica gel and the mixture was continuously stirred with heating for 1 hour so that the resins were adsorbed to the silica gel. After cooling for one overnight, the liquid portion was filtered. The silica gel was placed in the thimble of the soxhlet apparatus and extracted with toluene: methanol (90:10) solvent until the silica gel becomes free from resin. The solvent was evaporated in the water bath followed by drying in the oven, and eventually, the weight of the resin was determined [147].
The filtrate after removing asphaltene and resin was concentrated to about 50 ml on the water bath. The concentrate was mixed with 200 ml of methyl ethyl ketone (MEK) and cooled in a deep freezer. The mixture was kept overnight in the freezer to allow for possible wax precipitation. Finally, the mixture was filtered with Whatmann filter paper (size 42), dried in ambient temperature and weighted to determine the wax content [148].
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To measure the acid number of crude oil, 1 ml of the phenolphthalein solution was added to 50 ml of C2H5OH in a conical flask and the mixture was heated to 40-50 oC. The mixture was neutralized with standard 0.1 N KOH solution. The neutralized alcohol was added to 10 gm of crude oil in a 250 ml conical flask and the mixture was heated to boiling for 5 min with agitation. Further 1 ml of phenolphthalein solution was added to the mixture and cooled to 45 oC. Finally, the solution was titrated quickly with the KOH solution. The acid number of the oil was calculated using the formula [145]:
AN = 56.1 NV
W (2.8) where, AN = Acid number in mg KOH/g of sample
V = Volume of KOH solution in ml N = Normality of the KOH solution W = Weight of the sample in gm.
The crude oil was also analyzed with FTIR spectrophotometer to determine the functional groups present mainly the existence of carboxylic acids. For the analysis, analytical grade potassium bromide (KBr) pellets were prepared and a small drop of the crude oil was added to the prepared KBr disc.