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TPG 4510
Petroleum Production Specialization Project
Department of Petroleum Engineering and Applied Geophysics
Supervisor: Jon Steinar Gudmundsson
METHODS TO CLEAN PRODUCEDWATER
Carlos Arribas Miranda
Trondheim, Norway
June 2013
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Abstract
Produced water is the largest oilfield waste; the total amount rounds
250 Mbbl/day. It is a mixture of inorganic and organic compounds,
including oil, metals, chemicals, gases, microorganisms, etc.
This report is an overview of different methods to treat the components
and contaminants of produced water and the technologies applicable for
this purpose reducing the environmental impact of oil and gas industry.
Methods explained are physical, chemical and biological and the
facilities where those technologies could be applied. There are several
technologies described and compared according to the particles they
separate, size, applications, advantages and disadvantages, etc.
The limits of discharge and disposal are becoming more restrictive so
water treatment companies have to keep on researching and developing
new technologies in order to achieve those specifications.
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IndexLIST OF TABLES ................................................................................. VLIST OF FIGURES ............................................................................... VI1 INTRODUCTION ............................................................................ 12 DISPOSAL STANDARDS .................................................................. 23 CHARACTERISTICS OF PRODUCED WATER ....................................... 3
3.1 PRODUCTION AND SUSPENDED SOLIDS.................................................. 4
3.2 DISSOLVED SOLIDS.......................................................................... 5
3.3 DISSOLVED AND DISPERSED OIL.......................................................... 6
3.4 PRODUCTION CHEMICAL COMPOUNDS................................................... 7
3.5 DISSOLVED GASES........................................................................... 8
3.6 WATER IN OIL EMULSIONS.................................................................. 9
3.7 NATURALLY OCCURRING RADIOACTIVE MATERIALS (NORM) .................... 10
4 THEORY OF SEPARATION ............................................................. 114.1 PHYSICAL TREATMENT.................................................................... 12
4.1.1Gravity Separation ............................................................... 12
4.1.2Coalescence and Dispersion ................................................ 13
4.1.3Flotation ............................................................................. 13
4.1.4Membrane Treatment .......................................................... 14
4.2 EVAPORATION.............................................................................. 19
4.3 ADSORPTION............................................................................... 20
4.4 CHEMICAL TREATMENT................................................................... 21
4.4.1Ion exchange process ......................................................... 21
4.4.2Electrodialysis (ED) ............................................................. 23
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4.4.3Chemical Oxidation and Ozonation ..................................... 24
4.4.4Flocculants and Coagulants ................................................. 25
4.5 BIOLOGICAL TREATMENT................................................................. 26
5 BEST AVAILABLE TECHNIQUES BAT) ............................................. 275.1 SKIM TANKS................................................................................ 27
5.2 CORRUGATED PLATE INTERCEPTOR (CPI) ............................................. 29
5.3 UFWITH CERAMIC MEMBRANES......................................................... 30
5.4 DISK STACK CENTRIFUGES............................................................... 32
5.5 HYDROCYCLONES......................................................................... 34
5.6 IGF........................................................................................... 36
5.7 COMPACT FLOTATION UNIT (CFU) .................................................... 37
5.8 SAND CYCLONES........................................................................... 39
5.9 C-TOUR.................................................................................... 40
5.10 MPPE ...................................................................................... 42
5.11 WALNUT SHELL FILTERS................................................................. 43
5.12 MARES TAIL............................................................................... 45
5.13 BAF ........................................................................................ 45
5.14 ACTIVATED SLUDGE..................................................................... 46
5.15 MBR ....................................................................................... 47
6 FLOW DIAGRAM .......................................................................... 487 NEW CHALLENGES ....................................................................... 498 CONCLUSIONS ............................................................................ 52FIGURES .......................................................................................... 61TABLES ........................................................................................... 54
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REFERENCES .................................................................................... 61
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List of TablesTable 1: Worldwide produce discharges (Steward, 2008)............................ 54Table 2: Worldwide produce discharges (Neff, 2011)................................... 54Table 3: Constituents summarize from gas fields (Ahmadun, 2009)...... 55Table 4: Constituents summarize from oil fields (Ahmadun, 2009)........ 56Table 5: Chemical compounds in gas-oil fields (Steward, 2008).............. 55Table 6: Ceramical membranes characteristics (USBR)................................. 57Table 7: Ekofisk C-Tour performance (Phillips).............................................. 57
Table 8: BAT comparison........................................................................................ 58
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List of FiguresFigure 1: Water/Oil production profile (Ebenezer, 2012)............................ 61Figure 2: Drag force, Stokes law (Fluids Mechanics UPM, 2010)............. 61Figure 3: Membrane sketch (Cheryan 1998).................................................... 62Figure 4: ED sketch (EET corporation)................................................................ 62
Figure 5: Skim Tank configurations (Steward 2008)...................................... 63Figure 6: Down-flow CPI (ESI)............................................................................... 63Figure 7: Up-flow CPI (ESI)..................................................................................... 64Figure 8: Oil coalescence and solids settling (ESI).......................................... 64Figure 9: Disk Stack Centrifuge (Veolia)............................................................ 65Figure 10: Liner (Aker, Prosep)............................................................................ 65Figure 11: Hydrocyclone (NETL)........................................................................... 66Figure 12: Micron size separation Vs Oil viscosity (Cyclotech).................. 66Figure 13: Max and Min pressure drop operation Vs eff (Cyclotech)....... 66Figure 14: Micron separation Vs Droplet inlet size (Cyclotech)................. 67Figure 15: Hydraulical IGF (Unidro)..................................................................... 67Figure 16: Mechanical IGF (Unidro)..................................................................... 68Figure 17: CFU sketch (Statoil, 2010)................................................................. 69Figure 18: Sand Cyclone (Veolia).......................................................................... 70Figure 19: C-Tour flow diagram (Statoil, 2010).............................................. 70Figure 20: MPPE flow diagram (Veolia)............................................................... 71Figure 21: MPPE and flotation Comparison (Meijer 2010)........................... 71Figure 22: Walnut shell filter sketch (Siemens)............................................... 71Figure 23: Activated Sludge (Pipeline, vol 14, 2003)..................................... 72
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Figure 24: ubsea Separation sketch, Marlim Project (FMC, Orlowski)...... 72Figure 25: Flow Diagram Example....................................................................... 74
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1 Introduction
Oil and gas industry is one of the most important industries nowadays.
Since 1850 when Edwin Drake drilled the first oil well, oil demand has
been increasing thus, the oil and gas production. One of the problems
of this production increase is the big amount of water it is produced
with it, produced water is the largest byproduct stream associated with
oil and gas production [Duhon, 2012]. So that, produced water can be
defined, as the water that comes with oil and gas in the production
facilities and it needs to be treated for different purposes such as
reinjection or disposal.
Produced water is a complex mixture of inorganic and organic
compounds, including oil, metals, chemicals, gases, microorganisms,
etc [Neff, 2011]. The total amount of produced water is estimated in
250 Mbbl/d being the water oil ratio between 2 and 3 to 1 depending on
where and when the water is being produced [Ferro and Smith]. The
older the production field the bigger the WO ratio represented in figure
1 with an oil-water vs. time profile [Ebenezer, 2012]. The produced
water can be classified in formation water and injected water. Formation
water is the one trapped with the oil in the reservoir and since the well
starts to produce oil so it does. Injected water is the artificial via to
maintain reservoir pressure and lengthen the production of the field.
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The motivation of the project is to explain different methods to treat the
components and contaminants of produced water and the technologies
applicable for this purpose reducing the environmental impact of oil and
gas industry.
2 Disposal StandardsThe environmental impacts of discharging water without the appropriate
treatment are incalculable. That is why there are production water
disposal standards for produced water, both offshore and onshore,
according to the current water separation technology and the limits they
can achieve. Several techniques are being developed and investigated in
order to accomplish the zero content discharge. Both onshore and
offshore can treat the water also for water reinjection to maintain
pressure reservoir and lengthen the production of the field. The
treatment of the water includes oil removal but also production
chemicals, suspended solids, bacteria, etc.
