كيف تتكون مياه الصرف الصناعى فى مواقع استخراج الغاز الطبيعى والالبار البترولية

THE TYPE OF WASTE WATER PRODUCED FROM OIL,NATURAL GAS PRODUCTION

TECHNOLAB EL-BAHAA GROUP

GENERAL.DR

BAHAA BADR

CHEMICAL ADVISOR

WHAT IS PRODUCED WATER?

Natural water or formation water is always found together with petroleum in reservoirs.

It is slightly acidic and sits below the hydrocarbons in porous reservoir media .

Extraction of oil and gas leads to a reduction in reservoir pressure, and additional water is usually injected into the reservoir water layer to maintain hydraulic pressure and enhance oil recovery.

In addition to injected water, there can be water breakthrough from outside the reservoir area, and as oil and gas production continues, the time comes when formation water reaches production well, and production of water begins alongside the hydrocarbons.

This water is known as produced water or oilfield brine, accounting for the largest volume of by product generated during oil and gas recovery operations .

It is a mixture of injected water, formation water, hydrocarbons and treating chemicals , generally classified as oilfield produced water, natural gas produced water and coal bed methane (CBM) produced water depending on the source.

Oilfields are responsible for more than 60% of daily produced water generated worldwide .

The rate of oilfield produced water production is expected to increase as oilfield ages .

Generally, produced water is composed of dissolved and dispersed oil components, dissolved formation minerals, production chemicals, dissolved gases (including CO2 and H2S) and produced solids .

There is a wide variation in the level of its organic and inorganic composition due to geological formation, lifetime of the reservoir and the type of hydrocarbon produced.
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2.1 Dissolved and dispersed oil components

Dispersed and dissolved oil components are a mixture of hydrocarbons including BTEX (benzene, toluene, ethylbenzene and xylene), PAHs (polyaromatic hydrocarbons) and phenols.

Dissolved oils are the polar constituent organic compounds in produced water, while small droplets of oil suspended in the aqueous phase are called dispersed oil .

BTEX, phenols, aliphatic hydrocarbons, carboxylic acid and low molecular weight aromatic compound are classified as dissolved oil, while less-soluble PAHs and heavy alkyl phenols are present in produced water as dispersed oil .

Dissolved and dispersed oil content in produced water is dangerous to the environment and their concentration can be very high at some oil fields .

The quantity of oil present in produced water is governed by a number of complex but interrelated factors .
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2.2 Dissolved mineral

Dissolved inorganic compounds or minerals are usually high in concentration, and classified as cations and anions, naturally occurring radioactive materials and heavy metals.

Cations and anions play a significant role in the chemistry of produced water. Na+ and Cl− are responsible for salinity, ranging from a few milligrams per litre to ∼300 000 mg/l .

Cl−, SO42−, CO32−, HCO3−, Na+, K+, Ca2+, Ba2+, Mg2+, Fe2+ and Sr2+ affect conductivity and scale-forming potential.

Typical oilfield produced water contains heavy metals in varied concentrations, depending on the formation geology and the age of oil well .

Heavy metal concentrations in produced water are usually higher than those of receiving water (for enhanced oil recovery) and those found in sea water .

226Ra and 228Ra are the most abundant naturally occurring radioactive elements present in oilfield produced water .

Radioactivity of produced water results primarily from radium that is co-precipitated with barium sulphate (scale) or other types of scales.

The concentration of barium ions in produced water could give a strong indication of radium isotopes present in it .

In some oilfields, up to 21 Bq/l of 228Ra have been detected in produced water samples .

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2.3 Production chemicals

Production chemicals can be pure compounds or compounds containing active ingredients dissolved in a solvent or a co-solvent, and used for inhibition of corrosion, hydrate formation, scale deposition, foam production, wax deposition, bacterial growth, gas dehydration and emulsion breaking in order to improve the separation of oil and water .

These chemicals enter produced water in traces and sometimes significant amounts and vary from platform to platform.

Active ingredients partition themselves into all phases present depending on their relative solubilities in oil, gas or water.

The fate of these chemicals is difficult to determine because some active ingredients are consumed within the process .

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2.4 Produced solids

Produced solids include clays, precipitated solids, waxes, bacteria, carbonates, sand and silt, corrosion and scale products, proppant, formation solids and other suspended solids

Their concentrations vary from one platform to another.

Produced solids could cause serious problems during oil production.

For example, common scales and bacterial can clog flow lines, form oily sludge and emulsions which must be removed

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2.5 Dissolved gases

The major dissolved gases in produced water are carbon dioxide, oxygen and hydrogen sulphide.

