31 years experience with produced water technologies in the North sea

Produced Water & Environmental Conference

TUVNEL , Aberdeen 5-6 June 2013

Jon Berntsen, KANFA Mator AS

31 years. A short time in the history of oil, but technologically?

What was the typical NCS technology in the early 80’s?

CPI (Corrugated Plate Interceptor)

Flotation cells (WEMCO)

Skimmer/separator

What was the typical produced water flow rates?

Fairly low – moderate.

What was the prospects about water flow rates?

Rocketing!

Input data for design and equipment selection

• Characteristics and thereby treatability of produced water prior to treatment depends on:

− the type of fluid to be produced (oil field (saline) or gas field (no saline))

− the effect of choking (generation of small droplets)

− the water flow rate (able to handle high flow)

?

Hydrocyclone

Feed inlet via. tangential ports

Accelerating Helical

Flow Path

Reverse Flow in

Central Core

Dense Phase

Underflow

Light Phase

Overflow

Back - Pressure

Regulation

Advantage:

Handling high flow

Small footprint

Need to:

Understand design

Understand operation

Hydrocyclone: Understand design

Started with Serck Baker 60 mm oil spin Next

37 mm hydrocyclone Next

22 mm hydrocyclone Next

17 mm hydrocyclone Finally

14 mm hydrocyclone

The most promising was the 22 mm hydrocyclone since it provided a true operating

window. The smaller diameters (17mm and 14mm) was difficult to

operate, but showed good performance at one specific pressure drop!

Today’s design with D= 18 – 22 mm is a good choice.

Hydrocyclone: Understand performance

Parallel installation on

offshore platforms.

The shaded illustration

is what the vendors

tried to sell.

The performance is

real life.

PROSEP is the empirical software

we developed to calculate performance

Hydrocyclone: Understand performance

A closer look at the model shows

that flow rate (Q) is influential

for both flow pattern and efficiency.

Experience shows that optimal Q is very

important for good performance!

It is interesting to observe that the industry (suppliers) finally acknowledge

this fact and deliver vessel with multiple chambers to adjust capacity to fit

the actual Q.

Further increase in water production

The rapid increase in produced water production generated new

operational challenges.

• Increased use of corrosion inhibitor (surface active chemical)

• Reduced BS&W to export intensified the use of emulsion breaker

• Increased volume of low pressure produced water (carry over

from separators)

The increased use of production chemicals lead to periods with

difficulties to meet discharge requirements.

The industry grabbed this opportunity to develop something that

could boost the hydrocyclone performance:

Coalescing device(s)/methods

Use of inlet cyclones

It was not only increase in water production but also higher gas flow rates

that challenged the industry.

Higher gas rates and water rates increase problems with foaming in the

separators leading to compressor breakdown.

In an attempt to reduce this problem, inlet cyclones was installed in the

majority of inlet and secondary separators.

The result of this retrofits was EVEN MORE produced water problems!

Coalescence

Separation = Coalescence

Since oil is dispersed in the water as very small droplets there is a demand

that the oil droplets get in contact with each other and are able to coalesce.

One method, might be use of material that promote coalescence (oleophilic

surfaces).

The idea is good and various components is developed, but it did not met

the expectations.

• Produced water contain:

• High volume of solids (clay, silt, scaling products)

• Huge volume of fine dispersed gas bubbles

These coat the surface and inhibit contact with the coalescing material.

• Oliophilic material placed in water become hydrophilic with time.

C-Tour

Injection of condensate from the process to reduce oil over board.

Cons: Increased discharge of BTEX. Limited condensate availability.

Compact flotation unit

The oil industry has a long time experience with flotation but current (90’s)

technology represented huge weight and footprint and maintenance.

The development of a vertical column with more efficient design easy to

retrofit entered the marketplace.

CFU is a rather simple technology which resulted in a vast number of

suppliers.

It is normally possible to achieve very good performance, which is even

the history behind horizontal flotation cells (WEMCO’s), but it is easy to

end up with very bad or low performance.

Compact flotation unit

CFU require flotation gas and flocculants for reliable performance.

Produced water contain much particles, typically oil wetted. These particles

sticks to the flocculants together with oil and form a new component which,

over time, clog rejects.

Unfortunately we experience several

units that, periodically, have higher

oil concentration out of the unit than

into the unit !

We have had access to daily lab reports showing a CFU having lower oil

concentration out of the unit compared to into the unit for only

10 days in a 6 months period .

Compact flotation unit

Below are given two illustrations from control room monitor showing levels in

the CFU. This is an installation with two CFU’s in series.

