Editor’s Note: The oil and gas in-dustry is under public scrutiny like never before on a host of health, safety, and environmental issues. These concerns are already affecting how companies operate and interact with the public. This series is intended to shed light on how the industry is actively confronting these challenges or how it should address them going forward.
Produced water is an inevitable by-product of oil and gas production. It is estimated that for every barrel of oil produced worldwide there are roughly four to five barrels of water co-produced. As it contains a wide range of contaminants, the discharge of produced water could potentially cause harm to the environment, and therefore should be monitored closely.
For the offshore oil and gas industry, approximately 75% of the produced water is treated and then discharged into the ocean, with the rest being re-injected. While onshore, roughly 90% of this water is reinjected, with the rest being treated and reused or discharged. With the rapid development of shale oil and gas, and the demand for water for hydraulic fracturing, there is an increasing emphasis on flowback and produced water reuse.
The measurement of oil in produced water is vitally important, for both regulatory compliance monitoring and data collection for the development of future government regulations and operators’ corporate environmental policy. Of course, to calculate the total amount of oil that is discharged via produced water, the volume of that water must also be measured accurately. Measurement of the oil in produced water is also important for the control of the water treatment process itself.
Management of produced water varies with operators, geographical locations, and field history. Good produced water management practice should involve different disciplines, including reservoir engineers, production engineers, production chemists, and environmental engineers. It would also require an integrated approach, taking into consideration the different options, as well as production history and profile. Often water production becomes a limiting factor for continuing oil production economically for a specific field. How to best manage produced water in oil and gas production will therefore have a significant impact economically, socially, and environmentally.
The key to produced water management is to prevent water getting into the wellbore, and to minimize the water being brought to the surface. Failing this, produced water must then be treated, reinjected, and/or discharged or reused.
Using Online Oil-in-Water Monitors
The use of online oil-in-water monitors can play an important role in the management of produced water. There are many benefits in using online continuous oil-in-water monitors, but particularly as they provide continuous information on a minute-by-minute basis, if not more frequently. Thus, not only can one spot process upset conditions quickly and take actions to rectify the situation, but such monitors can also be used for process optimization, such as chemical dosing.
The use of online monitors will also significantly reduce the number of samples taken for laboratory analyses, and therefore reduce the use of solvents normally required for laboratory oil-in-water analyses. Furthermore, deployment of online monitors can potentially lead to more accurate oil-in-produced-water discharge data, when compared to taking manual samples and analyzing them daily.
Preliminary studies have also indicated that there is a large uncertainty associated with oil-in-water data obtained using existing laboratory-based methods, whether gravimetric or infrared absorption-based, or gas chromatography and flame ionization detection based. This uncertainty may be as high as ±50% (at 95% confidence level). The use of online oil-in-water monitors, particularly those installed inline, could potentially reduce the uncertainties associated with the current sampling and analysis practice.
Technology Options
There are a significant number of technologies available on the market for online continuous oil-in-water measurements. UV fluorescence-based technologies are probably the most commonly used. Laser-induced fluorescence (LIF) technology has been gaining acceptance and market share. This is due to the availability of LIF-based probes, which can be inserted directly into the process pipeline. Additionally, LIF-based monitors from the leading suppliers are equipped with ultrasonic cleaning capability, which helps mitigate fouling on the sensor optical windows.
It is worth noting, however, that all fluorescence-based monitors are affected by oil droplet size and the ratio of aromatic to total hydrocarbons in the produced water. Microscopy image analysis-based monitors offer the ad-vantage of providing both concentration and size of oil droplets and solid particles and are therefore popular for produced water re-injection operations. Since it is possible to view the particles/droplets present during measurement, the images produced in microscopy image analysis are being increasingly used for produced water treatment process optimization. Light scattering is a quick and robust process, and is well used in the shipping industry and is currently finding its way back for produced water applications.
However, industrywide acceptance of using online oil-in-water monitors faces several challenges. Historically, online continuous oil-in-water monitors are perceived to be unreliable and have poor performance. When it comes to reliability, like all online measurement devices, oil-in-water monitors require a good calibration and scheduled regular maintenance to ensure high-quality performance. These in-struments are not a type that one can fit and forget. However, one of the key issues related to reliable operation of these devices is fouling of the optical window. Consequently, new technologies have been developed to mitigate this problem, e.g., ultrasonic cleaning being incorporated into LIF devices, or a hydrodynamic mechanism being built into a light-scattering sensor. Some of the instruments also use a high-pressure jetting mechanism to mitigate fouling.
Evidently, there is a large uncertainty associated with oil-in-water figures obtained by sampling and laboratory analyses. It is quite possible that the perception of poor performance of online oil-in-water monitors might be linked to the way with which we assess their performance, by comparing results from the online monitors to those from using sampling and laboratory methods. With a relatively large uncertainty related to laboratory results, it is entirely possible that the performance of the online monitors might therefore have been wrongly judged in the past.
Online oil-in-water monitors have been predominantly used for process trending and optimization purposes for surface-manned installations, but there is now a strong desire from the oil and gas industry to utilize them for produced water discharge reporting purposes. To achieve this, there is a need to develop new guidelines and/or new approaches to be confident in their use for this application.
For unmanned and subsea applications, without a reliable and accurate online oil-in-water measurement instrument, discharge and/or reinjection of produced water from these installations would be extremely difficult, if not impossible.
The Future of Produced Water Measurement
Produced water is an inevitable by-product of oil and gas production, but its discharge could potentially harm the environment. Management of produced water is therefore critically important for the safe and economical production of oil and gas, which must involve multiple disciplines and use an integrated or holistic approach. With increasing water production from mature fields and the move toward deep water, chemical enhanced oil recovery, heavy oil, and unconventional oil and gas production, the oil and gas industry is facing a significant number of challenges to minimize the potential harm that can be caused by the discharge of produced water.
The uses of online oil-in-water monitors can play a significant role in production process control and optimization, injection water quality monitoring, and discharge reporting, and we will see their increased use for the management of produced water in the future.
| Ming Yang is principal consultant at TÜV SÜD National Engineering Laboratory. Since joining TÜV SÜD in 1998, he has participated in numerous conferences related to produced water, oil-in-water measurement, and multiphase separation. He has also initiated and led several joint industry projects, and has presented and chaired produced water-related events. He was one of two authors who originally drafted the UK guidance notes on sampling and analysis of produced water and other hydrocarbon discharges. Previously, he worked at Heriot-Watt University, where he was involved in research projects related to produced-water characterization and reinjection. He also conducted research projects related to production chemicals and multiphase separation at the University of Manchester. He received a BS degree in chemical engineering from South China University of Technology and an MSc degree and PhD in chemical engineering from the University of Manchester. |