Reuse and Recycling of Produced Water – Obstacles and Opportunities

1 Background

Produced water is used to describe water trapped in underground formations that rises to the surface during oil and gas exploration and production. (Igunnu & Chen, 2014). Produced water needs to be efficiently managed to develop these resources prosperously.

Natural or formation water is commonly found with petroleum in reservoirs. Its pH is slightly acidic, and it lies beneath the hydrocarbons in porous media, as identified in the image below. When extracting oil or gas, a reduction in pressure occurs. Water is re-injected into the reservoir water layer to maintain hydraulic pressure and improve oil recovery. Water breakthrough from around the reservoir area can occur. As time progresses, water formation reaches the production well, and water production occurs along with hydrocarbons. This water is classified as produced water (Igunnu & Chen, 2014).

Figure 1: Diagram of a typical reservoir (Igunnu & Chen, 2014)

Typically, produced water comprises dissolved and dispersed oil components, dissolved formation minerals, production chemicals, dissolved gases such as carbon dioxide and hydrogen sulphide and produced solids (Hansen & Davies, 1994: 176-188). Produced water is frequently considered wastewater. However, if managed as a resource, instead of a discarded waste, it provides possibilities. Prospects include ameliorating drought, reuse for hydraulic fracturing and new emerging solutions. The concept of produced water for reuse for activities other than secondary recovery has been discussed and promoted. However, beneficial use remains a challenge for various reasons that will be outlined in this article.

2 Water Recycle and Reuse Drivers

Internationally, approximately 250 million barrels of produced water are generated every day from oil and gas fields, and more than 40% of this is released into the surrounding environment (Igunnu & Chen, 2014). This waste presents many reasons why water reuse should be investigated, as outlined below.

2.1 Water and Wastewater Discharge Costs

Water cost challenges are not isolated to drought restricted areas or for high-density populations. The inflation rates of wastewater disposal are higher than the average freshwater inflation rates (Gupta & Berrum, 2017). Companies need to keep ahead of and monitor the cost of the waste. As produced water is the largest waste stream generated during oil and gas production, any costs incurred with its disposal are likely to have one of the most significant impacts on the process’s economics (Veil, 2015).

2.2 Water Scarcity

The scarcity of water during the dry season can cause companies to scrutinise their water footprint. Many regions globally are susceptible to seasonal droughts. If alternatives such as well-water or surface water are used, specific challenges may arise. Wellwater contains typically larger amounts of dissolved solids or other contaminants, and direct access to surface water may contain higher amounts of total suspended solids (TSS) (Gupta & Berrum, 2017). Increasing urban populations result in growing demands and puts a strain on the infrastructure that delivers fresh water. These factors may lead to water rationing, which is common during dry periods in the summer months. This may affect the company’s ability to deliver optimally.

2.3 Regulatory Challenges

General legislation for releasing produced water into the ocean has been outlined in regions globally, as illustrated in the table below for the oil in water (OIW).

Table 1: International regulations for permitted limits of total oil and grease (TOG) for produced water discharge (Neff, Lee and Deblois, 2011); (Gerardo, 2007); (Igunnu and Chen, 2012); (Jiménez et al.,2018).

The limits defined above and in any local legislation may hinder company growth. This is common when discharging to a local publicly owned treatment site. A rise in the volume of wastewater discharge or a change in water quality because of a change in production can cause fines. Modifying permits can become challenging to acquire. These factors may hinder company growth. Treating and reusing wastewater may be a great way to allow production increase without violating permit volume restrictions.

2.4 Sustainability

Sustainable manufacturing is how products are processed through economic means, which minimise negative environmental impacts whilst conserving energy and natural resources. Sustainable manufacturing maximises operational efficiency, reduces costs, reduces wastes, improves production capacity, and strengthens a company’s brand and reputation (US EPA, 2021).