Offshore regulations require total oil and grease content of the effluent
below the regulations; they vary from one country to another. They
range between 15 mg/l) in Argentina and Venezuela, up to 50 mg/l in
the Guinean Gulf [Steward, 2008] (Table 1); from 29 mg/l in U.S. up to
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40 mg/l [Neff, 2011] (Table 2). In the North Sea is regulated by OSPAR
commission (Oslo-Paris) and it is 30 mg/l. Despite he existence of these
standards, there are plenty of offshore facilities that do not achieve the
regulations registered in the OSPAR commission.
Onshore facilities normally treat onshore production wells and offshore
produced water that only has been treated superficially in order to be
transported to the onshore facility. Onshore plants normally discharge
by subsurface injection into rock formations, which has more restrictive
limits than the onshore facilities. The limits are higher because of the
risk of polluting fresh water in aquifers or soil pollution. Disposal in
freshwater streams or aquifers is generally forbidden.
3 Characteristics of Produced WaterAs it was explained, produced water is a mixture of organic and
inorganic materials that depends on several factors, for instance,
geographical location of the field, type of reservoir, lifetime of its
reservoirAlso type of hydrocarbons produced affect the chemical and
physical properties of the produced water [Veil, 2004]. Characteristics
will vary form oilfields, gas fields or oil and gas fields; tables 3 and 4
show a summarize list of the possible compounds that exist in the
different production fields. All parameters will be explained in this
chapter. It is important that the total amount of produced water in gas
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fields is much lower than in the oilfields, mainly because there is no
water injection in gas fields for gas recovery increase.
3.1 Production and Suspended Solids
Production and suspended solids include clays, scales, waxes, bacteria,
carbonates, sand, silt and asphaltenes [Veil, 2004]. Concentration of
the solids varies from one field to another depending on the reservoir
initial conditions. The general amount of suspended solids is small
except in wells that produce in unconsolidated formation, where large
volumes of sand and other suspended solids might be produced. In
order to accomplish the water disposal requirements, the solids cannot
affect oil measurement methods, and special equipment must be used.
When suspended solids are present, it is necessary to apply different
techniques in order to remove the solids. Chemical treatment is used to
separate the oil droplets form the solid particles and the equipment
must incorporate solids removal ports, jets and/or plates.
Precipitation solids or scales are the ions capable of reacting with
temperature, pressure or composition changes. This phenomenon can
occur in tubing, pipelines, vessels and water treatment equipment. The
most common precipitating solids are carbonates and sulfates.
Carbonate scales can occur in all systems containing CO2and ions, for
instance Ca2+, which will precipitate as CaCO3. Carbonate scale
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formation is mostly affected by changes in CO2pressure and
temperature, also by mixing different waters. Large pressure changes
happen in chokes or flash tanks while temperature variations will take
place in the heat exchangers [Sandengen, 2012].
Sulfates form in the same parameters variations but they are more
dependent on concentration than pressure or temperature changes.
They precipitate fast and cause big problems when they do it inside the
production wells. It is possible to control its formation with the
production temperature. For example, CaSO4reaches its highest
solubility at 38C (2150 mg/l), if the temperature is placed at 93C
solubility decreases until 1600g/l.
3.2 Dissolved solids
Dissolved solids are inorganic constituents that are predominantly
sodium (Na+) cations and chloride anions (Cl-). Other common cations
are potassium (K+), magnesium (Mg2+), calcium (Ca2+), barium (Ba2+),
Strontium (Sr+), iron (Fe2+), etc. There are also other anions such as
carbonates (HCO3-, CO32-) and sulfates (SO42-) [Steward, 2008]. Tables
3 and 4 enumerate the metals dissolved in both gas fields and oil fields.
These ions affect produced water chemistry in salinity and scale
potential principally [Hansen 1994].The amount of solids dissolved in
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the produced water can vary from less than 100 to over 300,000 mg/l
[Steward, 2008; Roach, 1994]. It is important to pay special attention to
dissolved solids in order to prevent scale formation in the piping,
wellbore-bore formation, etc. It would carry big costs in cleaning and
maintenance and the stop of the production in most cases.
3.3 Dissolved and Dispersed Oil
Dissolved and dispersed oil components are mixture of hydrocarbons
including BTEX (benzene, toluene, ethylbenzene and xylene), PAHs
(polyaromatic hydrocarbons) and phenols. Dissolved oil is composed by
polar constituents distributed between low and medium carbon ranges,
meanwhile the small droplets of oil suspended in the produced water
are called dispersed oil. The size of the oil droplets is between 0,5 m
and over 200 m [Steward, 2008].
The amount of dissolved oil depends on the type of oil, volume of
produced water and age of production [Ahmadun, 2009]. The
experience from the field tells that the temperature range where the
water is treated (25-75C), does not affect the solubility of oil.
Temperature only affects solubility above 75C. Phenols concentrations
are low normally, in the North Sea for instance they have been never
been detected over 20mg/l [Neff, 2010]. BTEX and phenols are the most
soluble compounds in produced water, followed by aliphatic
hydrocarbons, carboxylic acid and low molecular weight aromatic
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compounds. Typical gravitational separation is not enough to separate
dissolve oil from the produced water. Other technologies are needed
such as adsorption, filtration, biological treatment or membranes.
PAHs and heavier alkyl phenols (C6-C9) are related to the dispersed oil
because they are less soluble in produced water. They are considered
the greatest environmental concern because of its toxicity and
persistence in the environment. The total quantity of dispersed oil is
determined by the source of the produced water. For example, produced
water from gas/condensate fields exhibit higher levels of dissolved oil
[Neff, 2011]. Oil droplets size distribution is the most important
parameter, which affects oil and water separation treatments. It is
experimentally demonstrated that the bigger the droplet diameter is,
the better the equipment efficiency. The size distribution is influenced
by system shearing (pumping, pressure drop in the piping system, etc.),
oil-water interfacial tension, temperature, turbulence, density and other
factors.
3.4 Production chemical compounds
Chemical components are added to treat operational problems. They are
dissolved and used to prevent hydrate and scale formation, corrosion,
wax deposition, bacterial growth, gas dehydration and emulsion. The
totality of the chemicals varies from field to field and sometimes they
appear in insignificant amounts. These low concentrations are explained
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by the solubility of the chemicals in the oil phase, thus they are not
treated in the cleaning water systems.
Production chemicals can be very injurious in low concentration, 0.1
ppm [Glickman 1998]. Besides the danger it represents, some chemicals
like the corrosion inhibitor can reduce oil/water efficiency [Veil, 2004].
Table 5 shows the typical chemical production in oil and gas fields. The
most common chemicals used in oil/gas production that affect the
water facilities are the Scale inhibitors, scavengers, coagulants and
flocculants and finally some gas treatment chemicals because they
remain in water phase [Neff, 2011].
3.5 Dissolved Gases
The main gases, which are encountered in produced water, are natural
gas (methane, ethane, propane and butane), hydrogen sulfide, carbon
dioxide and oxygen. They are formed naturally, by chemical reactions
or bacterial activities. Most of the gasses are saturated at reservoir
conditions but as the well starts producing, most of the gases flash to
vapor phase [Arthur, 2005]. These gases are removed in separators and
stock tanks in most of the occasions.
The gas separation is influenced by the pressure and temperature in
which the process occurs. The higher the separation pressure the higher
the quantity of dissolved gasses will be. The opposite effect we get with
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the separation temperature, the higher the temperature the lower the
quantity of dissolved gasses.
Natural gas components are barely soluble in water at operation
pressures. This solubility is based on pressure, temperature and specific
gravity of the water. It is important to comment that these compounds
are attracted to the dispersed oil droplets, that attraction is taken into
account to design the flotation equipment for the water treatment
[Steward 2008].
Looking at the other common gasses, hydrogen sulfide is corrosive and
enables iron sulfide scaling, besides is extremely toxic if inhaled. It is
necessary to be especially careful if the sulfide is present in the flotation
cells when maintenance and adjustments are done. Carbon dioxide is
also corrosive and may originate CaCO3scaling. When the CO2and the
H2S are removed, pH increases so scale could also form.
It is relevant to comment the role of the oxygen. It is not found naturally
in produced water but produced water may absorb it when it comes to
surface. Water with dissolved oxygen causes corrosion and oil
weathering that difficult the separation.