The left illustration show first CFU with clogged reject having only gas in

the vessel. No water level established.

Right illustration show first unit with restriction in reject line. Only gas emerging.

Compact flotation unit

Location of CFU.

To sea

Gas release Henrys lov, metane

Pressure bar

Equlibrium HT 11 1,43E-02 mol ch4/liter water

Equlibrium LT 2,5 0,00325 mol ch4/liter water

differanse 1,11E-02 mol ch4/liter water

Flow rate 500 m3/t

139 l/s

Released mol metan 1,535 mol ch4/s

volumflow metan ved likevekt LT bar/50 deg 0,016 Am3/s

59 Am3/t

volum% gas 12 %

Oppløst gass specs:

Mw 0,018 ca molvekg kg/mol

0,027625 kg/s

99,45 kg/h

132,292368 Sm3/t

Locating a CFU directly after a

hydrocyclone installation might be

very challenging. The release of large

volumes of gas after the hydrocyclone

or control valve calls for an increased

gas handling capacity in the CFU.

We have experienced

several installations having

problem handling both

dissolved gas and added

flotation gas.

In combination with clogging

of reject this is a situation

where operators loose

control.

Handling low pressure produced water

• Low pressure produced water is often more stabilised by particles and chemicals

(lower interfacial tension thereby lower oil droplet coalescence), thereby more

difficult to treat.

• However, treatment systems for low pressure water are often less equipped than

systems for high pressure water → low pressure water has negative influence

on the discharge.

Recycling of rejects

• Rejects are often a mix of oil, particles and chemicals. Direct recycling of rejects to

the separation train may impair the oil/water separation.

• Many installations experience accumulation of rag-layers in the separators with

resulting need for manually carry-over (close water outlet and lift interface), and

experience unforeseen process up-sets, both on the oil and produced water side.

• Negative effects from direct recycling of rejects occur over time, and are difficult to

prove by short time testing.

• We strongly believe that alternative solutions to

direct recycling of rejects over time would make

a large difference and would reduce the number of

unforeseen and inexplicable increased discharges

of oil to sea.

Case: Recycling of rejects (cont’d)

• Direct recycling of rejects via the 57-tank → high OiW and particles to 3. stage separator.

• Resulted in bad quality water from 3. stage separator, however, better than the stream

from the 57-tank.

• Intermittent return directly to the degasser, i.e. treated by only one treatment stage.

• Large negative influence on discharge to sea.

Case: Recycling of rejects (cont’d)

• Build-up of rag-layer in coalescer and

intermittent (on/off) return of bad quality water

with flocculant and particles to 1. stage

separator.

• Periods with control of flocculant injection,

reduced skimming from degasser thereby

reduced recycle, and improved pumping

frequency of water return from coalescer to

1. stage separator.

Online OiW:

Online OiW:

Case: Recycling, but more controlled

• Reject water phase treated by dedicated

centrifuge. Discharge < 10 ppm OiW

• Improved solution for treatment of reject

and low pressure water from different

sources!

Generic production process

Equipment drain

HP separator LP separator Export

Overboard

PWT

system 44:Rejects

57:Closed drains

Export

Overboard

Solids/sludge

Export

Overboard

Solids/sludge

An alternativ treatment system for recycled fluid

An alternativ treatment system for recycled fluid

A major challenge for treatment of reject fluids is particles, typically oil coated

neutral density particles. This situation is frequently experienced on installation

processing high water cut wells.

This situation limits the use of filter, flotation and hydrocyclone technologies.

The only technology that has proven capable to treat such fluid, is centrifuges.

BUT, history tells that the operators are not satisfied. WHY?

The centrifuges are incorrect installed and used

The fluids to be treated are a mix of particles, stable oil droplets and chemicals.

Sometimes it can be low temperature fluids stored in a huge tank.

This situation represents a separation challenge and to be able to handle this

the centrifuge should, at least, have twice the capacity compared to the feed pump!

A very simple role of thumb.

31 years experience with produced water technologies in the North sea

A neutral review of lesson learned:

Hydrocyclone followed by degasser proved performance to less than 10

ppm OiW.

Low pressure water treatment is a challenge, should never be recycled.

Technology available – market not prepared.

Production chemicals challenging but with correct dosage and proper

understanding from operators it might be less a problem.

Lack of process understanding (incorrect location of technology).

Some produced water might need special treatment due to reservoir

specific properties (iron, barium …..) (correct chemical).

Accept that produced water contain lots of particles, many oil wetted.