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].
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.