3.6 Water in oil Emulsions
Emulsion is a mixture of two immiscible liquids. In the normal
emulsions, water is dispersed in small droplets from 100m to 400m in
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diameter. If the emulsion is unstable, the oil droplets will coalesce into
larger ones. This is a short time process. However a stable emulsion is a
suspension of the two liquids with a stabilizer that maintains a film
between the phases. This film may be removed so coalescence starts to
act. In order to break it down chemicals or heat are used. In water in oil
emulsions, the emulsion breakers must be oil soluble, so that, they have
more time to act during the separation processes.
3.7 Naturally Occurring Radioactive Materials (NORM)
NORM originates in geological formations and can be brought to surface
with produced water [Veil, 2004]. They can be found in production
wastes, equipment and solids at the production facilities. The most
abundant NORM compounds are 226Ra and 228Ra, the ambient
concentrations are ranged between 0,3 and 1,3 Bq/L and 16 to 21 Bq/L
[Gafvert, 2006]. In the North Sea, Utvik confirms that the measure
concentrations of NORM in produced water range from 0,23 to 14,7
Bq/L. Both compounds derive from uranium and thorium present in
hydrocarbon bearing formations. As the produced water approaches to
surface, temperature and pressure decrease so it may lead to a NORM
scale production. [Veil 2004, Steward 2008]. The scales and sludge
would accumulate in water separation facilities.
NORM regulations are more focused on the equipment accumulation
rather than produced water limits. It has been proved than seafood
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consumption from produced water disposal does not affect human
health. The specifications in NORM management are centered on
identification, control and volume reduction of the wastes and solids, in
order to diminish human exposure to radiation [Ebenezer, 2012].
4 Theory of Separation
The main goals for proper water treatment are nine [Arthur, 2005]. De-
oiling (removal of free dispersed oil and grease); dissolved organics
(bacteria and microorganisms) and gas elimination such as natural gas
or carbon dioxide; suspended solids removal (mostly sand and other
particles); desalination; sulfates and scaling agents clearance;
disinfection and softening, in order to adjust water hardness and make
it available for irrigation; and finally NORM removal. To meet this
achievements different methods can be used, mostly physical and in
less often chemical and biological procedures.
Physical procedures will separate contaminants and oil from water by
the application of different forces. Chemical system bases its separation
in the addition of components that will react with the contaminants
wanted to remove. Finally biological will be focus in the use of several
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types of bacteria and microorganisms.
4.1 Physical Treatment4.1.1 Gravity SeparationGravity separation is the most usual process in water treatment. As it is
known, oil is lighter than the volume of water they displace so, by
Archimedes principal, oil droplets experiment a buoyant force. But the
vertical movement of the particles through the water originates a drag
force that withstands the flotation described by the Stokes law and
sketched in figure 2. Droplets reach a constant velocity when the to
forces are equal [Fluid Mechanic Notes, UPM, 2010].
vs= particles velocity
g = gravity acceleration
r = particles radius
p= particle density
f = fluid density
= water viscosity
With the formula we can conclude that the bigger the droplet and the
density difference the higher the vertical velocity. If temperature is
increased, viscosity will reduce so a higher velocity is also obtained.
Stokes law may be applied to droplets never below 10m, but field
experience indicates that the lowest limit applicable is 30m[Devold, 2006].
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4.1.2 Coalescence and DispersionCoalescence is the process in which two or more droplets, bubbles or
particles merge during contact to form a single daughter droplet,
bubble or particle. If this occurs repeatedly, a continuous liquid phase
forms [Schlumberger, 2013]. Coalescence is a time dependent process,
the smaller de oil droplets diluted the greater the time to grow bigger
droplets. Dispersion is the act of breaking up particles into smaller
ones and distributing them throughout a liquid or gaseous medium.
This process occurs when a large amount of energy is input in the
system in a short period of time [Schlumberger, 2013]. This energy
applied minimizes the surface area between the two fluids, which favors
the separation between the droplets.
The coalescence and dispersion processes occur at the same time and
they are totally opposed. If the kinetic energy of the particles in the
system is larger than the difference in surface energy between the single
droplet and the two smaller droplets formed from it, dispersion process
is happening. In the other side, the motion of the smaller droplets
causes coalescence [Ebenezer, 2012].
4.1.3 FlotationFlotation consists in the injection and dissolution of air in the produced
water. Then, the small air bubbles adhere to the oil droplets increasing
its buoyancy, the specific gravity of the oil-gas bubbles combined is
significantly lower that the oil droplet alone. When the oil has floated to
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the surface it is normally skimmed and removed. Flotation process is
really effective, over 90% of the oil is removed in short periods of time
and can remove very small oil droplets. The droplets separation size is
lower if a chemical pretreatment is used to favor the flotation,
coagulants and flocculants for example. This process can also be used
to remove natural organic matter, volatiles, grease, etc.
The efficiency of the flotation process depends on specific gravity
difference, droplet size and temperature. They usually work better with
low temperatures, because at high temperature, dissolving air into the
water requires more pressure. It also depends on the air bubbles size,
the smaller the bubble size the more chances to adhere to the oil.
Flotation can operate as the principal separation force in two kinds of air
flotation systems: Dissolved Air Flotation (DAF) and Induced Air
Flotation (IAF), IAF will be explained later but DAF is barely used in
offshore facilities for its size and weight, operation at high
temperatures, etc [Unidro, Prosep]. It can function also as a secondary
force to help other separation principles to perform.
4.1.4 Membrane TreatmentMembranes are thin films of synthetic organic or inorganic materials,
which separate a certain fluid from its components. The separation is
achieved by diffusion through the membrane under pressure difference.
Several processes exist for this purpose, microfiltration (MF),
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ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO) and
polymeric or ceramic membranes. Membrane treatment is more suitable
for the stable oil water emulsions.
The process usually operates with a recycling water system that
maintains a constant water flow. The same amount of water introduced
in the tank at the same rate as it is withdrawn and clean. The process is
stopped when the limit of particles displaced reaches a certain level
concentration in the process tank. When the procedure stops, a clean in
place is carried out (CIP), process is sketched in figure 3. It is important
to mention that membranes normally need a pretreatment to remove
free oil and bigger particles to lengthen the life of the membranes.
These types of procedures have some advantages over the traditional
methods of flotation and separation. Using membranes for the
treatment reduces the oil concentration from 1/40 to 1/200 initial feed
and the total quantity of water used can be recycled [Madaeni, 2003].
They also have some disadvantages; it is very expensive to install
membranes over a certain size. Most of the membrane processes
include chemical pretreatments; to avoid scale formation (in NF and RO)
and is common the addition of coagulants. They also suffer high
degradation during their use so that they must be changed frequently in
order to avoid membrane fouling [Madaeini 2003]. The flux varies with
the time, the longer the time the more attached oil and solids stuck at
the surface of the membrane the lower the flux [Jiang, 2008]. The space
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of installation needed is higher than the traditional methods and
because of the chemicals used in the pretreatments several kinds of
impacts into the environment may occur.
4.1.4.1Filtration: MF, UF, NF and ROMF has the largest pore size (0,05m to 2m) and operating pressure
difference below 2 bars. MF is mainly used to remove suspended solids.
UF ranges from 2nm to 0,05m and operating pressure between 1-20
bars; it is used for colloids and solids separation [Martinous, 2001, Judd
2003]. Both systems used as a pretreatment for other cleaning
technologies NF, RO and electrodyalisis [Jurenka, 2010]. MF and UF can
treat any type of produced water; they can operate with high TDS and
salt concentrations.
NF is normally used for metals removal from produced water. It has
membrane pore size between 0,5nm and 2nm, pressure difference of
the process between 10 and 100 bars. It is used for multivalent ions and
charged polar molecules [Martinous, 2001, Judd 2003]. NF membranes
have negative charge at neutral pH; it is an important key for the
separation properties of the membrane [Sutherland, 2009]. RO is
capable of remove over 99% of the organic macromolecules and
colloids, besides inorganic ions are also removed over 0,1nm [Bilstad
1994]. The most important problem of RO and NF is the complex
pretreatment that needs to be done; NF and RO are mostly used for
human consumption in desalination processes.
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Membranes can operate either cross flow separation or dead end
filtration. Cross flow separation occurs in perpendicular direction with
the flow, gravity and density difference makes the particles fall to the
bottom of the flow and then be filtrated, only part of the feed water is
treated. In dead end separation all water is treated and flow and
filtration happen in the same direction.
