Flow Assurance and Multiphase flow - part 2 By Prof ... - Aker Solutions

Scandpower Petroleum Technology 

Flow Assurance and Multiphase flow 

part II 

Prof. Rune W. Time 

Department of Petroleum Engineering 

University of Stavanger 

Seminar at Aker Solutions, Stavanger – May 31st, 2011 

1

Outline and time schedule 

8.30 – 9.15 Flow regimes and impact on phase slippage, 

fluid concentrations and pressure drop in 

pipelines 

9.25 – 10.15 Hydrates, wax and asphaltenes 

10.25 -11.00 Multiphase flow – influence from interfaces, 

compression effects and waves 

Seminar at Aker Solutions, Stavanger - May31st, 2011 

2

PART II 

Hydrates, wax and asphaltenes 

+ 

Scale, Emulsions and Erosion 


3

Flow assurance projects –Norway 

1993-1997: 1997: Statoilst Multiphase Technology Development 

Programme 1993-97. Summary “Cost savings through 

Multiphase Solutions”. Products: 

• Subsea multiphase meters and pumps 

• Long-lasting multiphase chokes 

• Mobile multi-test unit and total fluid management procedures 

• New industry standard d multiphase flow simulator 

• Increased c-steel pipeline applicability (NORSOK) 

• Leading edge hydrate mitigation procedures 

• Heated pipeline concepts for hydrate and wax control 

1995: A dedicated joint R&D programme 

between Statoil, Saga and Hydro (SSH) 

launched in 1995 with objective to improve the methodology 

for characterisation of dispersed multiphase systems and 

technical ability to transport and effectively separate them. 


4

Flow assurance and precipitation of solids 

• Hydrates: Ice substance 

• Wax: Paraffine wax crystallic 

• Asphaltenes: Aromatic solids 

• Scale: Wall deposited heavy soluble mineral deposits 

In production oil systems very complex, heterogeneous and 

some times even difficult to discrimate at first sight. 


5

Wax, asphaltenes and more …. 

Mercaptans are the most odoriferous 

substances known to mankind and are the 

substances found in Skunk k spray and the 

chemicals used to odorize natural gas, but 

in extremely low parts-per-billion 

concentrations. 

http://www.spentcaustic.com/tragedy.htm 

http://tigger.uic.edu/~mansoori/HOD_html 


also called thiols or organic sulfides 

6

Phase transitions 

7

Relevant phase transitions for flow assurance 


8

The long and transforming travel of oil 

from reservoir to tank 

Hydrates 

Asphaltenes 

Wax 


9

HYDRATES (Hammerschmidt, 1934) 

• Gas hydrates are crystalline materials where water molecules form a 

framework containing cavities which are occupied by individual gases or 

gas mixtures (e.g. methane, ethane, propane, isobutane and inorganic 

molecules such as CO2 and H2S). 

Ref: SSH Multiphase program: 235 

Appearance 

Hydrate gas loading 

Burning ice 


"Ice that burns" could 

provide enormous amounts 

of energy, but can it10 

be made 

environmentally friendly? 

(Image: USGS)

versus ice: 

Hydrate (clathrate) structures 

http://www.pet.hw.ac.uk/research 

/hydrate/hydrates_what.cfm 

”Cages” fit different molecules. How to know? 

- Chemical modelling (molecular dynamics) 

- Experiments (inhibitors) 


11

Hydrate flow assurance issues 

Hydrates? 

Gulf of Mexico – ”Horizon” 2010 

(Exam problem - Multiphase course) 

Environmental 

issues 

The drilling rig “Horizon” experienced a gas-kick k while drilling 

for BP in the Gulf of Mexico on April 21 st 2010. The rig sank and 

oil leaked out from the well at a flow rate Q L = 10 L/s. Several 

methods have been proposed to prevent the oil from spreading 

on the sea surface. In one concept a large hood (funnel) as 

shown in the figure above will be positioned over the well head 

with a vertical pipeline system leading the oil up to the sea 

surface for collection. 

12 

Seminar at Aker Solutions, Stavanger - May31st, 2011

Hydrates and inhibitors 

• Hydrates form when light hydrocarbons meet with water, 

typically at T < 15° -25°C at elevated pressures. 

Conventional methods of 

hydrate control typically costs 

NOK 300 million for a field. 

Use of conventional inhibitors 

requires large storage tanks on 

the platforms and costly 

systems for injection and 

possible regeneration. 

