Produced water is an inevitable by product of oil and gas production, and includes formation water, condensation water, and re-produced injection water. It may also include water used for desalting.
In a mature field, a water cut level of 95% or more is not uncommon. Worldwide, on average it is estimated that for every barrel of oil produced, there are roughly 4 – 5 bbl of water co-produced.
In the US, a review study found that some 21 billion bbl of water was produced in oil and gas production in 2012, which is equivalent to some 57.5 million bpd. The average water to oil ratio in the US in 2012 was 9.2 bbl of water per 1 bbl of oil. In Oman, data from the Petroleum Development of Oman (PDO) shows that the water to oil ratio is approximately 10 bbl of water per 1 bbl of oil. In the North Sea, official figures from the Oslo-Paris (OSPAR) Commission confirm that on average some 7.1 million bbl of produced water was generated every day in 2014. The water to oil ratio in the North Sea in 2014 was approximately 2.3 bbl of water per 1 bbl of oil. Often water production becomes a limiting factor for continuing oil production economically for a specific field.
Discharge of produced water is strictly regulated, although discharge standards may vary from location to location. The quality of produced water is most widely expressed in terms of its oil content, which acts as a ‘surrogate’ for other pollutants. Table 1 shows examples of the discharge limits currently set around the world.
Table 1. Examples of current discharge limits
As a result, the measurement of the concentration of oil in produced water has become an important subject, not only for regulatory compliance monitoring but also for operations and data collection for the development of future government regulations and corporate environmental policy. To calculate the total amount of oil that is discharged in produced water, the volume of produced water also needs to be measured accurately.
As a National Measurement Institute (NMI), over the past two decades TUV SUD NEL has supported many produced water related activities, aimed at improving produced water measurement and management practices. Ultimately, these activities help minimise the environmental impact of produced water discharge.
Potential harm to the marine environment
Produced water contains a wide range of contaminants. These include:
- Hydrocarbons: aliphatic and aromatic hydrocarbons including polycyclic aromatic hydrocarbons (PAHs). They are present in dispersed and dissolved forms in produced water.
- Soluble organics: naphthenic acids, carboxylic acids and phenols. They may be present in both dispersed and dissolved forms, depending on the pH of the water.
- Total dissolved solids (TDS).
- Total suspended solids (TSS).
- Production chemicals.
- Heavy metals.
- Radioactive materials.
- Dissolved gas, e.g. H2S, CO2.
With the contaminants listed above, it is not difficult to see that any discharge of produced water could potentially cause harm to the marine environment.
For the offshore industry, approximately 75% of the produced water is treated and then discharged into the ocean, with the rest being re-injected. Whilst onshore, roughly 90% of this water is re-injected, with the rest being treated and re-used or discharged. With the rapid development of shale oil and gas around the world, and the demand for water for hydraulic fracturing, there is an increasing emphasis on flowback and produced water re-use.
Getting it right
Management of produced water varies with operators, geographical locations, and field history. Options include using water shut-off technologies inside the reservoir, to prevent water getting into the wellbore and downhole, and subsea separation, to stop water being brought to the surface. If water is brought to the surface, then it must be treated and either discharged or re-injected.
Good produced water management practice should involve different disciplines, including reservoir engineers, production engineers, production chemists, and environmental engineers. It also requires an integrated approach, taking into consideration the different options mentioned above, as well as production history and profile.
The key to produced water management is to prevent water getting into the wellbore, and to minimise the water being brought to the surface. Failing this, produced water must then be treated, re-injected and/or discharged or reused.
Industry takes steps
To help industry tackle produced water management and oil-in-water measurement challenges and issues the Produced Water Club (PWC) was established in the early 2000s, welcoming more than 100 organisations from around the world including government bodies, oil and gas operators, technology and service providers and academic institutions. The PWC is of benefit to both individual member organisations and the industry as a whole, by facilitating business developments, knowledge sharing, formulation of new research ideas, and the promotion of industry-wide best practices. Its core aims are to:
- Keep up to date with legislative and technology developments.
- Provide a central contact point on the subjects of produced water management and oil-in-water measurement.
- Promote best practices aimed at reducing oil pollution.
- Bridge the various industry stakeholders, including regulators, operators, technology providers and R&D organisations.
In an open industry forum, club members discuss measurement and management challenges and technology developments, sharing knowledge and current practice. From these meetings, research projects including joint industry projects (JIPs) have been formulated to tackle vital industry challenges identified by the group.
This is part one of a two-part article. Part two will be available shortly.
Written by Dr. Ming Yang, TUV SUD NEL.
Read the article online at: https://www.oilfieldtechnology.com/special-reports/16072019/rise-to-the-challenge--part-one/