The membranes applicable for oil separation purposes are polymeric or
ceramic, being the second ones more expensive but capable of treating
more water. Therefore a cost/benefit analysis must be done. Both
membranes are explained below.
4.1.4.2Polymeric/Ceramic MembranesCeramic (or inorganic) membranes have attracted interest due to their
superior mechanical, thermal, and chemical stability. The primary
advantage of using ceramic membranes is the ability to accomplish the
current and pending regulatory treatment objectives with no chemical
pre-treatment [Ebrahimi, 2010]. Ceramic membranes are made from
alumina, titanium, silica and zirconium oxides and carbides. They are
tubular and consist of a porous support material (-alumina), a
separating layer and a decreasing pore diameter layer. Different
materials applied for the different range of filtrations used by Ebrahimi..
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UF with ceramic membranes has been shown to be very effective in
treating waste oil, grease and effluents and can compete against
traditional separation techniques [Fabish, 2001].
Polymeric membranes are made from polyacrylonitrile (PAN) and
polyvinylidene fluoride (PVDF). The main problem of the stable organic
materials is the hydrophobicity of their basic materials. This results in a
low water permeation rates. PAN membranes combine chemical stability
and good aqueous filtration [Scharnagl, 2001]. The gravest problem of
the polymeric membranes is their integrity. As a consequence to that,
the integrity of the membrane must be tested to ensure the process.
This test can be done with a pressure decay test. In this test,
pressurized air is applied to the membranes at a pressure less than
would cause the air to flow through the membrane, and the pressure
decay is measured [Colorado School of Mines, 2009].
Current experiments mixed both types of membranes, a PVDF
membrane with nano-sized aluminum particles improving antifouling
performance of the membranes. In the UF experiment the removal
efficiency of COD was 90% and in TOC 98%; oil residue was less than 1%
[Lia, 2006]. This shows that UF process is the most competitive
compared with the traditional systems of wastewater treatments. The
main challenges of the membrane treatments to consolidate are the
scale forming and the clogging. On their side, they have good efficiency
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and capacity; and also they are flexible and can accept well variations in
flow and quality [Statoil, 2010].
4.2 EvaporationThe processes for produced water cleaning, which include steam
formation in some way, are also called thermal technologies. Different
evaporation systems has been tested and proposed for water treatment.
They use few or none chemicals so waste sludge is cleaner. They also
reduce equipment needed for the process, therefore O&M costs
decreases substantially. The most applied thermal technologies used
nowadays are multistage flash (MSF), multieffect distillation (MED), vapor
compressor distillation (VCD), AltelaRainSMand freeze-thaw evaporation
(FTE). The thermal technologies have being applied for water
desalination and solids removal for human consumption sin the middle
XX century. The application of these technologies has increased lately
for produced water with the proliferation of shale gas in the United
States. They have high O & M costs and energy consumption. In their
advantage they do not need pretreatment and can handle over 100.000
of TDS [Dores, 2012].
Evaporation can also be used in traditionally in evaporation ponds where
solar energy efficiently evaporates de water placed in artificial ponds.
They have no mechanical systems, so low O&M costs. It is the cheapest
facility for saline water disposal in the applicable areas. The main
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disadvantages of the ponds are that they need vast extensions and dry
weather. These means they are only suitable in dry areas with high
evaporation rates and availability of lands at low cost [Ahmed 2000].
4.3 Adsorption
Adsorption is the process where a special solid used for removing
substances from the water. For oil and other non-polar substances
presents in the oil, BTEX and PAHs the active carbon is the most used
solid. It is made in order to achieve big internal surface, which improves
the adsorption process. There are two kind of active carbon, Powder
Activated Carbon (PAC) and Granular Activated Carbon (GAC). The one
employed in the oil-water treatment is the GAC, it normally gas greater
diameter than 0.1 mm [EPA, 2009].
GAC can be regenerated removing the adsorbed compounds through
steam, thermal o physical/chemical procedures. The first two are
common methods to recycle the active carbon. Steam regeneration is
only suitable option when the carbon has only retain volatile products.
Thermal regeneration is based in pyrolysis (burning the organic
substances). It is a very effective regeneration process but it also has
two big inconveniences, high carbon losses and cost [lennthec library,
2010]. Other adsorbents have been found in order to substitute the
active carbon, specially the GAC. These materials are the organoclays,
hydrophobic zeolite or polymer adsorbents.
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Organoclays present several benefits in comparison with the GAC. They
have higher adsorption capacity of hydrocarbons and they are very
effective in removing soluble and dispersed hydrocarbons. Organoclays
are used mainly in two purposes. The first one is as a pretreatment for
membrane filtration systems UF / RO and also for ion exchange resins
method. The second is a post treatment for oil and water separators
[Islam, 2006]. Organoclays are manufactured by modifying bentonite
with quaternary amine. Bentonite is basically montmorillonite; there are
two types, sodium bentonite and calcium bentonite. Quarter amines
used as oil-wetting agents, corrosion inhibitors and bactericides.
Zeolite is an alumina-silicate crystal with uniformly sized pores. It is
naturally hydrophilic (affinity for polar molecules), after it is treated and
the aluminum is removed, it becomes hydrophobic (affinity for non-
polar molecules). Polymers are manufactured with pore ranges from
macro to almost micro pores. They worked in polymer beds and they are
proved to adsorb faster than the active carbon.
4.4 Chemical treatment
4.4.1 Ion exchange process
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Ion exchange is the process where an ion replaces another one in an
aqueous solution. The synthetic materials specially designed for these
purpose are called ion exchangers or resins; resins developed for the
water treatment purposes are IX resins [Colorado School of Mines,
2009]. These resins are capable of capturing the contaminant cationic
ions dissolved in water, Calcium, Magnesium and be substituted by
exchange cations from the resin.
Resins used in produced water treatment are known as Strong Acid
Caution (SAC) where hydrogen and sodium cations highly dissociate and
remain ready for the exchange (Equation 1) [Arthur, 2005].
() ( )
()
This process is only applied for hardness water removal and it can also
be named as water softening. When the resin cannot exchange more
ions it must be regenerated. The resin is backwashed with the typical
cations that form the resins, Na cations, so that it is ready to begin the
process again; there are regeneration looses, around 2% [Colorado
School of Mines, 2009]. Ion exchange process is typically used for
drinking water or discharge to environment and also is a usual process
in nuclear power plants.
The major benefits of the process are low energy consumption; high
efficiency in the resin regeneration process and TDS values manageable
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up to 7000mg/l. Important disadvantages are the needs of pre-
treatment and post- treatments that increase significantly the O&M
costs and high sensitivity to fouling. Therefore, its main application is in
the coal bed methane produced waters because they are free of the
contaminants, which affect ion exchange performance.
4.4.2 Electrodialysis (ED)
ED and electrodialysis reverse (EDR) are processes where dissolve
inorganic ions from salts are separated from the water through ion
exchange membranes. The membranes are placed in between two
electrodes and allow the ions to pass through. If the membrane is
positively charged the negative ions will be separated and in reverse as
shown in figure 4. That is why several membranes are positively or
negatively charged alternately so every ion can be removed. EDR and ED
are very similar processes; the difference lies in the electrodes. In EDR
electrodes polarity is reversed in order to free accumulated ions in the
membrane surface.
A pretreatment is needed for both ED and EDR, because suspended
particles above 10 microns will block the membrane pores [lenntech
treatment solutions], also potential scaling minerals must be removed.
But they present some benefits, low-pressure requirements, no chemical
addition and long membrane working life. ED process is able to
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eliminate from 59% to 94% of dissolved solids and up to 12.000 mg/L of
TDS (normal operational conditions are 1.200 mg/l) [Jurenka, 2010].
4.4.3 Chemical Oxidation and Ozonation
The main objective of the chemical oxidation is to generate a powerful
oxidizing hydroxyl (OH-), which reacts rapidly but non-selectively with
nearly all organic compounds, formatting carbon dioxide and inorganic
salts or less toxic products. Typical chemical oxidation processes are
Advance Oxidation Processes (AOPs) and Ozonation.