They are formed naturally, by the activities of bacterial or by chemical reactions in the water.

PRODUCED WATER MANAGEMENT TECHNOLOGIES

The general objectives for operators treating produced water are:

de-oiling (removal of dispersed oil and grease), desalination, removal of suspended particles and sand, removal of soluble organics, removal of dissolved gases, removal of naturally occurring radioactive materials (NORM), disinfection and softening (to remove excess water hardness)

To meet up with these objectives, operators have applied many standalone and combined physical, biological and chemical treatment processes for produced water management.

Some of these technologies are reviewed in this section.

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4.1 Membrane filtration technology

Membranes are microporous films with specific pore ratings, which selectively separate a fluid from its components.

There are four established membrane separation processes, including microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO) and nanofiltration (NF)

RO separates dissolved and ionic components, MF separates suspended particles, UF separates macromolecules and NF is selective for multivalent ions

MF and UF can be used as a standalone technology for treating industrial wastewater, but RO and NF are usually employed in water desalination.

Membrane technology operates two types of filtration processes, cross-flow filtration or dead-end filtration that can be a pressure (or vacuum)-driven system
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4.1.1 Microfiltration/ultrafiltration

MF has the largest pore size (0.1–3 µm) and is typically used for the removal of suspended solids and turbidity reduction.

It can operate in either cross-flow or dead-end filtration.

UF pore sizes are between 0.01 and 0.1 µm.

They are employed in the removal of colour, odour, viruses and colloidal organic matter

UF is the most effective method for oil removal from produced water in comparison with traditional separation methods and it is more efficient than MF for the removal of hydrocarbons, suspended solids and dissolved constituents from oilfield produced water

Both MF and UF operate at low transmembrane pressure (1–30 psi) and can serve as a pre-treatment to desalination but cannot remove salt from water

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4.1.2 Polymeric/ceramic membranes

Polymeric and ceramic membranes are used for UF/MF treatment of water.

Polymeric MF/UF membranes are made from polyacrylonitrile and polyvinylidene and ceramic membranes from clays of nitrides, carbides and oxides of metals

Ceramic UF/MF membranes have been used in a full-scale facility for the treatment of produced water

Product water from this treatment was reported to be free of suspended solids and nearly all non-dissolved organic carbon.

Ceramic UF/MF membranes can operate in both cross-flow filtration and dead-end filtration modes and have a lifespan of >10 years.

Chemicals are not required for this process except during periodic cleaning of membranes and pre-coagulation (used to enhance contaminants removal).
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4.1.3 Reverse osmosis and nanofiltration

RO and NF are pressure-driven membrane processes. Osmotic pressure of the feed solution is suppressed by applying hydraulic pressure which forces permeate (clean water) to diffuse through a dense, non-porous membrane

Seawater RO can remove contaminants as small as 0.0001 µm, but its major disadvantage is membrane fouling and scaling

Early studies on using RO to treat produced water failed due to insufficient process integration and poor treatment

Capital costs of RO membrane systems vary depending on the size of rejection required, materials of construction and site location.

Operating costs depend on energy price and total dissolved solid (TDS) level in the feed water. RO membrane systems generally have a life expectancy of 3–7 years

NF is a robust technology for water softening and metals removal and is designed to remove contaminants as small as 0.001 µm

It is applicable for treating water containing TDS in the range of 500–25 000 ppm.

This technology is similar to RO

NF membranes were employed for produced water treatment on both bench and pilot scales

4.2 Thermal technologies

Thermal treatment technologies of water are employed in regions where the cost of energy is relatively cheap.

Thermal separation process was the technology of choice for water desalination before the development of membrane technology.

Multistage flash (MSF) distillation, vapour compression distillation (VCD) and multieffect distillation (MED) are the major thermal desalination technologies

Hybrid thermal desalination plants, such as MED–VCD, have been used to achieve higher efficiency

Although membrane technologies are typically preferred to thermal technologies, recent innovations in thermal process engineering make thermal process more attractive and competitive in treating highly contaminated water
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4.2.1 Multistage flash

MSF distillation process is a mature and robust technology for brackish and sea water desalination.

Its operation is based on evaporation of water by reducing the pressure instead of raising the temperature.

Feed water is pre-heated and flows into a chamber with lower pressure where it immediately flashes into steam

Water recovery from MSF treatment is ∼20% and often requires post-treatment because it typically contains 2–10 mg/l of TDS

A major setback in operating MSF is scale formation on heat transfer surfaces which often makes this process require the use of scale inhibitors and acids.

