SECOND TECHNICAL SEMINARDATE: 25.03.2014

VENUE: PS3 JORHAT

1

CRUDE OIL RHEOLOGY & FLOW IMPROVER MECHANISM

ALAKESH DEKAME(O)PS2 MORAN

CRUDE OIL RHEOLOGY & FLOW IMPROVER MECHANISM

1. Crude oil characteristics & composition.

2. Relationship between different properties & their effect during pumping operation.

3. What are flow improvers (PPD)?

4. Working principle of PPD chemicals.

5. PPD dosing techniques in OIL (PS1 & PS2).

6. Effect of varying temperatures on PPD chemical at PS2 (Case Study).

7. Pour point of OIL, ONGC crude & line temperatures throughout OIL’s pipeline a general overview.

POINTS TO BE COVERED

CLASSIFICATION OF FLUIDS:

• Newtonian fluids: The magnitude ofthe viscosity is not dependent onshear rate or time. It exhibits constantviscosity. It shows a linear relationshipshear stress & shear rate and passesthrough the origin. Water and Honeyare Newtonian liquids.

CRUDE OIL CHARACTERISTICS & COMPOSITION

Time-Independent Non-Newtonian Fluids

• Shear thinning (“pseudo plastic”)The apparent viscosity of the fluid decreaseswith increasing shear rate. No initial stress(yield stress) is required to initiate shearing.

• Shear thickening (“dilatant”)The apparent viscosity of this fluid increaseswith increasing shear rate and no initialstress is required to initiate shearing.

• Bingham plastics are a special class ofviscoplastic fluids that exhibit a linearbehaviour of shear stress versus shear rateonce the fluid begins to flow. Some finiteshear stress must be applied before thematerial will flow. This minimum stressrequired is known as the yield stress.

Wet sand

Time-Dependent Non-Newtonian Fluids

• ThixotropicA thixotropic liquid will exhibit a decreasein apparent viscosity over time at aconstant shear rate. Once the shear stressis removed, the apparent viscositygradually increases and returns to itsoriginal value.

• RheopecticA rheopectic liquid exhibits a behaviouropposite to that of a thixotropic liquid, i.e.the apparent viscosity of the liquid willincrease over time at a constant shearrate. Once the shear stress is removed,the apparent viscosity gradually decreasesand returns to its original value.

At constant shear rate . . .

ha

Time

Thixotropic

Shear thinning

Rheopectic

Shear thickening

Shear on Shear off

• Assam crude oil is waxy in nature (wax content 12 – 20 %w/v, pour point 21-27oC) and thus has poor flow behaviour at low temperature. n-Alkanes are main constituents of these waxes.

• Digboi Crude oils have high wax contents (8-17% w/v) and high pour point (typically 18oC).

COMPOSITON

CRUDE OIL (based on polarity & solubility)

Asphaltenes (6%)

Aromatics (15 %)

Saturates (78%)

Resins (1%)

Saturates are non-polar hydrocarbons including straight chain and branch alkanes, and cycloparaffinscompounds (naphthenes).

Aromatics refer to all compounds with one or more aromatic nuclei, generally are the lightest fractions of the crude oil.

Asphaltenes are the heaviest, most aromatic components of the crude oil with the largest percentage of heteroatom (O, S & N).

Polar molecules containing hetroatoms such as N, O or S

Precipitation cause severe problems in pipelines

WAXComposition

Paraffins 30%

number of carbon atoms in the range of 18–36

Naphthenes 49%(Cycloalkanes)number of carbon atoms in the range of 30–60

Asphaltenes 6%composed of aromatic rings containing sulfur, nitrogen and alkyl side chains up to C30

• PARAFFINS: or alkanes, are saturated hydrocarbons that result fromcombining CH 2 groups in succession. The additional CH 2 groups areadded to form straight chain paraffins.

• The distribution of n-alkanes in a paraffin wax sample:

• The crude oils having high paraffin content are called waxy crude oils.• At higher temperature, the high molecular weight paraffins (wax) are

dissolved in the crude oil.• when the temperature of the pipeline drops below the CLOUD POINT which is

also known as the Wax Appearance Temperature , Paraffin wax precipitatesand forms deposits on the pipe wall.

• Further decreasing the temperature, a stage is reached when crude oil ceasesto flow and is called POUR POINT.