• “Thermodynamic” inhibitors (methanol or glycols) require concentrations 

around 30-70 % wt of water. 

• New low concentration inhibitors (LCI), conc. < 0.5 %wt , T< 5°C. 


13

Low Concentration Inhibitors (hydrate control) 

LCIs work in two different manners: 

1. The kinetic inhibitors impact on the kinetics, preventing or 

delaying hydrate formation. 

2. Hydrate modifiers allow hydrates to form, but as 

transportable particles. 


14

Other hydrate mitigation techniques 

Direct electrical l heating: 

Åsgard field 

15 


Cold flow principle - Sintef 

Stable dry hydrate 

slurry forms 

Flow from 

wells 

Hydrate 

recycle loop 

Hydrate recycle ’crash 

cools’ the incoming 

warm well stream 

16 


Definition? 

ASPHALTENES 

(J.B. Boussingault , 1837) 

Asphaltenes are: 

- High molecular weight polycyclic organic compounds with nitrogen, 

oxygen and sulphur in their structure, in addition to carbon and hydrogen 

- Presence in petroleum fluids is defined as the fraction of petroleum fluid 

(or other carbonaceous sources such as coal), which is soluble in benzene 

and deposits, by addition of a low-boiling paraffin solvent. 

- Not crystallised upon deposition from petroleum fluids and as a result, its 

phase-transition from liquid to solid does not follow the same route as 

paraffin wax. 

- Not easily separated into individual purified components or fractions. 

Ultimate analysis not very significant, since resins are strongly adsorbed by 

asphaltenes - and not easily quantitatively separated from them. 

Ref: 09.IJOGCT.020203.MANSOORI 


17

Asphaltenes 

Benzene rings 

Cyclohexane rings 

Four different asphaltene 

structures separated 

from different natural 

petroleum fluids. 

Ref: 09.IJOGCT.020203.MANSOORI 


18

Asphaltenes - appearance 

Asphaltenes in North Sea crude oils North 

Sea crude oils generally contain 0.1-1.5 wt% 

asphaltenes (n-pentane insolubles). 

There are crude oils which contain more 

than 10 wt% asphaltenes. 

Asphaltenes may precipitate in pores 

near the well bore 

Asphaltenes in Ula separator 


19

Asphaltene risk in reservoirs 


Ref: SSH 

20

Dried asphaltenes 

http://baervan.nmt.edu/Petrophysics/group/intro-2-asphaltenes.pdf 

Figure 1. Examples of the appearance (magnified about 15 times) of asphaltenes separated from 

Mars-P crude oil with an excess of (a) n pentane (n-C5) and (b) n-heptane (n-C7). 

Some would argue that the n-C7 asphaltenes are the “real” asphaltenes, whereas the n-C5 material is a 

mixture of asphaltenes and resins. 

The “high end materials” in molecular weight, polarity and aromaticity — may separate into an 

asphaltene-rich phase in response to changes in pressure, composition, and/or temperature. 


21

When can asphaltenes cause problems? 

• Asphaltenes can cause problems in oil production, transportation, and processing. 

Amount of asphaltene in oil less important than asphaltene stability. 

• Stability depends on asphaltene properties, and solvent properties of the oil. 

• Light oils with small amounts of asphaltenes more likely to cause problems than 

heavy oil with larger amounts of material in the asphaltene fraction. Heavier oil also 

contains intermediate components that are good asphaltene solvents whereas the light 

oil consist largely of paraffinic materials in which, by definition, asphaltenes have very 

limited solubility. 

• Asphaltenes in heavier oils can also cause problems if they are destabilized by 

mixing with another crude oil during transportation or by other steps in oil processing. 

• Unstable asphaltenes can form separate phase that might plug the oil-bearing rock 

formation near a well. Can also aggregate at oil/water interfaces, stabilizing water-in-oil 

emulsions or at oil/solid interfaces 

• Can alter surface wetting properties or accumulate and plug well bores and flow lines. 

The first step toward predicting and avoiding any of these problems is knowing how to 

evaluate asphaltene stability. 


22

WAX 

Definitions 

What is wax? 

Mainly long-chain alkanes with 20-50 carbon atoms, but also contain minor quantities of 

branched and cyclic hydrocarbons. Typical content t in North Sea oils is 1-15 15 weight%. 

What is cloud point? 

Cloud point is the first temperature where wax starts to precipitate when an oil is cooled. 