AOPs present some advantages; the most important benefit is its
capability of oxidation of organic compounds. Main disadvantage of the
process is the addition of chemicals that increases the cost. AOPs have
been tested in labs and in fields for produced water treatment but it is
not applied commercially [Dores, 2012]. Its application for other
wastewater treatment makes this process potentially valid for oilfield-
produced water when biological treatment cannot be used.
Ozone gas is created with electrical discharges in the ozone generator
and then it is pumped into the tank. Inside the tank the ozone bubbles
flows into a contactor where adsorption takes place. This process is
called Ozonation. It is more effective than chlorine destroying bacteria
and no harmful removal is needed (ozone decomposes rapidly). It is
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complicated process that needs lots of technology to be applied. It is
also corrosive, non-suitable with suspended solids, possibly toxic, etc.
Both processes provide high removal efficiency of toxic compounds,
specially alkylated phenols but in the other hand some other toxics
forms (Chlorinated and brominated phenols) in low concentrations
[Grini, 2002]. Biological treatments are preferred over chemical
oxidation because they are non-environmental friendly, complex and
expensive to operate and maintain.
4.4.4 Flocculants and Coagulants
Coagulation is the process in which it is reduced the electric repulsion of
particles (same electrical charge) with the addition of salts; then
particles aggregate because of the remaining forces that attract the
particles. Flocculation causes the aggregation with polymers aid.
Coagulants and flocculants are the agents that cause respectively both
processes. They normally remove efficiently heavy weight organic
particles being incapable of removing low molecular weight and non-
polar particles. Those particles can be collected by biological systems.
Flocculants and coagulants miust be non-hazardous and biodegradable.
They are designed to aid in the oil-water separation processes; typically
they are ammonium and acrylamide. There are also several types of
coagulants, which can be cationic (positively charged), anionic
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(negatively charged) and nonionic (neutrally charged). Primary
coagulants are made to neutralize the charges while secondary or
coagulant aid mission is to maintain the flocs together so that they will
not break during the process. Some primary coagulants are aluminum
sulfate, ferrous sulfate or artificial polymers. Secondary coagulants can
be sodium silicate or charged polymers [Minerallurgy notes, UPM 2010].
4.5 Biological treatmentBiological treatment is normally used for organic material removal with
bacteria and other microorganisms; it is the latest process of produced
water before discharge or reuse. It is very important to know the
composition of the water in order to plan a specific treatment, for
instance in oil industry there is a special high demand of oxygen from
de bacteria to process the water [Schultz, 2005]. There are three basic
biological treatment groups, aerobic (presence of oxygen), anoxic
(oxygen deficient) and anaerobic (lack of oxygen). This oxygen quantity
is directly linked with the type of bacteria involved in the degradation of
the contaminants.
Aerobic treatments will take place in air presence with microorganisms,
which use the oxygen molecules to assimilate the organics creating
other compounds; they are also called aerobes. Anaerobic and anoxic
microorganisms or anaerobes will process in air absence to assimilate
the impurities. The compounds generated usually are carbon dioxide,
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water and biomass for the aerobics processes and carbon dioxide,
methane and biomass for the anaerobic [Mittal, 2011].
The main biological techniques are activated sludge, Sequenced Batch
Reactors (SBRs) and Membrane Bioreactors (MBRs) and Biological
Aerated Filters (BAFs). Because of its size and time of operation (days),
they are impossible to install in offshore platforms with the usual flux of
the production offshore facilities. They are also used in downstream oil
treatment.
5 Best Available Techniques BAT)This chapter of the report is a description of the best available
techniques for produced water in the oil & gas applications applying the
principles explained in chapter 4. There are many different technologies
to be able to cover all kind of diverse produced waters, varying its
characteristics not only from one field to another but also the variation
during the production time.
5.1 Skim TanksSkim tanks are the simplest and primary treatment of produced water.
They are designed for long time residence (up to hours) where
coalescence and gravity separation occur. They can have vertical or
horizontal configuration and work at atmospheric pressure or under
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pressure. These tanks can have several purposes, dispersed oil removal
(Skim tanks), solids removal (Settling Tank) and when oil and water ratio
is high, in order to make a bulk separation (Wash tanks).
In vertical skimmers oil droplets rise upward, meanwhile in the
horizontal vessels, the droplets rise in a perpendicular direction with the
water inlet flow. In both configurations, the air released during the water
injection in the vessel helps the droplets to float. Figure 5 shows both
configuration sketches. Vertical skimmers can include a spreader that
helps the distribution of the flow. The oil is skimmed at the surface in
both shapes. In order to control the oil level in the weir, a water leg
could be used. Horizontal skimmers are proved to be more efficient
than vertical skimmers. But vertical skimmers present useful features
when sand and other particles must be handled because a sand drain
can be added at the bottom. Also, vertical skimmers are less sensitive to
flow variations.
Pressure vessels might be used when the water has to be pumped for
any reason or there is a gas blow that creates difficulties in the water
injection into the system. Otherwise, atmospheric tanks should be
installed because of its lower cost.
Skimmers can remove droplet size above 150 m and a minimum time
residence of 20 minutes [Steward, 2008]. The vessels are highly affected
by temperature and they are not suitable for cold produced water.
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Horizontal baffles can be installed to perform a better separation. They
can treat high oil concentrations with solid contaminants.
5.2 Corrugated Plate Interceptor CPI)CPIs are coalescers, coalescers are devices that use gravity separation
like the skimmers but they also induce coalescence to improve the
separation. CPI is a basically certain number of parallel-corrugated
plates with 2.5cm distance between them, where the oil water
separation takes place called CPI pack (figure 6). Figure 7 shows a
down-flow through the CPI pack, the process can happen the other way
round call up-flow process (figure 8). It also exists crossed-flow devices
that they work under pressure. It allows both horizontal and vertical
configuration systems.
Process begins when the water enters into the nozzle (1), over there
biggest solids will sink and settle for posterior collection (2). Water and
oil will pass through a perforated distribution baffle plate (3). The CPI
pack (4) receives the oily water, where the oil rises to the peaks of the
corrugations (figure 8) and coalesces (5), it keeps moving upwards
exiting the pack reaching the surface at the top of the chamber (6),
where it flows over a weir (7) until the oil compartment (8). Water exits
the pack (9) where the smaller solids settle and they are also removed
(10). Water flows upward (11) into the clean water compartment (12).
There is a secondary oil outlet adjacent to the water outlet (13) and
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valve to ensure a gas blanket in the camber (14) [Energy Specialties
International].
Down-flow and Up-flow processes have some differences. The
inclination of the pack is usually 45and 60 respectively (see figures
7and 8) and the droplets size separation achieved is better in the down-
flow system, around 50m but solid removal is not important;
meanwhile in the up-flow is always above 50m but the solids size
removal cut off is up to 10m [Veolia, 2013]. Therefore, for oil and
water separation, if solids content is insignificant down-flow might be
used and in the opposite way. The inlet oil influent accepted can be as
high as 3000mg/l within a flow rate variation from 20 m3/h to 200m3/h
[Veolia, Paramount].
CPI exhibits many advantages, little operation and maintenance costs, it
is simple and it has no moving parts, so that no energy requirements. It
offers a continuous processing with high oil and solids efficiency (up to
150mg/l). The main disadvantage for oil wastewater is that this
technology is inefficient with high amount of solids and sometimes it
requires a post treatment if the disposal specifications are not reached.
5.3 UF with Ceramic membranes
Membrane process, as it explains chapter 4.1.4 of the report, basically
consists in the filtration of the produced water through a membrane
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with specific pore size because of a pressure drop between both sides of
it. The application of the UF/MF water treatment for produced water has
become a successful discovery, which can compete with traditional oil
wastewater processes. It has been proved in various studies and field
trails [Dores, 2010, Szp 2010]. MF is also a possible process but
sometimes it does not reach the water disposal requirements; therefore
UF is more popular. UF membranes are suitable for suspend solids oil
and grease, organic carbons removal and metals; dissolved ions and
organics will not be separated.
Ceramic membranes can have multiple pore sizes and configurations.
Table 6 Summarizes filtration range, membranes materials and pore
shapes for different manufacturers. The filtration size ranges from 5nm
to 1.4m depending on the different technologies developed for the
companies. All the membranes are built with alumina oxide and the
filtration channel can have several shapes hexagonal, round, squared,
etc [Benko]. The most important operating parameters for a ceramic
membrane process are the volumetric flow rate of the water per
filtration area, the trans-membrane pressure (average of feed and reject
pressure minus filtrate pressure) and the back pulse of the water from
the filtrate side to the feed side.