• Asphaltenes are composed of aromatic rings containing sulfur, nitrogen andalkyl side chains up to C30 . Functional groups like ketones, phenols, andcarboxylic acids are observed as elemental functional groups of asphaltene.They are extremely complex structures.

• Asphaltene deposition has been found to increase with a decrease in pressuren-C7 Asphaltenes & n-C5 Asphaltenes

WAX

Macro crystalline, paraffin or distillate waxes

Microcrystalline or amorphous waxes

composed of mainly straight-chain paraffins (n-alkanes) ranging from C20 to C50

containing higher proportions of isoparaffins (branched chain alkanes) and naphthenes (cyclic alkanes) with somewhat higher carbon numbers ranging from C30 to C60

2 TYPES

FLOWING CRUDE OIL

OIL THICKENS WITHOUT

SHEAR

OIL SOLIDIFYING SEMI SOLID

WAX

SOLID & HARD WAX

• Crude oil shows a non-Newtonian shear thinning behaviour over the range of shear ratesfrom 0.6 s-1 to 720s-1, this means that the flow encounters less resistance at higher shearrates.

• Crude oil show thixotropic properties below their pour pointThe characterization of thixotropic properties correspond to the gel structure graduallybreaking down under the action of a constant shear stress or shear rate . When subjectedto varying rates of shear, a thixotropic fluid will demonstrate a "hysteresis loop".

• Pre-treated waxy (equilibrium state) oils behave as Bingham plastic fluids.Heating the sample to a sufficiently high temperature such that the wax crystals fullydissolve then cooling it.The Bingham yield value is defined as the shear stress required for initiating flow and it isimportant because it measures the ability of fluid to restart its flow after shutdown

CORRELATION

• Drop in pressure decreases melting point temperature and allows someof the larger hydrocarbons to precipitate out of the oil solution.Decreasing the temperature also allows some of these paraffins to createorthorhombic shaped wax crystals which are carried through the oil.

• The aforementioned wax crystals create networks and trap oil molecules.This amalgam of wax crystals and oil then creates a gel matrix. This gelmatrix ends up trapping other wax crystals present, and the amount ofwax in the pipes increases over time. With a further decrease intemperature, this waxy, gel-like substance hardens and deposits on thewalls of the pipe in which the hydrocarbons are being transported , thusreducing its fluidity

Figures: Paraffin Wax Deposition in a Pipeline

• Crude viscosity governs the pressure drop in a pipeline.

• Increasing line average temperature reduces the crude oil viscosity. The viscosity reduction caused higher Reynolds number and in effect lowered pumping power requirements.

• Increasing crude oil API decreases crude oil viscosity thereby reducing pumping power.

Relationship between different properties & there effect during pumping operation

22.2

33.3

45.5

0

5

10

15

20

25

30

35

40

45

50

242118

Plastic viscosity

Crude temperature

Series1

85

58

0

10

20

30

40

50

60

70

80

90

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

Apparent viscosity

Crude oil temperature

Series1

Crude temperature & apparent viscosity

Crude temperature & pressure drop

72

64.8

60.3

54

56

58

60

62

64

66

68

70

72

74

18 19 20 21 22 23 24

Calculated pressure

drop

Crude Temperature

Series1

Pipeline exp data Feb 1999 at PS4-PS5 sector

Effect of Flow Rate on Wax Deposition

The yield stress & viscosity may be used to predict the restart pressure& flow after restarting of a pipeline.

As the shear rate increases, the chain type moleculesdisentangled, stretched, and reoriented parallel to the driving force, andhence reduced the crude oil viscosity.

Above the yield stress point, the applied stress leads to unlimiteddeformation which causes the sample to start flow. The yieldpoint, which is required to start the flow, decreases with temperature.At a higher cooling rate, the rate of wax precipitation is higher. Hence, ahigher stress is necessary either to aggregate the crystals, or tobreakdown the structure.

Polymers designed to interfere in the wax crystallization process, thusmodifying the crystal structure of the paraffin present in the crude oil.

These polymers are structured so that part of the molecule is likeparaffin wax crystals and acts by providing nucleation sites and cocrystallizes with the waxes. The other part of the structure is dissimilarto the wax crystals and blocks the extensive growth of wax matrices.Thus change in the crystal shape diminishes the ability of waxyaggregates to inter grow and inter lock, resulting in lowering the pourpoint of the crude for transportation of oil.