2.5 Social Pressures/Responsibility

Protecting the world’s resources is a global concern. Negative publicity around a company’s water use can impact a company’s growth. Many large corporations now regard water conservation as their corporate social responsibility. Consumer perception of a company is becoming more prominent as consumers now pay closer attention to a company’s message about its environmental impact.

2.6 Wastewater processing Limitations

In a variety of industries, wastewater treatment capacities have not increased in direct proportionality with plant production. Consequently, treatment facilities struggle to accommodate larger flows and higher concentrations of waste. The elimination of specific contaminants may need a specified amount of contact and mixing time. Hence, an increase in wastewater volume can negatively influence removal efficiencies. Dilution is not the solution to pollution. For many effluent types, dilution is prohibited by local council regulations (Gupta & Berrum, 2017).

Whilst arrangements can be made to invest in upgrading and adding equipment to treat more flow, another approach could be to reduce a portion of the wastewater and recycle it to other plant processes.

3 Aspects of Implementation of Produced Water Reuse and Feasibility

1) Treatment

Produced water is a combination of inorganic and organic materials. Conditions that influence the properties of the produced water include the location of the oil or gas field, the lifespan of the reservoir, and the variety of hydrocarbon generated. Other factors include the operational environment and chemicals used in the processing facilities. The following substances exist in produced water and occur in various concentrations. The number of components in the produced water will correlate to the amount of treatment necessary. The principal components in produced water will be outlined below (Igunnu & Chen, 2014).

2) Dissolved formation minerals

Examples of inorganic dissolved components are anions, cations, radioactive substances, and heavy metals.

Cations and anions influence the buffering capacity, salinity and amount of scale that could form. Heavy metals can be present in the water. The concentrations measured depend on the geology and age of the site. Radium is a common radioactive substance present in produced water that needs to be treated before water is reused for agricultural or livestock purposes.

3) Dissolved and dispersed oil components

The amount of dissolved and dispersed oil components in produced water is dependent on the oil composition, the oil to water ratio, temperature, TDS, pH, salinity, type, and amount of oilfield chemicals. Aromatic components play the most significant part in environmental toxicity and cannot be removed efficiently by simple oil to water separation methods.

4) Suspended solids

Suspended solids include solids formed as a result of corrosion or scale, bacteria, or waxes. Bacteria need to be removed as they can obstruct or cause corrosion in pipelines and equipment.

5) Produced chemical compounds.

During the oil and gas generation process, chemical additives are used to mitigate operational concerns. Chemical additives include scale and corrosion inhibitors, biocides, emulsion breaks, antifoam, and water treatment chemicals.

6) Dissolved gaseous compounds

Carbon dioxide, oxygen, and hydrogen sulphide are commonly identified in produced water.

The primary outcomes for treating produced water are (Davarpanah, 2018):

1. Removal of dispersed and dissolved oils
2. Desalination
3. Removal of TSS and TDS
4. Disinfection
5. Softening
6. Ultimately, the end-use will determine the treatment steps undertaken.

Liability, risk perception and the environment

The ownership of the produced water and the mineral rights to the produced water needs to be clear. The owner would need to take responsibility and would be liable should any issues occur.

Long-term sustainability studies should be conducted for scenarios where the amount of produced water generated becomes inadequate. Hydrological studies and basin data should be collected, as the produced water should not intrude on local freshwater supplies. Policies such as water rights, rights of capture, inter-basin transfers should be appraised. Regulations promoting or inhibiting use must be studied to assess whether the project remains feasible (Gupta & Berrum, 2017). A market analysis should also be conducted.

Risks of Poorly managed Oil and Gas wastewater

Despite the advantages of water reuse, the scientific community has raised questions regarding the challenges for the environment and local communities. There are two broad categories associated with the risk of reuse and recycling. The first is the risk related to spills or leaks from minimally treated produced water, and the second broad category relates to the uncertainty about the constituents of wastewater. The risks from poorly managed wastewater include chemical hazards from organic chemical effluents. These chemicals pose a risk for terrestrial ecosystems, aquatic ecosystems, and human health. The contaminants may taint local soil and can cause leaching into local water sources, which may compromise drinking sources.

