Scale Formation problems in Oil & Gas Industry : Its reduction

procedures by Chemical Introduction

Oilfield scaling is serious for oil and gas industry. Every year problem with scale, costs industry millions

of dollars in damage and the lost production. The scale is one of the leading cause of worldwide decline.

In the North Sea area, 28% decline is related to the formation of scale.

The scale is an assemblage of deposits that clog perforations , casings, production tubings, valves ,

pumps and down holes, completion equipment, thereby clogging the well-bore and preventing fluid-

flow. Most scales found in oil fields forms either by direct precipitation from water that occurs naturally

in reservoir rocks or as a result of produced water becoming over-saturated with scale component when

two incompatible glasses of water meet downhole. Whenever an oil or gas well produces water or water

injection is used to enhance recovery, there is the possibility that scale will form.

The global cost of scale is estimated in more than USD 4 billion a year. Scale control form can for some

fields’ be the single biggest operational cost. The economic consequence of scale has estimated to have

the highest impact on North America and South America. In the coming years, scale costs will increase

as more as reservoir become mature and require pressure maintenance by water flooding to increase

recovery. The formation of scale may occur in the reservoir, in the wellbore or in the surface facilities.

Scale deposits may cause Formation damage by blocking pore throats, Flow restriction by blocking flow

lines and tubing, Completion damage by plugging perforations, screens, advanced completions, and

gravel packs, Choke and safety valve failure, Pump wear, Flow meter and instrumental failure,

Corrosion underneath scale deposits etc. Suspended solids can cause Plugged formation, Reduce

oil/water separator efficiency and Settlement in topside equipment.

This paper is a literature review on methods to control scale formation using various chemicals; and its

economic feasibility in the petroleum industry.

ANNEXURE 1 Scale Formation

ANNEXURE 2 Problems caused by scales

ANNEXURE 3 Potential scale formation site

ANNEXURE 4 Experiment Conducted

CONCLUSION

OUTLINE

ANNEXURE 1 : SCALE FORMATION

PRINCIPLE

In a hydrocarbon reservoir, before a well is drilled and completed, the fluids in the formation is saturated

with dissolved salt. After the well is drilled the fluids are no longer remain in equilibrium and salts may

start to precipitate. This means that scale begins to form when such the solubility limit for one or more

than one components is exceeded .

Formation of scale depends on parameters, such as

change in pressure and temperature

degree of agitation/turbulence during formation of crystals

size and number of seed crystals

degree of super-saturation

change in pH of solution.

Where, and represent the salts where M being the cation with a positive charge and X is the

anion with a negative charge

Ksp is called the solubility product( equilibrium constant for the dissolution of the salt). The solubility

product is a measure of how many moles of ions per unit volume of solvent there can be in a system

before a salt precipitates out . If the saturation ratio equals 1.0 the solution is saturated and neither

precipitation nor dissolution of the salts will occur.

When the SR is less than 1.0 the solution is under-saturated and precipitation will not occur.

When the SR is greater than 1.0 the solution is oversaturated and precipitation of the salts may

occur ( SCALE FORMATION )

This will, however, depend on the kinetics of the precipitation reaction. Some salt do not start

spontaneous precipitation even if they are many hundred times super-saturated .

Produced water that goes through a pH shift, a temperature or pressure change or are in contact with

incompatible water, do not always produce scale, even though the produced water has become

oversaturated. This is because scale must grow from solution to form. The process is called nucleation

Saturation Ratio for salts = [ ][ ]/ Ksp

and is the first stage in forming scale. Nucleation is the creation of a sub particle or ion cluster consisting

of several individual scaling ions. There are two different nucleation processes called homogeneous

nucleation and heterogeneous nucleation. Homogeneous nucleation is a process where scale growth

starts in a supersaturated solution with ion pairs forming single crystals in solution Heterogeneous

nucleation is a process where scale crystals start to grow on substrates like metallic surfaces, sand grains

or on pre-existing surface defects.

PROCESSES OF SCALE FORMATION

Incompatible mixing

Scale from incompatible mixing occurs when two incompatible waters like injected seawater and

formation water gets mixed downhole. The produced water then gets oversaturated with scale

components. This happens because seawater has a high content or sulfate (SO4-2

) and formation

water is rich in ions such as calcium (Ca+2

) and barium (Ba+2

). Mixing of these two waters leads

to precipitation of sulfate scales, such as BaSO4.