It is often referred to as the wax appearance (or precipitation) temperature. Cloud point is 

typically 30-40°C, but may be as high as 50-55°C. 

What is wax melting point? 

The melting point of wax deposits is normally about 20°C higher than the cloud point. 

What is pour point and yield stress? 

Pour point is the temperature where sufficient amount of wax (about 4 weight %) is 

precipitated to make the oil take on a solid-like (gel) structure. The pour point of North 

Sea oils may be as high as 35°C and lower than -50°C. Below the pour point, the oil has a 

yield shear stress, i.e. the oil cannot flow unless it is subjected to a certain minimum 

shearing force (shaking or pumping). p Figure 3 shows a typical relationship between yield 

stress and restart pressure. 

23 


Wax plugs 

Figure 1 Part of a wax plug retrieved from the pig trap at Statfjord B after pigging 

the pipeline from Snorre BtoStatfjord B (Sept. 2001) 

2006-Aberdeen-1-Tordal 


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WATER-OIL EMULSIONS control 

Mixture 

versus 

emulsion 

Pipeline oil-water flow 

Ref: L. Amundsen: PhD Thesis 2011, Helseth 2001 

What are emulsions? 

When water and oil are mixed, one of the phases is dispersed as droplets into the 

other. In oil production, water is most often the dispersed phase. Depending on the 

supply of mixing energy, the dispersion may separate readily into pure water and 

pure oil, or it may stay as a stable emulsion. 

Emulsions are stabilised by components naturally present in the oil, such as 

asphaltenes, resins and organic acids. Solid particles like wax, scale and fines may 

also act as stabilising agents. 

25 


Water-oil emulsions li 

“ A dedicated di d joint R&D programme between Statoil, t Saga and Hydro (SSH) was 

launched in 1995 with the clear objective to improve the methodology for 

characterisation of dispersed multiphase systems and the technical ability to 

transport and effectively separate them ….” 


26

SCALE PRECIPITATION 

Figure 6.6 SrSO4 crystals 

precipitated from brine of 

SrSO4 supersaturatiori=5 

Figure 7.4 Crystals 

precipitated from static 

brine BSS3 at 70°C, 

(Ba+Sr)/SO4=1, 

Sr/Ba=1000 

What is oilfield scale? 

Oilfield scale is mainly deposits of 

inorganic salts such as carbonates and 

sulphates of barium, strontium or 

calcium. Scale may also be salts of iron 

like sulphides, carbonates and hydrous 

oxides 

Figure 7.58 Crystals grown 

from 50:50 mixed North sea 

water and South Brae under 

static condition, 'tree leaves' 

crystals 

Ref: (Statoil,Saga,Hydro – Multiphase Technology Program) 

Ref: M. Yuan: PhD thesis, Heriot Watt, 1989 


27

Scale formation mechanisms 

Oilfield scale can form in one of 

two ways: 

Brine (e.g. formation water) may 

undergo change in conditions 

such as temperature or pressure. 

This generally gives rise to 

carbonate scales. 

Two incompatible ibl waters (e.g. 

formation water rich in calcium, 

strontium and barium and sea 

water rich in sulphate) mix.This 

generally gives rise to sulphate 

scales. 

The curves illustrate that calcite precipitation 

occurs by a positive feedback mechanism; 

pressure drop gives precipitation DP1 , 

deposition gives a new pressure drop, DP2 , 

which in turn gives increasing deposition,DP3 . 


28

Scale mitigation 

What is a scale inhibitor? 

Scale inhibitors are chemicals which stop or interfere with the 

nucleation, precipitation and adherence of mineral deposits. 

What is a scale dissolver? 

Scale dissolvers are chemicals which dissolve scale by complexing 

with ions like barium, strontium, calcium and iron. 

What is chelation or sequestration? 

Chelation or sequestration is the formation of soluble metal ion 

complexes in the presence of substances which normally would give 

a precipitate. 

Other techniques ? 

Electromagnetic 

Inhibition 


29 

http://www.ed2000.net/york/ 

yk_theo_disc.htm

EROSION 

Erosion mechanisms and materials 


30

Erosion test of “IPC” downhole 

instrumentation system 

Project with Aker Maritime Well Services, 1998 

Inflow, choke 

Multiphase 

metering 

device 

Erosion test 

Flow regime 

dependence! 

Flow 

direction 

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Flow Assurance and Multiphase flow - part 2 By Prof ... - Aker Solutions

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