Furthermore, irrigated agriculture could become tainted with effluent. This may accumulate in the edible components of the plants and fruits, and vegetables. These hazardous organic components may build up in the food chain and can harm humans and livestock.

Personnel in the plant need to be trained on handling produced water as spills of brackish water cannot be compared to spills handling freshwater. Any spills in the storage or transport of the produced water need to be cleaned up appropriately. Corrosion in wells or pipelines can cause problems, and appropriate materials should be used in pipelines and storage containers. Other factors that should be considered when conducting a feasibility assessment for produced water are infrastructure investment and expenses. The necessary infrastructure includes tanks, pipes, and treatment facilities. Things to consider with costing include expenses to access the produced water (pumping from subsurface), transport costs and treatment costs, including disposal costs of the solid and liquid concentrate waste.

Financing should be investigated into whether it will be public or private, and partnerships with industries and localities should be considered (Gupta & Berrum, 2017).

4 Alternative Uses for Produced water

4.1 Reuse in oil and gas production and Hydraulic fracturing

Hydraulic fracturing is a controlled process that pumps fluid and chemical additives to target geological formations at high pressure to produce fractures and enable the generation of hydrocarbons. Millions of litres of water are used for hydraulic fracturing. Water is essential in the lifecycle of the oil and gas industry. It is also used to help cool equipment in mud circulation, which helps cool the drill bit and carries rock cutting out of the borehole; it is also used to improve oil recovery techniques. Produced water can also be reused for maintaining pressure and integrity during oil and gas operations. Produced water can be processed and recycled for hydraulic fracturing to minimise the need for fresh water and minimise the need to inject produced water.

4.2 Irrigation (after dilution and treatment)

Irrigation of food crops in dry land areas is still not feasible or sustainable as the produced water quality, treatment and transport costs restrict any reuse potential. The soil irrigated with produced water has a high sodium content, boron content, and salinity that could affect crops’ quality and safety (Echchelh, Hess and Sakrabani, 2018). However, the produced water can be used for the irrigation of other crops. Cotton is considered to be a drought and salt-tolerant crop. Research conducted in Texas, USA, concluded that produced water blended with freshwater with a 4:1 ratio of groundwater: treated produced water resulted in the successful growth of crops and did not reduce cotton yield or lint quality. The lint yield for groundwater was 658 kg/ha compared to 637 kg/ha for blended water. Other studies have been conducted in the USA to analyse irrigation for biofuel crops such as switchgrass and rapeseed. These crops were explicitly selected as they can tolerate a wide variety of pH and salinity conditions. It was determined from this case study that the produced water should be treated enough to reduce the TOC to 3500 mg/l and to reduce the concentrations of organic substances to permit water reuse for irrigation (Alexopoulou et al., 2008).

4.3 Algal production

Traditionally, mechanical, chemical, and physical techniques have been used to treat, and process produced water. These methods have several disadvantages: high costs and intensive energy input, generation of scale, excessive pre and post-treatments, disposal of solid concentrate waste, and low water recovery efficiencies. However, biological treatments are both economical and efficient in removing toxins and contaminants from the environment. Many microorganisms thrive in saline environments and therefore make them perfect candidates for bioremediation of produced water. Algae are proficient in removing contaminants such as organic chemicals and heavy metals from water sources. They use some constituents in the produced water for their growth, and therefore the cultivation of algae in produced water help decontaminate it. The amounts of nutrients available in produced water are often in concentrations higher than in the commercial growth media. As the algae nutrient’s supplement for cultivation makes up approximately 50% of the costs involved, these elements aid in the investment return. The algae could provide a sustainable option in produced water treatment as they integrate resource recovery to generate biomass which acts as a biofuel resource. Algal biofilms are also used to generate many products such as bioplastics, pharmaceutically active compounds, high-value chemicals, biofuel feedstock, and animal feed. End products of algal biofilms also have applications in the cosmetic, medical, biotechnological and food industries.