Overall costs vary depending on the size, site location and materials of construction

Its energy requirement is between 3.35 and 4.70 kWh/bbl
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4.2.2 Multieffect distillation

MED process involves application of sufficient energy that converts saline water to steam, which is condensed and recovered as pure water.

Multiple effects are employed in order to improve the efficiency and minimize energy consumption

A major advantage of this system is the energy efficiency gained through the combination of several evaporator systems.

Product water recovery from MED systems are in the range of 20–67% depending on the type of the evaporator design employed

Despite the high water recovery from MED systems, it has not been extensively used for water production like MSF because of scaling problem associated with old designs.

Recently, falling film evaporators have been introduced to improve heat transfer rates and reduce the rate of scale formation

MED has a life cycle of 20 years and can be applied to a wide range of feed water quality like MSF.

It is good for high TDS produced water treatment

Scale inhibitors and acids may be required to prevent scaling and pH control is essential to prevent corrosion.

Power energy consumption is in the range of 1.3–1.9 kWh/bbl , operating cost is ∼$0.11/bbl and total unit cost is $0.16/bbl

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4.2.3 Vapour compression distillation

VCD process is an established desalination technology for treating seawater and RO concentrate

Vapour generated in the evaporation chamber is compressed thermally or mechanically, which raises the temperature and pressure of the vapour.

The heat of condensation is returned to the evaporator and utilized as a heat source

VCD is a reliable and efficient desalination process and can operate at temperatures below 70°C, which reduces scale formation problems

Energy consumption of a VCD plant is significantly lower than that of MED and MSF.

The overall cost of operation depends on various factors, including purpose of plant, zero liquid discharge target, size of plant, materials of construction and site location.

Cogeneration of low-pressure steam can significantly reduce the overall cost.

Although this technology is mainly associated with sea water desalination, various enhanced vapour compression technologies have been employed for produced water treatment
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4.2.4 Multieffect distillation–vapour compression hybrid

Hybrid MED–VCD has been recently used to treat produced water.

Increased production and enhanced energy efficiency are the major advantages of this system.

It is believed that this new technology would replace the older MSF plants

GE has developed produced water evaporators which uses mechanical vapour compression.

These evaporators exhibit a number of advantages over conventional produced water treatment methods, including reduction in chemical use, overall cost, storage, fouling severity, handling, softer sludge and other waste stream

4.3 Biological aerated filters

Biological aerated filter (BAF) is a class of biological technologies which consists of permeable media that uses aerobic conditions to facilitate biochemical oxidation and removal of organic constituents in polluted water.

Media is not more than 4 in in diameter to prevent clogging of pore spaces when sloughing occur

BAF can remove oil, ammonia, suspended solids, nitrogen, chemical oxygen demand (COD), biological oxygen demand (BOD), heavy metals, iron, soluble organics, trace organics and hydrogen sulphide from produced water

It is most effective for produced water with chloride levels below 6600 mg/l

This process requires upstream and downstream sedimentation to allow the full bed of the filter to be used.

Removal efficiencies of up to 70% nitrogen, 80% oil, 60% COD, 95% BOD and 85% suspended solids have been achieved with BAF treatment

Water recovery from this process is nearly 100% since waste generated is removed in solid form

BAF usually have a long lifespan. It does not require any chemicals or cleaning during normal operations.

Its power requirement is 1–4 kWh/day, and capital accounts for the biggest cost of this technology.

Solids disposal is required for accumulated sludge in sedimentation basins and can account for up to 40% of the total cost of this technology

4.4 Hydrocyclones

Hydrocyclones use physical method to separate solids from liquids based on the density of the solids to be separated.

They are made from metals, plastics or ceramic, and usually have a cylindrical top and a conical base with no moving parts

The performance of the hydrocyclone is determined by the angle of its conical section

Hydrocyclones can remove particles in the range of 5–15 µm and have been widely used for the treatment of produced water

Nearly 8 million barrels per day of produced water can be treated with hydrocyclones

They are used in combination with other technologies as a pre-treatment process.

They have a long lifespan and do not require chemical use or pre-treatment of feed water.

A major disadvantage of this technology is the generation of large slurry of concentrated solid waste.

4.5 Gas flotation

Flotation technology is widely used for the treatment of conventional oilfield produced water.

This process uses fine gas bubbles to separate suspended particles that are not easily separated by sedimentation.

When gas is injected into produced water, suspended particulates and oil droplets are attached to the air bubbles as it rises.