For instance, Veolias CerMem technology offers two different channel
sizes 2mm and 5 mm with a dimension of 8.64m/1.42m. That makes a
filtration area of 10.7m2and 5 m2 respectively. And the pressure drop
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is 1.3 bar in the first one and 0.5 bar in the second. Shows de
approximated values of the water flow for the membranes described.
Cross flow velocity should be between 3 to 4 m/s. Membrane
component materials determine the pH range, 0 to 14 for silica
membranes and 2 to 13 fro alumina and titania. The production rates
depends of the number of modules installed, the biggest flow available
is 170m3/h (30 or 52 modules installed, determined by the channel
size) and it needs pump power up to 170 kW.
5.4 Disk Stack CentrifugesIncreasing the acceleration the droplets are subjected to can enhance
the settling velocity of oil droplets achieving its separation from water,
this can be realized in a centrifuge [Van den Broek, 1996]. For oil/water
separation the centrifuges used are the Disk Stack Centrifuges. They
consist in a frame, a motor with a transmission, separator bowl (double
conical shape) and the inlet feed. The bowl has special inserts, the
gravity discs with conical shape that establish the oil water interface. It
is where the separation takes place; the distance in between the discs is
less than 1 mm. The centrifugal force generated ranges from 5000 to
6000 gs.
The feed is introduced in the bowl and is accelerated to maximum
rotational speed. The discs distribute the water due to the centrifugal
force and separate oil, water and solids. The oil flows towards the center
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of the bowl to the upper side of the discs; meanwhile the water and
sediments flow in the opposite direction. The liquids are led to the neck
of the bowl where they are removed. At the bottom of the bowl, in it
widest point, some solids discharge ports are installed. A piston moves
these ports, when the piston is at its lower position sediments are
released [Faucher and Sellman, 1998].
An example of this system is the X20 developed by Alfa Laval. It is a
special centrifuge system adapted to the oil and gas separation industry.
It can process 170 m3/h and its energy consumption is 150kw/h. The
small dimensions (3.15 m tall, 2.34 m long and 1.53 m wide) make it
suitable for offshore purposes. Represented in figure 9 Disk stack
centrifuges are capable of separate droplets with an approximately size
of 5 to 15 microns and solids from 3 to 10 microns and above. In the
solids removal, density is an important factor and it may be 1.4 g/ml or
higher in order to have a proper separation [Miedek and Fislage].
Centrifuges system present some benefits, the most important ones are
its efficient removal of smaller oil particles and solids and its application
for heavy oil de-oiling (up to 11.5 API) [Alfa Laval]. Centrifuges do not
need demulsifiers and also the rag layers found in traditional vessels are
eliminated. But they have high maintenance and operational cost
because of the rotation parts and also higher energy consumption. They
are meant for small water streams [Statoil, 2010].
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5.5 Hydrocyclones
Hydrocyclone vessels are units formed with conical devices where
centrifugal force and the specific gravity difference separate oil and
water. Individual hydrocyclone conical devices are called liners (figure
10). The quantity of liners varies depending on the produced water
characteristics and the water amount that needs to be treated. Figure 11
shows a hydrocyclone vessel with the liners inside.
Produced water is introduced under pressure into the hydrocyclone
vessel, and makes its way to the water/oil inlet ports; placed at the
larger diameter end of each liner. Pressure drop between the inlet ports
and the outlet ports of the liner ensures the flow path. A swirl
positioned axially in the liner induces a rotation flow throughout it. The
conical shape of the liner increases de fluid speed rotation. As the
diameter of the liner gets narrower the speed increases. Therefore the
centrifugal forces also augment resulting in the separation of light oil
and gas and heavy water and solids. The heavier materials move to the
walls of the liner towards the outer port. Meanwhile, the oil moves in a
closer vortex to the axis moving in the opposite direction towards the
inlet port [FMC, Veolia brochures, 2012]. The functioning of a liner is
sketched also in figure 10.
There are different factors that influence the separation performance,
such as the operating temperature that affects the viscosity, usually the
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higher the temperature the lower the viscosity. The decrease in water
viscosity favors the droplet settling velocity and the coalescence activity.
Figure 12 shows a comparison between different oil at constant
efficiency, as the viscosity increases, the droplet size of the separation is
also bigger. It can vary from 30 m to 10 m from heavy oil to light oil.
The pressure drop in the liners is a very important factor. The higher the
pressure drop the higher the tangential velocity is; and the hydrocyclone
performance is better. But if the hydrocyclone operates at maximum
flow rate, some turbulence may appear and it lows the efficiency of the
process as it shows figure 13. Another important agent is the droplet
inlet size. Cyclotech Technologies affirms that there is a critical droplet
size around 10 m to 15 m where the efficiency drops notably (figure
14).
Several oil & gas companies have elaborated different designs, Siemens,
Veolia, FMC because of the advantages they provide. They are
compact modules with high efficiency and the can reduce oil
concentration to 10 ppm. They do not need any pre treatment and
energy consumption is very low, they only use energy to pump the water
into the vessel. The main disadvantages of this system are that the
solids can block the inlet and scale formation could happen increasing
the maintenance cost. Besides they can treat any kind of produced
water, a post-treatment may be needed in order to remove other
dissolved components to achieve the disposal standards.
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5.6 IGFIn the IGF units, the water is injected into the floatation tank but the
bubbles are generated by physical procedures.it can be several gas
injected such as nitrogen, natural gas, carbon dioxide or air (it is
necessary to be extremely careful with the air ant its oxygen content for
its explosion potential). There are two types of IGF, Hydraulic and
Mechanical. In both of them a coagulant pretreatment could be use to
favor the flocculation.
Hydraulic typical system is showed in Figure 15. It shows a flotation unit
with three cells. The recycled water flows through the venture eductors,
where the gas is sucked and the mixture is released into the chamber
where flotation occurs. Then the oil is skimmed and removed and the
water is pumped into the recycling system [Natco]. Normal
manufacturers design each cell with around 50% efficiency that makes
not cost-effective to install more three or four cells because more cells
efficiency increase is too low [Steward, 2008].
Mechanical system includes a rotating impeller, driven by an electric
motor that creates a vortex, which introduces de gas into the vessel.
The gas mixes with the water and originates the bubbles. Figure 16
sketches the process. Hydraulic system is less expensive and involves
less maintenance than the mechanical system because of the rotating
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parts. In the other hand mechanical procedure allows to control bubble
size and usually they are more efficient.
IGF can handle low oil concentrations from 15mg/l up to 500 mg/l,
mechanical IGF are more efficient with lower concentrations (below
150mg/l) [Aker, Unidro]. It offers really high efficiency, over 98% in the
separation sizes over 15m. They are not affected by flow rate variations
and they can operate heavy oils with not big density difference achieving
good separation results. They have high power consumption and a
proper chemical pretreatment can increase their efficiency. They can
operate onshore and offshore but in no floating facilities, as a
consequence of skimming over a weir.
5.7 Compact Flotation Unit CFU)The CFU is a vertical separator vessel, which separates the three phases
oil/water/gas by using centrifugal force and gas flotation. It has no
moving parts and is capable of achieving high standards of oil removal.
It has smaller volume and shorter retention than traditional flotation
units (Statoil, 2010). That is why CFU suits ideally offshore applications,
it reduces the size and weight of the oil/water separation facilities
compared old systems [EPCON, Siemens]. CFU was developed by EPCON
that nowadays is owned by MISwaco (Schlumberger), other companies
have built similar systems such as Siemens with its Vorsep technology
or OPUS. CFU systems are capable of reducing the oil content below 10
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ppm [EPCON, Opus, VWS westgarth], and if two CFU systems work
together, this content can be reduced to 5 ppm (EPCON).
Small oil droplets are made to coalesce, creating larger droplets, which
are easier to remove. The droplets because of specific gravity difference
form a continuous layer at the top of the vessel. Oil water separation is
helped with a simultaneous flotation effect, caused by the release of
residual gas from the produced water. In some occasions the gas
flotation is increased with external gas injection and flocculants.