Produced water in the brackish range of TDS is the best resource for algae cultivation, along with the following factors: dry climate, solar irradiance, availability of large and affordable land (Rahman et al., 2020). Iran, a famous oil-producing country, conducted a study using available technology, using produced water for biodiesel and estimated the country could produce 795 x 10³ tons/month of biodiesel, which could fulfil approximately 26.5% of Iran’s diesel needs (Talebi et al., 2016). Cases studies for microalgae production have also been concluded in Qatar, with some toxic elements having 100% removal. Microalgae cultivation from produced water presents a promising application for the future.

5 Treatment Options

Potable reuse of produced water faces obstacles comparable to other varieties of wastewater. Examples of obstacles include expensive treatment, potential chronic toxicity of the treated produced water, and public approval. The amount and characteristics of the produced water vary over time, making a “one size fits all” solution improbable.

The methods of treatment for produced water are outlined in the image below (O’Hara, 2020):

Two categories are available for treatment and reuse of produced water: conventional treatment and advanced treatment. Both categories have energy, environmental, and economic impacts that directly influence water quality. Conventional treatment examples include flocculation, coagulation, sedimentation, and filtration. These are generally effective in removing water quality parameters, including suspended solids, oil, hardness compounds, and other undissolved parameters. Produced water quality can vary from brackish to saline to brine, and the water’s salt content may need to be addressed. (Matthew & Mantell, 2011). Advanced treatment options include:

1. Membrane methods such as reverse osmosis,  nanofiltration, microfiltration and ultrafiltration.
2. Thermal methods, including multistage flash distillation, multiple-effect distillation, and solar thermal methods.
3. Electrocoagulation and capacitive deionisation.

5.1  Conventional Treatment

I. Primary

The primary stage of conventional treatment aims to remove approximately 90% of the dispersed oil. Effluent from this treatment level contains 50 to 200 ppm oil (Azam et al., 2019). Primary treatment equipment examples include skimmers, plate interceptors and hydrocyclones.

Skimmers

The principle behind skim tank separation is gravity. It is a piece of equipment designed for the continuous separation of oil droplets from the produced water. Droplets larger than a determined diameter will rise to the surface and will be skimmed off. Skimming tanks provide long residence times. Various designs exist, and in some, the produced water first flows into a degassing chamber where liquids settle and leave under a cone or spreader. In other designs, the oil and water layers are distinguishable within the tank, and interface control is used when skimming.

Skimming tanks cannot accommodate many solids coming in, and frequent de-sludging can be a cumbersome process.

Figure 2: Skimming tank schematic (Azam et al., 2019)

Plate Interceptors

Corrugated plate interceptors also use gravity separation as the fundamental principle. Fluid enters the unit at one end and separates along the length of the bed. The unit contains corrugated plates stacked at a 45° angle, as illustrated in the diagram below. These plates allow for a larger surface area for suspended oil droplets to coalesce and form bigger globules. After separation, oil globules rise to form, and the oil layer is skimmed off. The sludge settles at the bottom. Plate interceptors are better suited to accommodate smaller sized droplets. The corrugated plates improve the separation efficiency and have a small footprint. However, a degasser needs to be installed upstream. Other disadvantages are that the plates can become plugged and that the unit contains less sludge storage space, therefore needing frequent maintenance (Dey, 2021).