This results into the formation of foam on the surface of the water which is skimmed off as froth

There are two types of gas flotation technology (dissolved gas flotation and induced gas flotation) based on the method of gas bubble generation and resultant bubble sizes.

In dissolved gas floatation units, gas is introduced into the flotation chamber by a vacuum or by creating a pressure drop, but mechanical shear or propellers are used to create bubbles in induced gas flotation units

Gas floatation can remove particles as small as 25 µm and can even remove contaminants up to 3 µm in size if coagulation is added as pre-treatment, but it cannot remove soluble oil constituents from water

Flotation is most effective when gas bubbles size is less than oil droplet size and it is expected to work best at low temperature since it involves dissolving gas into water stream.

Flotation can be used to remove grease and oil, natural organic matter, volatile organics and small particles from produced water].

It does not require chemical use, except coagulation chemicals are added to enhance removal of target contaminants.

Solid disposal will be necessary for the sludge generated from this process and the estimated cost for flotation treatment is $0.60/m3 of produced water

4.6 Evaporation pond

Evaporation pond is an artificial pond that requires a relatively large space of land designed to efficiently evaporate water by solar energy

They are designed either to prevent subsurface infiltration of water or the downward migration of water depending on produced water quality

It is a favourable technology for warm and dry climates because of the potential for high evaporation rates.

Evaporation ponds are typically economical and have been employed for the treatment of produced water onsite and offsite.

Ponds are usually covered with nettings to prevent potential problems to migratory waterfowl caused by contaminants in produced water

All water is lost to the environment when using this technology which is a major setback when water recovery is an objective for water treatment.

4.7 Adsorption

Adsorption is generally utilized as a polishing step in a treatment process rather than as a standalone technology since adsorbents can be easily overloaded with organics.

It has been used to remove manganese, iron, total organic carbon (TOC), BTEX, oil and more than 80% of heavy metals present in produced water

There are a variety of adsorbents, such as activated carbon, organoclays, activated alumina and zeolites

Adsorption process is applicable to water treatment irrespective of salinity.

It requires a vessel to contain the media and pumps to implement backwashes which happen periodically to remove particulates trapped in the voids of the media.

Replacement or regeneration of the media may be required depending on feed water quality and media type.

The rate of media usage is one of the main operational costs of adsorption technology

Chemicals are used to regenerate media when all active sites are blocked which often results in liquid waste disposal, and media replacement results in solid waste management.

4.8 Media filtration

Filtration technology is extensively used for the removal of oil and grease and TOC from produced water

Filtration can be accomplished by the use of various types of media such as sand, gravel, anthracite, walnut shell and others.

Walnut shell filters are commonly used for produced water treatment. This process is not affected by water salinity and may be applied to any type of produced water.

Media filtration technology is highly efficient for the removal of oil and grease, and efficiency of more than 90% has been reported

Efficiency can be further enhanced if coagulants are added to the feed water prior to filtration. Media regeneration and solid waste disposal are setbacks to this process.

4.9 Ion exchange technology

Ion exchange is a widely applied technology in industrial operations for various purposes, including utilization for the treatment of CBM produced water.

It is especially useful in the removal of monovalent and divalent ions and metals by resins from produced water

Ion exchange technology has a lifespan of ∼8 years and will require pre-treatment options for solid removal.

It also requires the use of chemicals for resin regeneration and disinfection.

The operating cost accounts for more than 70% of the overall cost of this technology
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4.10 Chemical oxidation

Chemical oxidation is an established and reliable technology for the removal of colour, odour, COD, BOD, organics and some inorganic compounds from produced water

Chemical oxidation treatment depends on oxidation/reduction reactions occurring together in produced water because free electrons cannot exist in solution

Oxidants commonly used include ozone, peroxide, permanganate, oxygen and chlorine.

The oxidant mixes with contaminants and causes them to break down.

The oxidation rate of this technology depends on chemical dose, type of the oxidant used, raw water quality and contact time between oxidants and water

Chemical cost during this process may be high

Energy consumption accounts for ∼18% of the total cost of operations and maintenance

It requires minimal equipment and has a life expectancy of 10 years or greater and solid separation post-treatment may be employed to remove oxidized particles

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4.11 Electrodialysis/electrodialysis reversal

Electrodialysis (ED) and ED reversal (EDR) are mature electrochemically driven desalination technologies.

These processes involve separation of dissolved ions from water through ion exchange membranes.

They use a series of ion exchange membranes containing electrically charged functional sites arranged in an alternating mode between the anode and the cathode to remove charge substances from the feed water

If the membrane is positively charged, only anions are allowed to pass through it.

Similarly, negatively charged membranes allow only cations to pass through them.