As figure 17 sketches, the produced water enters the CFU tank
horizontally, in a tangential direction. The distributor situated at the top
chamber dispersed the water. The majority of any entrained gas is
released at this point. Produced water makes its way under gravity
trough the eductors towards the bottom chamber. The design of these
eductors ensures that the gas from the upper chamber is drowning
down into the eductors where it mixes with the water. One of the
important features of the CFU is the perfect mix between gas and
produced water. The shape of the eductors creates a vortex in the lower
chamber, which favors the coalescence of the oil droplets, and a toroidal
flow is created. The oil floats with the help of the gas and it moves to
the top where it is removed with a skimmer. The water exits the vessel
through the bottom [Veolia].
CFU system has many advantages; most of the companies confirm that
it is a robust system with small footprint and low weight. Because they
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have no rotating parts, it is easy to operate, no energy is required, and
maintenance costs are also lower. It has a high flow capacity with low
volume, for instance, an EPCON CFU system can operate a water flow up
to 220 m3/h with a vessel volume of 2.4 m3.
5.8 Sand cyclonesThe desander vessel is ideal for the inline desanding of produced water
and is the most important element in the sand management system
[Aker, 2012]. The vessel consists in two sections, the upper section
where the separation occurs and the bottom section where the sand is
removed. The principle of operation is the same as in every
hydrocyclone. Separation happens due to the pressure drop in the liners
inlet and outlet ports that creates two different vortexes. In the sand
cyclones the solid particles move to the walls of the liner and the water
flows in the smaller vortex. The sand will be accumulated in the
catchment chamber at the bottom and discharged intermittently,
meanwhile the desanded water discharge continuously.
The liners, very similar to de-oiling hydrocyclones are placed in the
upper section between two support plates. They must be manufactured
with special ceramic materials such as alumina ceramic (standard) or
bonded silicon carbide [Cyclotech] in order to resist the erosion
provoked by the solid particles [Aker, Veolia]. A sand cyclone sketch is
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shown in figure 18; they can have different diameter size depending on
the size of the particles that need to be removed. For instance, Aker
suggest 1.5-inch diameter for 4 mm particles and a separation size
ranged from 10 to 20 microns. For bigger particles, around 6 mm they
put forward a 3-inch liner that can separate up to 40 microns.
5.9 C-TourC-tour system technology developed to extract dispersed and dissolved
and dispersed oil, reducing the environmental impact in the North Sea.
The participants were Statoil, Norsk Hydro, BP, Shell, etc. It was
conducted at the Rogaland Research I Institute and Norsk Hydro
research center. The name comes from the French scientist who
discovered the phenomena of super critical fluids in XIX century,
Cagniard de la Tour in 1822 [Voldum and Garpestad, 2008].
The principle of the process is to use natural gas liquid-condensate as a
solvent to extract the hydrocarbons contaminants in produced water
[Descousse, 2004]. The process includes several steps; the first is to
collect the condensate from the production extreme. This can be done
in the gas compression train scrubbers [Grini, 2002]. Then the collected
condensate is injected at small rate into the produce water line (0,3-2%
volume/volume). The second is the extraction of the hydrocarbons from
the water into condensate phase; this process might take couple of
seconds. And the last step is the separation of the condensate from the
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water in a hydrocyclone system. Recycling of the rejected water must be
done. Figure 19 is a diagram of the Ctour process.
The condensate should accomplish some features in order to achieve
good results in the process. The condensate must remain in liquid
phase during the injection and following extraction. Composition of the
condensate is a very important factor [Voldum, 2008]. The condensate
may contain some aromatic components, which, could be present in
higher concentrations than in the produced water. It can end in an
increase of the heavy aromatic compounds in the produced water. If the
condensate does not reach the needed characteristics some
pretreatments can be used. Some of them are increasing the processing
pressure to match the liquid phase or flashing the condensate to reduce
bubble point [Voldum and Garspetad, 2008]. Condensate injection and
mixture is another key element for the process. The system must ensure
a homogeneous dispersion throughout the produced water stream,
providing the highest possible surface favoring coalescence process.
Besides the dispersion, the higher the turbulence the better the mass
transfer will be, which also helps the performance of the system.
Table 7 presents the results of the Ctour system in the Ekofisk, offshore
oilfield in the North Sea. It is important to notice the low efficiency of
the process in the C4-C5 phenols. Voldum explains this phenomenon
affirming that those phenols are highly soluble in water with low
bioaccumulation. The average of oil in water discharge ranges 1-2 ppm
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and it is never higher than 2.2 ppm. Statoil confirms that the removal
average of the Ctour system is: 95% for Dispersed oil, 92% in
Naphthalene, 97% in PAH, 0% C0-C3 phenols (very important, it meets
the zero content discharge), 50% C4-C5 phenols, 97% C6-C9 phenols
and 10% to 80% BTEX.
5.10MPPEMPPE or macro porous polymer extraction is an Akzo Nobel technology
elaborated in the 1990s. It is capable to withdraw dispersed and
dissolved hydrocarbons to very low levels with flow rates from 200 m3
to 250m3. This technology is a liquid-liquid extraction performed by a
macro porous polymer particle. MPPE system is commonly placed after
the first separation processes, in gas/condensate fields after
degasser/skimmer and in oil fields after hydrocyclones.
The MPPE usually consist in two columns that ensure a continuous
operation, one is destined to extraction and the second for
regeneration. The contaminated water passes through the first column
packed with MPPE particles that contain specific extraction liquid. The
particles have a diameter of 1m with pore size range 0,1 to 10 m. The
hydrocarbons with high affinity for the liquid are removed. In order to
clean the extraction liquid, low-pressure steam strips the hydrocarbons
that are condensed and separated later in the second column (figure
20). It is a long process that takes usually one hour for each column
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[Meijer, 2004]. Most of the oil/gas field components can be removed
with a very high efficiency, BTEX and PAH`s, reach 99999% removal
from 2000 to 3000ppm concentrations. It has been proved that
chemicals such as scale corrosion inhibitors, demulsifiers or H2S
scavenger have no negative effect on the performance. To improve the
MPPE process it is necessary to optimize de steam consumption used in
the regeneration tower and some solids pre-filter can be added to avoid
blocks.
MPPE presents many advantages to become an even more important
technology. It is a robust system with long life and flexible, it is capable
of treating different kinds of water (oilfield/gas-field) with high
efficiency removal, it presents 84% in EIF (Environmental Impact Factor)
analysis [Meijer, 2004] (almost reaching the zero discharge goal). It has
been demonstrated in Kvitebjrn that bioactivity in the field stopped
during year 2005, when the MPPE system was installed; bioactivity of the
field was restored in less than 3 months because of the high
hydrocarbon elimination. Figure 21 presents a comparison between
flotation and MPPE process. The worst disadvantage of this unit is its
high price. Other disadvantages are the relatively high-energy
consumption and the cost of the pre-treatment in the oilfield produced
water.
5.11Walnut shell filters
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Walnut shell filters consist in filtration and scrubbing processes in the
same vessel. Filtration usually occurs down flow, as the liquid passes
through the media, oil and solids are efficiently attached (coalesced) in
bed. This process is based on time or pressure difference. Often, air or
gas is added to create an airlift pump.
Then the scrubbing system starts, the scrub pump is opened and the
media starts circulating in the scrubbing system. During this circulation
the media is positively cleaned because of the turbulence of the
backwash water and the air (if added) [Siemens, Cameron]. For the
backwashing of the media, Cameron proposes a rotating media, which
also allows a horizontal configuration process but it adds rotation parts
to the system increasing O&M costs.
The addition of air into the vessel reduces considerably the backwash
water required. Figure 22 sketches both processes by Siemens. The
filtration needs the backwash cleaning approximately every 24 hours
and it will take 15 to 20 to clean the media. The media is highly
regenerated and only 5% per year will be lost.
Walnut filters can process maximum oil loading of 100ppm of oil and
suspended solids with a high efficiency removal of 99% for insoluble
hydrocarbons over 2 m and 90 to 99% for solids over 5 m. These
filters are suitable for every oil gas production facility, either offshore or
onshore. A typical flux rate would be 33m3/h. No pretreatment and low
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energy needed for the process, only for the scrub pump and rotating
part if existing.