Figure 3: Corrugated plate interceptor (Dey, 2021)

Hydrocyclones

Hydrocyclones employ physical separation techniques to divide liquids and solids based on the density of the solid components. They are composed of plastic, ceramics or metals and have no moving fragments. They are primary or secondary treatment facilities and are used with other pieces of equipment as a pre-treatment. Hydrocyclones have a lower capital expense than other pieces of equipment, are durable, and offer a chemical-free process (Igunnu & Chen, 2012). They can accept high inlet concentrations of TSS. However, they generate a substantial amount of concentrated solid waste and require a steady flow of produced water at the inlet. They also present high operational expenditure costs because of de-oiling chemicals that need to be dosed upstream for a high oil removal efficiency (Dey, 2021).

Figure 4: Hdyrocyclone schematic adapted from Dey (2021)

II. Secondary

Gas Flotation

Flotation technology is a classic conventional treatment technology: small gas bubbles are employed to separate suspended particles through a gas injection into the produced water. This results in TSS and oil globules attaching themselves to the air bubbles as they rise and leads to the development of foam on top of the water, which is removed as froth.

Two types of gas flotation exist: induced gas flotation and dissolved air flotation. The type of flotation depends on the procedure of gas bubble formation and sequential bubble size.

In dissolved gas flotation, gas is brought into the flotation bed by a vacuum or generating a pressure drop. Whereas in induced air flotation units, bubbles are formed by mechanical shear or propellers.

Gas flotation can eliminate particles as small as 25 μm. If coagulation is included as pre-treatment, particles larger than 3 μm can be removed. However, gas flotation cannot separate soluble oil components from produced water. Flotation is most efficient when the gas bubble size is less than the oil droplet size. Flotation can be used to remove grease, oil, volatile organics, small particulates, and organic material. However, flotation is negatively affected by the effluent’s high salinity and may require additional treatment methods to treat the salinity.

The best operational conditions are at low temperatures. Chemical use is unnecessary. However, coagulation is recommended to aid the removal of contaminants. Disposal of solid concentrates is necessary, and a cost will be incurred in the disposal (Igunnu & Chen, 2012).

III. Tertiary

Coalescers

Coalescence is a process whereby two or more emulsion droplets converge into a single droplet. These droplets combine to form bigger droplets that can be removed more easily.

Figure 5: Operating configuration of a horizontal bed coalescer (Almeida et al., 2019)

Emulsions are classified as mixtures with two immiscible liquids, such as oil and water. For an emulsion to be created, contact between the two liquids combined with an emulsifying agent is necessary. Mechanical energy is also needed to merge the tiny droplets. Treatment by this method is highly effective when removing oil and grease in emulsions. It provides the advantages of low energy expenditure and does not require excessive chemical reagents. The bed can operate continuously, is simple to install, and only needs essential maintenance. There is also the prospect of self-cleaning beds. However, it has several downfalls. Removing the separated oil from the equipment may clog the bed when the solids concentration is high. The bed may also need to be routinely replaced.

Media Filtration

Filtration is another conventional classical technique used to remove oil, grease, and total organic compounds from produced water. Various media can be used, such as sand, walnut shells, gravel, and cartridges.

Walnut shells are typically used for produced water treatment. This procedure can be used on any type of produced water and is not influenced by water salinity. The crushed walnut shell media (3 mm size) is light, oleophilic (can adsorb oil), and has a high modulus of elasticity (can withstand rigorous backwashing). It removes oil mostly without chemical destabilisation (Azam et al.,2019). Media filtration is incredibly efficient, with efficiencies greater than 90 % reported. Efficiencies can be improved by introducing coagulants to the inlet water stream before filtration. Disadvantages of this method include solid concentrate disposal and media replacement (Igunnu & Chen, 2012).

Chemical oxidation

Chemical oxidation is a tried and tested technology used to eliminate odour, colour, COD, BOD, organics, and some inorganic compounds from produced water. This treatment is subject to oxidation/reduction reactions simultaneously as free electrons are removed from the produced water. Examples of chemical oxidants used are ozone, permanganate, oxygen, chlorine, and peroxide. The chemical oxidant incorporated into the water breaks up the impurities. The rate at which this occurs is determined by the chemical dose, variety of oxidant used, quality of the incoming produced water and the time the oxidant and water may react. This treatment does not need a lot of equipment, has a lifespan of approximately ten years and has a 100% water recovery rate. Unfortunately, the price of chemicals used may be high and periodic maintenance is necessary. Chemical metering equipment is also essential (Igunnu & Chen, 2012).