EDR uses periodic reversal of polarity to optimize its operation

EDR and ED technologies have only been tested on a laboratory scale for the treatment of produced water.

ED/EDR membrane lifetime is between 4 and 5 years, but major limitations of this technology are regular membrane fouling and high treatment cost

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4.12 Freeze thaw evaporation

Freeze thaw evaporation (FTE®) process developed in 1992 by Energy & Environmental Research Centre (EERC) and B.C.

Technologies Ltd (BCT) is a mature and robust technology for produced water treatment and disposal

FTE® process employs freezing, thawing and conventional evaporation for produced water management.

Naturally, salts and other dissolved constituents in produced water lower its freezing point below 32 F.

When produced water is cooled below 32 F but not below its freezing point, relatively pure ice crystals and an unfrozen solution are formed.

The unfrozen solution contains high concentration of dissolved constituents in the produced water and it is drained from the ice.

The ice can be collected and melted to obtain clean water.

About 50% of water can be recovered from this process during winter, but at other seasons, no water is recovered because FTE® works as a conventional evaporation pond.

FTE® can remove over 90% of heavy metals, TDS, volatile and semi-volatile organics, total suspended solids and total recoverable petroleum hydrocarbons in produced water

FTE® does not require chemicals, infrastructure or supplies that limit its use.

It is easy to operate and monitor, and has a life expectancy of ∼20 years

However, it can only work in a climate that has substantial number of days with temperatures below freezing and usually requires a significant amount of land.

Waste disposal is essential when using FTE technology because it generates a significant amount of concentrated brine and oil.
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4.13 Dewvaporation: AltelaRainSM process

Dewvaporation is a desalination technology.

Its principle of operation is based on counter current heat exchange to produce distilled water

Feed water is evaporated in one chamber and condenses on the opposite chamber of a heat transfer wall as distilled water

Approximately 100 bbl/day of produced water with salt concentration in excess of 60 000 mg/l TDS can be processed by this system

High removal rates of heavy metals, organics and radionuclides from produced water have also been reported for this technology.

In one plant, chloride concentration was reduced from 25 300 to 59 mg/l, TDS from 41 700 to 106 mg/l and benzene concentration from 450 µg/l to non-detectable after treatment

energy requirements of this system are low because it operates at ambient pressures and low temperatures.

This makes it a viable alternative water treatment at remote oil wells where there is no high power grid but there is no information on the overall cost of the system which is likely to be its major disadvantage.

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4.14 Macro-porous polymer extraction technology

Macro-porous polymer extraction (MPPE) is one of the best available technologies and best environmental practices for produced water management on offshore oil and gas platforms

It is a liquid–liquid extraction technology where the extraction liquid is immobilized in the macro-porous polymer particles.

These particles have a diameter of ∼1000 μm, pore sizes of 0.1–10 μm and porosity of 60–70%. Polymers were initially designed for absorbing oil from water but later applied to produced water treatment in 1991

In 2002, the first commercial MPPE unit offshore was successfully installed on platforms in the Dutch part of the North Sea.

MPPE was used for the removal of dissolved and dispersed hydrocarbons, achieving >99% removal of BTEX, PAHs and aliphatic hydrocarbons at 300–800 ppm influent concentration.

It was also reported that removal efficiency of 95–99% for aliphatics below C20 and total aliphatic removal efficiency of 91–95% was possible

In the MPPE unit, produced water is passed through a column packed with MPPE particles containing specific extraction liquid.

The two columns allow for continuous operation with simultaneous extraction and regeneration

Almost all hydrocarbons present in produced water can be recovered from this process which can in turn be disposed or recycled.

Stripped hydrocarbons can be condensed and separated from feed water by gravity, and product water is either discharged or reused.

This technology is essentially used to reduce the toxic content of produced water and can withstand produced water containing salt, methanol, glycols, corrosion inhibitors, scale inhibitors, H2S scavengers, demulsifiers, defoamers and dissolved heavy metals.

Pre-treatment through hydrocyclones or other flotation methods is however necessary before letting produced water from oilfields flow into the MPPE unit.

Studies have shown that in gas/condensate produced water streams pre-treatment is not required and MPPE can remove the whole spectrum of aliphatics, as well as BTEX and PAHs

As international legislations seek ‘zero discharge’ of contaminants into the environment and focus on the EIF of contaminants, MPPE will be a major produced water treatment technology in the future.

A study carried out by Statoil to compare the effect of different treatment technologies of oilfield produced water on EIF found that the MPPE technology had the highest EIF reduction of ∼84%

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