Evaporation

When a mixture of hydrocarbon gas and formation water is produced simultaneous, evaporation-

induced scale may occur. A pressure drop caused by reduced hydrostatic pressure leads to an

expansion of the hydrocarbon gas and the hot brine phase evaporates. The salt concentration will

then increase above the solubility limit and salt will precipitate. Halite (NaCl) scale in High

temperature High pressure (HTHP) wells is the most common scale type to be formed this way.

Auto-Scaling

This occurs when the natural water in the reservoir undergoes a change in pressure and/or

temperature when it is produced. Normally an increase in temperature tends to increase water

solubility of a salt which implies more ions gets dissolved at high temperatures. Similarly,

decrease in pressure tends to decrease the water solubility.

ANNEXURE 2 : PROBLEMS CAUSED BY SCALES

The formation of scale may occur in the reservoir, in the wellbore or in the surface facilities. Scale

deposits may cause

Formation damage by blocking pore throats.

Flow restriction by blocking flow lines and tubing.

Completion damage by plugging perforations, screens, advanced completions, and gravel packs.

Choke and safety valve failure

Pump wear

Flow meter and instrumental failure

Corrosion underneath scale deposits.

Suspended solids can cause

Plugged formation

Reduce oil/water separator efficiency

Settlement in topside equipment

ANNEXURE 3 : POTENTIAL SCALE FORMATION SITES

Case 1 : At the surface water injection facility where incompatible sources of water are mixed prior to

injection

Case 2 : In injection wells where the injected water starts to mix with the reservoir formation water.

Case 3 : Downhole in the reservoir where the injected water displaces formation water

Case 4 : Downhole in the reservoir where the mixed injected water and formation water are about to

reach the range of producing wells

Case 5 : Production tubing

Case 6 : At the connection of a branched zone where each branch produces different waters

Case 7 : At the manifold of producing zone where water is produced from different blocks within the

same producing zone.

Case 8 : At topside facility where produced fluids are mixed with different zones to separate oil and gas

from produced waters, or in pipelines that transport produced fluids to on-shore processing facilities.

ANNEXURE 4 : EXPERIMENTS CONDUCTED

• Collection of samples of casing and tubing was done by PDPU Alumni from ONGC Ahmedabad

(samples of KG Basin)

• Analysis of samples of casing and tubing for the identification of minerals was done in PDPU Solar

Department via X-Ray Diffractometer ( measuring instrument for analyzing the structure of a material from the scattering pattern

produced when a beam of radiation or particles (such as X-rays or neutrons) interacts with it )

• The analysis of the peaks obtained was done using the HIGHSCORE PLUS ( comprehensive phase

identification software used in XRD with additional functionalities of profile fitting, crystallographic and extended cluster analysis )

• The casing and tubing samples were tested in the chemical laboratory for the removal of scales by

increasing concentration of acids

• The variation of the removal of scales with concentration of acids was also noticed and the de-scaling

rate was calculated.

• After the completion of chemical experiment the samples underwent through XRD again and no or

fewer peaks were obtained which assured the removal of scales.

Scale inhibition is a chemical treatment used to control or prevent scale from forming in a

producing well. Scale inhibitors are water-soluble chemicals that are designed to prevent or

retard the nucleation and the crystal growth of inorganic scales. They can reduce the rate of scale

formation to almost zero. For a scale inhibitor to be considered as a good inhibitor it must be

Stable : It must be sufficiently stable tinder the conditions imposed.

Compatible : It must not interfere with the action of other oilfield chemicals, nor be affected . It must be

compatible with the chemical injection system under operating them.

Efficient : It must be able to inhibit the scale in question, irrespective of the mechanisms operating.

N

Y

N

Y

Y

Scale samples from Casing and Tubing

X-Ray Diffraction Graph : Intensity vs. 2 θ/degree

Peaks obtained

Identification of scale type

Removal Techniques by use of HCl, HNO3,

HCOOH, H2SO4 , Oxalic acid and sulphamic

acid

Analysis of New Peaks

If Changes in peak visible

Experiment successful

Feasibility and Viability Parameters with effective cost reduction, optimizing removal of scales

Scales not removed by chemical Introduction ; Alternate methods recommended

Absence of potential scales

CASE 1 : CASING

1. Effect of concentration of HNO3 on weight loss of the sample

Concentration Time

(minutes)

Initial

weight

sample

( g )

Final

weight

sample

( g )

Dissolved

scale ( g )