5.2 Advanced Produced Water Treatment Options

Figure 6: Diagram illustrating the need for Advanced Treatment (Azam et al., 2019)

Advanced treatment is required to remove the dissolved or soluble portion of oil and grease measured in produced water.

I. Membrane Methods

The membrane pressure-driven process depends on the membrane’s pore size to divide the feed components as per the pore size. There are four accepted membrane separation methods: microfiltration (MF), ultrafiltration(UF), reverse osmosis (RO) and nanofiltration (NF). MF divides suspended particles, UF segregates macromolecules, RO separates dissolved, and ionic compounds and NF is used for multivalent ions (Igunnu &Chen, 2012).

Microfiltration/ultrafiltration

MF and UF are used as single separation systems in treating wastewater and are typically used to eliminate odour, viruses, colloidal organic material, and discolouration in the water. UF is the most efficient technology for separating oil from produced water when weighed against conventional separation methods. There is no need for chemical additives, energy expenses are minimal, and the unit is compact. UF is also more useful for the removal of TSS and TDS when compared to MF. Both UF and MF function at low transmembrane pressures and can operate as preparation for desalination but cannot separate salt and water.

Reverse Osmosis and Nanofiltration

These are pressure-driven membrane processes. Hydraulic pressure is applied to the feed stream; this compels the produced water through a dense, non-porous membrane. Reverse osmosis can eliminate contaminants as minuscule as 0.0001 μm. However, the biggest drawback for RO is the scaling and fouling on the membrane. Studies have proven that RO is successful when appropriate pre-treatment is conducted to reduce water salinity or when the produced water is brackish.

Capital expenditure in RO varies depending on the materials used in construction, the site’s location and the size of particles processed. Operational costs rely on energy prices and the amount of TDS in the inlet produced water.

NF is an excellent selection for water softening and metal removal. It can remove particles larger than 0.001 μm and is typically used to remove TDS with concentrations ranging from 500 ppm to 25000 ppm (Igunnu &Chen, 2012).

II. Thermal Methods

Thermal treatment was the method of choice before membrane technologies became more readily available and are commonly used when the cost of energy is inexpensive. Advancements in technology had resulted in thermal technologies becoming more enticing and competitive when processing significantly contaminated produced water. Thermal technologies available include Multistage Flash (MSF), Multi-effect Distillation (MED) and Vapour Compression Distillation (VCD) (Igunnu & Chen, 2012).

Multistage flash

This is a robust desalination treatment that has been used for many years. It is also relevant for highly contaminated produced water with TDS below 40 000 mg/L. Reducing the pressure, as opposed to raising the temperature, causes the water to evaporate. The inlet water stream is pre-heated. It then flows to a chamber with a lower pressure, where it instantly evaporates. With this method, labour costs are less expensive than using membrane technology and require a less meticulous pre-treatment process. The lifespan of the equipment is 20 to 30 years. However, water recovery is estimated between 10% and 20%, and post-treatment is recommended. A significant disadvantage is the scale formation that requires chemical treatment (scale inhibitors and acids). pH control is also vital to inhibit corrosion.

Multi-effect distillation

This method requires energy to convert saline water to steam. This water is then condensed and recovered as pure water. Water recovery is estimated to be between 20 – 67 % depending on the multiple effects employed. Technology innovation has now resulted in evaporators with improved heat transfer rates that mitigate scale formation and increase operating water recovery efficiencies. MED has a similar lifespan to MF distillation and requires chemical treatment to manage scale formation and pH.