5.12Mares TailMares Tail technology has been developed by Opus, it is basically an
inline coalescer. Its purpose its to coalesce the very small droplets
(smaller than 10m) of the produced water to improve the posterior
performance of the hydrocylones or other separation technologies.
Opus affirms the droplets size increase ranges 400to 500%.The unit contains a spool fibrous element fixed in the inlet. The dirty
fluid enters the nozzle and flows along the spool in the same direction
as the coalescence medium. As the fluid travels throughout the fibers,
the small droplets are attracted to the surface and coalesce.
The main advantages are its high coalescing efficiency, tolerant to solids
and unaffected by motion (available for floating offshore facilities).
Compact and easy to operate and maintain. OPUS ensures it is its best
cost effective technology. They do not need pretreatment but it is made
as a pretreatment itself for a posterior more efficient removal.
5.13BAFIt consists of a permeable media such as rocks, graves or plastic, which
combines aeration and separation facilitating biochemical oxidation and
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as a consequence the organic removal. BAF can remove oil, ammonia,
suspended solids, nitrogen, heavy metals and hydrogen sulphide
[Ebenezer, 2012]. The approximate removal capabilities are 60% to 90%
of nitrification, 70% to 80% oil and 75% to 85% of suspended solids.
They feed water of oil can never be over 60 mg/l [Colorado School of
Mines, 2009]. Water recovery is 100% and it has low energy
consumption. Cost is low as in most biological treatments and it is
adaptable to wide range quality and quantity water flux.
5.14Activated sludgeConventional Activated Sludge Process (ASP) is the oldest and the most
common biological wastewater treatment. In the ASP, as the
microorganisms grow, they from particles that gather (flocs), settle to
the bottom of the tank leaving a liquid free of organic material. An ASP
is basically composed of an aeration tank where the biological reaction
happens; an aeration source of oxygen; a clarifier where solids separate
and settle and finally a tank to collect the solids and return them into
the tank (called return activated sludge or RAS) or remove them form the
process (waste activated sludge or WAS).
The process begins when the influent is injected into the aeration tank.
Bacteria rise up as they move through it where the air is pumped in fine
bubbles at the bottom, for the oxygen requirements of the process.
Then the aerated mixed water, also called mixed liquor, overflows by
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gravity to the clarifier tank where bacteria separate the organisms and
settle to the bottom. The sludge is either pumped back to the tank with
the new income of wastewater (RAS) or removed from the system (WAS);
figure 23 sketches the process. According to Fakhrul Razi ASP can
remove from 98% to 99% of total hydrocarbons with twenty days
duration process. Mittal ensures that conventional ASP will meet
specified discharge standards.
The evolution of the conventional ASP ended in the sequencing batch
reactors in which all the treatment occurs in one singular tank. The
process starts with the filling of the reactor with the influent, it can be
aerated or anoxic. Afterwards, the react phase begins where aeration
and mixing continue until the full biodegrading finishes. When
biodegrading has ended, aeration and mixing are turned off and
biomass settles down to the bottom of the tank. Then effluent is
removed in the decant phase and then the sludge is also discharged.
SBRs can operate 200.000 m3a dayensuring denitrification and
phosphorous removal, basic compounds needed to be discharge for
human or agricultural consumption [Mittal, 2011]. SBEs are flexible and
can vary and adjust shortly to the configuration of the process.
5.15MBR
MBR process combines anoxic and aerobic treatment with an integrated
immersed membrane. It is a similar process to the ASP with mixed liquor
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in the aeration tank but they differ in the separation. In the MBR process
the separation occurs because of the action of a polymeric membrane
with MF/NF, meanwhile gravity settling occurs in the same system as in
an ASP. Thus MBR provides an extra filter for the water that makes this
process achieve better results in bio-solids removal (less than 1 mg/l)
[Siemens]. It is small for being a biological process and it also shortens
the performance time to 6 hours if the membrane is correctly clean with
appropriate CIP. It also requires high maintenance of the membrane,
which is the key to lengthen the life of it, such as dehydration besides
the CIPs.
Table 7 shows a comparison of the BAT explained on the report, main
characteristics and suitable operations.
6 Flow DiagramProduced water facilities use various methods to treat the produced
water. But every production facility despite its differences can be divided
in several steps depending on the technologies applied and the limits
they want to reach. Those steps are bulk removal or primary, secondary
and tertiary or advanced treatment. Because of obvious issues offshore
and onshore facilities are different in size and technology used, being
the offshore more compact and lighter than the typical onshore plants.
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Onshore primary treatment normally uses separators and skimmers; it
can also use hydrocyclones, CPI and ceramic membrane processes.
Secondary is characterized by CFU and IGF and finally for tertiary
treatment Walnut filter or MPPE for example. Also for advanced
treatment all biological treatments and ion exchange for water
softening. Offshore treatment needs to use the most compact and
lighter systems necessary to reach the specifications for injection or
disposal. They can use hydrocyclones, C-Tour, IGF, etc.
Figure 25 shows a possible flow diagram for a full line produced water
treatment. The water could be used for reinjection for disposal or
reservoir pressure maintenance and also for surface discharge. A
hydrocyclone has been used as primary treatment after the 3 phase
separators of the production train. The oil separated is redirected to the
production train before the low-pressure separator. As a secondary
treatment an IGF unit has been placed. The skimmed oil must be
pumped into the production line. Another pump has been placed for the
water recycling system in the IGFs units. Finally a walnut shell filter will
cover the tertiary treatment for reinjection or disposal of the water.
7 New challengesProduced water technologies has become an even more important cost
issue in the oil and gas production industry since the new laws have
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lowered the disposal standards not only for dispersed oil but aromatic,
production chemicals, etc. These new limits for discharge have
increased the cost of produced water treatment and nowadays the oil
and gas are lowering the price because of the global crisis since 2008.
As a consequence of the increasing of the difficulties for disposal and
the decreasing of the benefits, many efforts have been made for
developing new BAT that can compete with traditional oil and water
separators reaching the new standards at the lowest possible price.
Everyone must agree in the best via to save costs in produced water
treatment is reducing the amount of produced water that has to be
administered. But this is a difficult task, because water injection is a
really common procedure for oil production and water is also need for
water flooding. Other solution might be over treat the water achieving
human consumption standards so freshwater would be obtained and
many possibilities offer for freshwater utilizations.
One of the ways to reduce the cost is to separate the oil and the water in
subsea facilities with simultaneous reinjection so the amount of water
that needs to be treated at the surface is only the water in oil emulsions.
[Ogunsina, 2005; Sheridan, 2013]. The first subsea hydrocyclone was
installed by FMC; it is capable of separating heavy oil from water and
reuses the water for reinjection to boost oil production of the field. It is
called Marlim Project and it is built for PetroBras. It also includes other
equipment for the sand treatment [FMC, 2011]. The systems consists
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basically in a sand remover, a set of vertical pipes (free gas removal), a
pipe separator (60m long) and separation vessel (oil/water separators),
another sand remover and an hydrocyclone (figure 24) [Orlowsky, 2012].
The other future goal of produced water companies is the final
developing of the membrane RO and NF for oilfield wastewater. Many
experiments have been carried out and ceramic membranes are being
tested for RO and NF but they were not yet successful. Both systems will
decrease considerably the chemicals, solids, metals, etc. The main
objective of these technologies is to reduce the pretreatment that both
of them need to perform and the integrity of the membrane. For such
thin filters, there is a high risk of membrane fouling with oil and gas
produced water. The possibility of application of RO for produced water
means that produced water can be clarified to the limits of human
consumption and agriculture irrigation. This capability will open
produced water to a whole new market with new possibilities that could
make produced water treatment profitable.
For instance, there is an emergent market of fresh water consumption in
developing countries in desert areas where water supply is becoming a
big problem. Countries such as Saudi Arabia or The Emirates, which are
also oil producers. These countries, as a solution for the human fresh
water consumption could treat oilfield, produced water with the new
technologies, membranes generally, solving both problems, fresh water
consumption and oilfield produced water.
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8 ConclusionsThere are many different produced water techniques to optimize the
treatment and make the process the best cost-effective. This is even
more important in offshore platforms because of the additional difficulty
of the weight, supplies issues, maintenancewhich makes only a few
technologies available at a high price. In order to reach the zero
disposal concentration, some