%-

dissolution

Rate of

de-scaling

1% 60 5 4 1 20 0.3333

2% 60 5 3.6 1.4 28 0.466

3% 60 5 2.2 2.9 58 0.9666

4% 60 5 1.7 3.2 64 1.066

5% 60 5 0.9 4.1 82 1.366

2. Effect of concentration of HCl on weight loss of the sample

Concentration Time

(minutes)

Initial

weight

sample

( g )

Final

weight

sample

( g )

Dissolved

scale ( g )

%-

dissolution

Rate of

de-scaling

1% 60 5 3 2 40 0.666

2% 60 5 2.5 2.5 50 0.833

3% 60 5 1.4 3.4 68 1.133

4% 60 5 1 4 80 1.333

5% 60 5 0.3 4.7 94 1.566

3. Effect of concentration of H2SO4 on weight loss of the sample

Concentration Time

(minutes)

Initial

weight

sample

( g )

Final

weight

sample

( g )

Dissolved

scale ( g )

%-

dissolution

Rate of

de-

scaling

1% 60 5 5 0 0 0

2% 60 5 5 0 0 0

3% 60 5 4.4 0.6 12 1.932

4% 60 5 4.5 0.5 10 0.166

5% 60 5 4.5 0.5 10 0.166

4. Effect of concentration of HCOOH on weight loss of the sample

Concentration Time

(minutes)

Initial

weight

sample (

g )

Final

weight

sample (

g )

Dissolved

scale ( g )

%-

dissolution

Rate of

de-scaling

1% 60 5 4 1 20 0.333

2% 60 5 4 1 20 0.333

3% 60 5 3.5 1.5 30 0.5

4% 60 5 3 2 40 0.666

5% 60 5 2.7 1.3 26 0.433

5. Dissolved mass of scale in casing in different acid solution for 5g sample in 60 mins.

Concentration Time

(minutes)

% - dissolution

HNO3 HCl H2SO4 HCOOH

1% 60 1 2 0 1

2% 60 1.4 2.5 0 1

3% 60 2.9 3.4 0.5 1.5

4% 60 3.2 4 0.5 2

5% 60 4.1 4.7 0.5 2.3

0

20

40

60

80

100

1 2 3 4 5

Dis

solv

ed

sca

le (

%)

Chemical Treatment on casing scale sample

HCOOH

H2SO4

HCl

HNO3

Formulas

Average specific rate of de-scaling ( R ) = ( mloss / msample ) x t

CASE 2 : TUBINGS

1. Dissolved mass of scale in casing in different acid solution for 5g sample in 60 mins

Concentration Time

(minutes)

% - dissolution

HNO3 HCl H2SO4 HCOOH

1% 60 0.5 1.5 0 0

2% 60 0.92 2 0 0.24

3% 60 1.4 2.9 0.2 0.76

4% 60 1.98 3.5 0.34 1.12

5% 60 2.77 4.2 0.51 1.76

0

10

20

30

40

50

60

70

80

90

1 2 3 4 5

Dis

solv

ed

Sca

le (

%)

Chemical Treatment on tubing scale samples

HCOOH

H2SO4

HCl

HNO3

CASE 3 : On an average scale dissolution ( % ) vs Concentration of various acids can be inferred as

CONCLUSIONS

In order to prevent scale economically, inorganic acids ( except H2SO4 ) proved to be more effective as

compared to organic acids. HCl and HNO3 are better candidates to be used for chemical de-scaling of

tubing and casing. HCl may provide 30-50% saving on circulation time as compared to HNO3 based on

rate of dissolution. XRD plots also shown great variation after chemical treatment signifying scale

removal.

Fig. represents XRD plots before and after acid treatment in CASINGS

Fig. represents XRD plots before and after acid treatment in TUBINGS

REFERENCE

1. Femier WW and M. Ziauddin, removal and inhibition of organic scale in oilfield environment

2008.

2. Kelund M.A Productions chemical for oil and gas Industry 2009

3. Norwegian Petroleum Directorate

4. Al. Salami, AR and AA Momen, Downhole and Topside scale challenge “ Removal, prevention

and inhibition technology for scales “ , 2000

5. Crabtree, fighting scale- Removal and Prevention in Oilfield review , 1999

6. petrowiki.org/Scale_problems_in_production

7. www.kemira.com/en/industries-applications/Pages/scale-inhibition-production.aspx

8. Prediction of Scale Formation Problems in Oil Reservoirs and Production Equipment due

to Injection of Incompatible Waters Authors J. Moghadasi, A. Sharif, H. Müuller-Steinhagen,

M. Jamialahmad