Vapour Compression Distillation

VCD is another option used to treat water. Vapour produced in the evaporation chamber is thermally or mechanically compressed. These conditions increase the temperature and pressure of the vapour. VCD presents minimal scaling issues. VCD is a dependable and efficient desalination method and can function at temperatures below 70°C. Consequently, VCD is more energy-efficient than other thermal methods. In the past, VCD has been mainly linked with desalination but recently has been used to treat produced water.

III. Electrocoagulation, capacitive deionisation

Wastewater treatment using electrochemical technologies has become more attractive due to high energy efficiency, ease of automation and safety. Capacitive deionisation (CDI) is an emerging technology that employs electrophoretic driving forces to attain desalination. No chemical reagents are needed for this process. CDI uses direct current to remove charged dissolved ions from water. Ions are adsorbed onto the outer surface of porous electrodes. The anode attracts positively charged ions such as sodium, magnesium, and calcium, whilst the cathode attracts negatively charged ions such as chlorides, nitrates, and sulphates. Short-circuiting the electric field temporarily permits ions to desorb from the electrodes’ surface and regenerates the electrodes.

Various materials and configurations of the electrodes exist. CDI technology is comparable with reverse osmosis when analysing costs incurred. However, this comparison is only applicable to low TDS concentration ranges. CDI is recommended to treat produced water with TDS < 3000 mg/L. CDI also has a lower energy consumption than thermal treatment processes and has a product water recovery of approximately 80%. The unit has a small footprint and does not require monitoring, control, or skilled labour. The setback is that CDI requires additional treatment to remove uncharged species such as Boron and organics (Azam et al., 2019).

Figure 7: Steps involved in CDI (Azam et al., 2019)

Electrocoagulation (EC) has also increasingly been used due to its enhanced separation of organics. The ease of access to electrode materials, non-requirement for pH control, minimal requirements for reagent chemicals, minimal sludge production and minimal OPEX, makes it an appealing treatment option. EC uses soluble metals such as Aluminium and Iron for the electrodes. These generate metal hydroxides when a current is applied. The metal hydroxides act as coagulants or adsorbents to remove bacteria, inorganic and organic contaminants. Electrocoagulation techniques may include redox, decomposition, deposition, coagulation, adsorption, precipitation, and flotation. Typically, direct current is used whereby an impenetrable oxide layer is created on the cathode surface, and corrosion occurs on the anode surface because of oxidation. This inhibits efficient current transfer between cathode and anode, thereby deteriorating the treatment process. Innovative development has resultant in pulse electrocoagulation (PE), which utilises an on-off-on power supply similar to CDI. This helps electrode passivation and minimises energy consumption, as the electrification time is less than the EC reaction time (Ren et al., 2011).

5.3 Technology Specification

The factors affecting technology selection decisions include:

1. Produced water specification—the composition of the produced water going into the treatment plant and the characteristics of the produced water leaving.
2. Flow rate—both at the start of the field’s life and at the end of its life operation. This can vary significantly when re-injecting water. More water is produced as the well matures. Therefore, flexibility needs to be built into the process. Process equipment where the performance efficiency is independent of the flow rate should be selected. Examples include plate separators or filters.
3. Offshore challenges—Several factors should be considered, such as the vessel motion when there is a floating installation which may impact a liquid-liquid separation process, such as in coalescence. The footprint and weight of the units installed should also be assessed as this both increases the cost of the support of the vessel and causes challenges in transporting and setting up the unit offshore.
4. Remote operation—where installations are geographically remote and equipment that does not need frequent human interactions should be selected.
5. CAPEX vs OPEX—the right balance should be found; flowrates should be considered in this decision.
6. Sludge – Sludge provides a significant challenge, as this can blind the surface of a filter. It can also settle and interfere in the operation of other equipment.