WHAT ARE FLOW IMPROVERS (POUR POINT DEPRESSANTS)?

They typically have a wax-like paraffinic part that co-crystallises with wax-forming components of oil, and a polar component limiting the degree of co-crystallisation.

Polymers such as vinyl acetate copolymer, acrylate copolymer and their derivatives are the main additives used to improve the flow-ability of waxy crude oil at low temperature.

Flow improvers are very selective; that is, not all additives are sufficiently effective for every crude oil.

• These additives function by mechanisms like nucleation, adsorption, co-crystallization and improved waxy solubility.

• The mechanism for improving the crude oil viscosity by using additivescan be referred to its ability to disperse saturates & asphaltenemolecules. The additives disperse the saturates & asphaltenes andconcentrate them on wax crystals to impede the formation of largecrystals. Therefore, the size of the wax crystals is in a reduced form tolower the viscosity of crude oils.

• The crystal growth rate of these waxes is slower than that of the normalwax crystal without PPD.

WORKING PRINCIPLE OF PPD CHEMICALS

• The PPDs on adsorption on the surface of the wax renders its nucleiinactive for further growth. The waxes then occur in small sizedparticles distributed within the crude oil samples, thus cannot form network like structure required for solidification and deposition.

• Asphaltenes appear to play an important role in pour point reduction.They either symptomize waxes that are susceptible to chemicaladditives, or else interfere with the congealing mechanism. Thepresence of asphaltenes with waxes indicates that the waxes will bemodified by chemical additives.

Generally, the polar component of the additive creates the barrier to the formation of the interlocking crystal wax network. As a result, the altered shape and smaller size of the wax crystals reduce the formation of the interlocking networks and reduces the pour point.

The structures involved in this process are: the pendant chains to co-crystallize with the wax and the polar end groups which are responsible for disrupting the orthorhombic crystal structure into a compact pyramidal form. This process prevents the crystals from agglomerating and forming a gel-like structure to deposit on the pipeline surface

Figure 2: PPD Inhibition mechanism of wax modification. 2a) Chemical structure of wax 2b) Crystal shape of wax structure 2c) Crystal structure of growing wax lattice 2d) Polymeric Additive with wax-like components 2e) Co-crystallization of wax and PPD 2f) satirically hindered wax structure

Figure : Prevention mechanism of interlocking wax crystals by polymer additives. a) nucleating site interaction (red) to asphaltene and wax molecules (blue) b) Polar component of additive (green) hinder co-crystallization of asphaltenes and wax.

Figure : Prevention mechanism of interlocking wax crystals by polymer additives.

• It was observed that dosing techniques at PS1 & PS2 were varying.

• At PS1 dosing is carried in the line carrying crude oil from CTF topipeline boosters.

• PPD as seen is directly injected into pipeline.

• At PS2 dosing is carried in the line carrying mixed crude from OCS toCTF.

• Due to various process like tank heating, electrolytic treatment the PPDtraverses through different equipments before entering the pipeline.

• Similarly for ONGC crude at PS-2.

PPD DOSING TECHNIQUES IN OIL (PS1 & PS2).

16” pipeline

Aldrich Pumps

CTF Tank (48-52 C)

CTF Tank ( 42-48 C)

EET (62-68 C)

PPD tanks

Metering pump

Water Cut

PPD dosing150 ppm

Mix crude OCS 1

Steam

CTF MORAN CRUDE OIL CONDITIONING

60 C

CRUDE OIL FROM SHALMARI & MORAN OCS

Dosing facility at Moran CTF

16” pipeline

Aldrich Pumps

CTF Tank

Mix crude Steam

CTF DULIAJAN DOSING

PPD tanks

Metering pump

PPD dosing ( 150 ppm & 100 ppm)

60 C

PPD from tank to line using Dosing pumps at PS1

PPD from drums to storage tanks

BRANCH LINE DOSING

MAIN LINE DOSING

• As the PPD had to traverse through different process it was decided to test samples during different periods of crude oil treatment at PS-2.

• Samples▫ Crude oil with PPD mixed collected at CTF tank.

▫ Crude oil coming to tank after traversing through EET .

