Produced Water Treatment

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escveritas
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Produced Water Treatment

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Article courtesy of OUP, link as follow: HERE

Produced water is a complex mixture of organic and inorganic compounds and the largest volume of by-product generated during oil and gas recovery operations. The potential of oilfield produced water to be a source of fresh water for water-stressed oil-producing countries and the increasing environmental concerns in addition to stringent legislations on produced water discharge into the environment have made produced water management a significant part of the oil and gas business.

This article reviews current technologies for the management of produced water, examines how electrochemical techniques may be used in these areas and compares the prospects for future development. It suggests that treatment technologies based on electrochemistry could be the future of produced water management, since produced water is a potential electrolyte because it has a relatively good conductivity. It also explains that by applying photoelectrochemistry, water electrolysis, fuel cell and electrodeposition, electrochemical engineering could achieve energy storage, production of clean water and recovery of valuable metals from produced water with minimal or no negative impact on the environment.

Introduction

Petroleum is a major source of energy and revenue for many countries today, and its production has been described as one of the most important industrial activities in the twenty-first century [1]. Since late 1850s when Edwin Drake drilled the first oil well, demand for petroleum has continued to rise. It is estimated that world daily petroleum consumption would increase from 85 million barrels in 2006 to 106.6 million barrels by 2030 [2]. Despite its significance, petroleum is produced with large volumes of waste, with wastewater accounting for more than 80% of liquid waste [3] and as high as 95% in ageing oilfields [4]. Generally, the oil/water volume ratio is 1:3 [5].

Produced water has a complex composition, but its constituents can be broadly classified into organic and inorganic compounds [6], including dissolved and dispersed oils, grease, heavy metals, radionuclides, treating chemicals, formation solids, salts, dissolved gases, scale products, waxes, microorganisms and dissolved oxygen [5–8]. Globally, ∼250 million barrels of water are produced daily from both oil and gas fields, and more than 40% of this is discharged into the environment. Currently, oil and gas operators treat produced water via one or more of the following options [9]: Environmental concerns and the prospect of beneficial uses have driven research into the treatment of produced water. Current conventional treatment technologies are targeted at removal of heavy metals, oil and grease, suspended solids and desalination, which often lead to the generation of large volumes of secondary waste. For instance, heavy metals are removed as sludge using current treatment technologies [10]. This article reviews current produced water treatment technologies and examines the ability of electrochemically driven technology to store energy, produce clean water and recover valuable materials from produced water with minimal negative impact on the environment.
  • Avoid production of water: water fractures are blocked by polymer gel or downhole water separators, but this option is not always possible.
  • Inject into formations: produced water may be injected back to its formation or into other formations. This option often requires transportation of water, and treatment to reduce fouling and bacterial growth. In the long term, the stored produced water may pollute the underground waters.
  • Discharge to the environment: produced water may be discharged to the environment as long as it meets onshore and offshore discharge regulations.
  • Reuse in petroleum industry operations: minimally treated produced water may be used for drilling and workover operations within the petroleum industry.
  • Apply in beneficial uses: produced water may be consumed for irrigation, wildlife consumption and habitat, industrial water and even drinking water. However, beneficial uses of produced water may involve significant treatment [5, 9].
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 (Figure 1). 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 [11, 12]. It is a mixture of injected water, formation water, hydrocarbons and treating chemicals [13], 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 [5]. The rate of oilfield produced water production is expected to increase as oilfield ages (Figure 2). Other factors have been reported to affect the quantity of produced water generated in an oilfield [11].

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 [14]. 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.
escveritas
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Re: Produced Water Treatment

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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 [6, 10, 15]. 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 [16]. Dissolved and dispersed oil content in produced water is dangerous to the environment and their concentration can be very high at some oil fields [6, 16–18]. The quantity of oil present in produced water is governed by a number of complex but interrelated factors [6, 14, 17].

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 [19]. 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 [5, 20]. Heavy metal concentrations in produced water are usually higher than those of receiving water (for enhanced oil recovery) and those found in sea water [19].

226Ra and 228Ra are the most abundant naturally occurring radioactive elements present in oilfield produced water [20]. 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 [21]. In some oilfields, up to 21 Bq/l of 228Ra have been detected in produced water samples [5]. Table 1 lists typical composition and properties of oilfield produced water [5].

3. Production chemicals

Production chemicals (Table 2) 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 [14]. These chemicals enter produced water in traces and sometimes significant amounts [18] 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 [18].

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 [5]. 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 [22].

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.
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