6 Case Study: Water Use in Shale Development

Chesapeake Energy Corporation has developed and implemented the recycling of produced water in shale development. The challenges presented and the reuse strategies employed are presented in the following case study.

Drilling a typical Chesapeake shale well requires between approximately 250000 litres to 2 300 000 litres of water. Hydraulic fracturing requires, on average, 19 million litres of water. Produced water plays a fundamental role in the environmental and economic viability of shale oil and gas development. The Barnett, Fayetteville, and Marcellus Shales all produce a substantial amount of produced water, enabling reuse.

Barnett Shale Reuse

Barnett Shale wells produce the largest volume of produced water. The energy required to treat Barnett Shale produced water (apart from direct filtration and blending) is considerable. As all energy sources create air emissions or require water use or generate waste, reusing produced water in this area using an advanced treatment method may result in more negative environmental impacts than saltwater disposal. Barnett Shale water has higher levels of TDS, low levels of TSS and intermediate scaling predisposition.

Chesapeake is treating and reusing 6% of the total water required to fracture and drill. The logistics, economics, and urban curfew limitations prevent higher levels of reuse in the southern region of the area.

Fayetteville Shale Reuse

The water produced in this region is of superior quality for reuse, and the quantity of water is adequate. The produced water in this region has exceptionally low TDS, TSS and has a low scaling tendency. This well is currently meeting 6% of the drilling and fracturing requirements with a target of 20% of produced water reuse set. Currently, Fayetteville is experiencing the same limiting factors as Barnett.

Haynesville Shale Reuse

This well site generates a lower quantity of produced water when compared to the other significant plays and has inferior water quality. TDS and TSS levels are high, and the water has a high scaling tendency. Produced water reuse at Haynesville is difficult because of the low quality and volume. Furthermore, existing saltwater disposal infrastructure already exists in the area. The resulting economics have led to the unsuccessful reuse of produced water. Nevertheless, substantial volumes of superior quality drilling wastewater produced during drilling have resulted in the Chesapeake actively searching for methods to reuse this in further drilling and fracturing tasks.

Marcellus Shale Reuse

The Marcellus shale is ideal for produced water reuse, and its reuse plan has flourished. The produced water has intermediate to high TDS levels, low levels of TSS, and intermediate scaling tendency. TDS are controlled by accurate blending with fresh water, and TSS are controlled by conventional treatment systems. Scaling and bacterial are accurately managed, monitored and tested. The Marcellus shale well reuses nearly 100% of all produced water and drilling wastewater. This minimises the amount of fresh water needed to drill and hydraulically fracture additional wells by 10% to 30%. This has reduced road deterioration and traffic on public roads and has also influenced air quality. The reuse program is attractive because it assists in the operational, disposal, water supply and transportation expenses (Matthew & Mantell, 2011)

7 Conclusion

Energy, environmental and economic considerations must be carefully contemplated when discussing potential reuse and disposal options for produced water. A lot of technology development has focused on treatment technologies that can treat produced water so that it is appropriate for reuse. These options include reuse in oil and gas operations, agricultural, biofuel or industrial operations. Lower dissolved solids may be feasible for treatment beyond the oil and gas industry. Higher dissolved solids should only be reused where the salt content can be kept in solution to avoid the intense energy input to remove the salts. Operators have successfully demonstrated this ability by using conventional treatment processes on produced water with high TDS concentrations rather than control the TDS levels by blending the water in hydraulic fracturing operations.

The feasibility of depending on high TDS levels produced waters for potential agricultural water supply does not make sense from an energy, economic, or environmental perspective due to the availability of alternative lower quality water resources that could be treated to acceptable standards with significantly less energy input. This includes municipal wastewater, brackish groundwater, and even seawater when logistically feasible. Therefore, environmental and economic benefits may directly correlate when evaluating reuse versus disposal. Ultimately, preparing a reuse feasibility study before freshwater deficits or disposal concerns arise will give the industry an advantage when it is necessary to react to change.

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