EFFECT OF VARYING TEMPERATURES ON PPD CHEMICAL AT PS2 (CASE STUDY)

DatePERIOD Pour Point TEMPERATUREs

28 25 22

Plastic viscosity Yield stress

25.11.13 BEFORE EET 21 6 8 12 2 4 8

27.11.13 AFTER EET 24 6 11 19 2 2 4

• From the test result we observe that the pour point has increased afterthe crude oil has undergone the conditioning process.

• This may be due to▫ PPD chemical loss in water draining during the conditioning process.

▫ Due to repeated heating & cooling of crude oil PPD may loss its properties.

▫ Transferring crude oil through EET may cause altering of PPD propertiesdue to the high current involved in it.

Pour point of OIL, ONGC crude PS-2 Lab Test

Date Crud e oil Pour PointTEMPERATUREs

28 25 22

Plastic viscosity (cP) Yield stress

25.11.13 OIL 27 4.5 4.5 11 <2 <2 2

27.11.13 ONGC 24 4 5 10 <2 <2 4

29.11.13 ONGC 24 8 9.5 18 <2 1 11

02.12.13 OIL 24 4.5 8.5 15 <2 2 6

08.12.13 ONGC 24 11 16 29 <2 4 12

11.12.13 ONGC 27 5 7 14 <2 2 8

19.12.13 ONGC 24 8 9 17 <2 <2 12

20.12.13 ONGC 27 6 12 21 <2 8 22

21.12.13 OIL 24 6 10 27 <2 2 8

24.12.13 ONGC 24 10 14 20 <2 2 12

26.12.13 OIL 27 10 12 15 <2 2 8

Date Crud e oil Pour PointTEMPERATUREs

26 24 22

Plastic viscosity(cP) Yield stress (Pas)

16.11.13 OIL 9 12 14 18 2 2 4

22.11.13 OIL 12 12 14 18 2 2 4

16.12.13 OIL 12 12 14 18 2 2 4

28.12.13 OIL 9 11 13 18 2 2 4

06.01.14 OIL 12 11 14 18 2 2 4

28.01.14 OIL 9 12 14 18 2 2 4

25.02.14 OIL 9 12 14 17 2 2 4

06.03.14 OIL 15 12 15 17 2 2 4

Pour point of OIL crude PS-1 Lab Test

DATE PS1 M/L PS2 PS3 PS4 PS5

08.09.2012 37 32 26.5 28 27

06.10.2012 38 31 26.5 29 29

10.11.2012 37 28 25 27 26

8.12.2012 40 27 26 26 26.6

12.01.2013 40 26 23 21 23

02.02.2013 37 26 23 23 23.5

09.02.2013 38 25 25 24 22

09.03.2013 40 28 25 24 23

06.04.2013 42 26.5 25 24 25

11.05.2013 38 29 26 25 24

08.06.2013 42 30 26 25 27

06.07.2013 38 31 28.5 29 27.5

03.08.2013 45 32 29 30 31

07.09.2013 43 31 29 29 28

Line Temperature OIL’s M/L

What we observe from the table above that the crude oil can reachtemperature of 21 deg centigrade during winters.

Due to 150 ppm dosing at PS1 pour point is maintained well below pourpoint of crude oil.

While at PS2 both pour point of ONGC (200ppm) & OIL (150ppm) areabove the line temperature during winter which may cause waxdeposition in the line.

Also it has been observed that due to the oil treatment at PS2 OIL thepour point somewhat increases, causing the use of PPD chemicalinsignificant.

INFERENCE

THANK YOU

• Shear strain is the relative deformation per unit length. The length is the one over which the deformation occurs.

• γ = dx /dy dimensionless• Shear rate is the speed of deformation: (dγ/dt) in s-1• Viscosity is the resistance of a material to flow under stress.• Viscosity in one-dimensional shear flow

• Kinematic viscosity κ = η / ρ• 35, 25, 15 and 8 sec-1, this shear rate range covers the normal flow

rates and resultant shear• encountered by crude in OIL’s main pipeline

Viscosity, η= τ / (dγ/dt)

• Shear stress Pa = N/m2

• Viscosity = Pa s or (N·s)/m2 or 1 cP = 1 mPa·s = 0.001 Pa·s = 0.001 N·s/m2

• Kinematic viscosity = 1 cSt = 1 mm2·s−1 = 10−6m2·s−1

• Yield stress = 0 newtonian, >0 bigham plastic