In oil and gas production, produced water brought to the surface is treated and then either discharged to the environment or re-injected back into a formation, either for disposal or pressure maintenance purposes. However, regardless of which option is taken, the quality of the treated produced water must be measured.
Whilst laboratory benchtop measurement methods are available, they require samples to be taken and brought into the laboratory for analysis, which is now understood to incur large uncertainties on the measured oil-in-water (OiW) results. Also, laboratory methods tend to be laborious, time consuming, and usually require the use of solvents.
In addition, with a decline in oil and gas production in mature basins, there is an increasing emphasis on maximising the recovery of the remaining oil and gas reserves. Subsea separation and produced water re-injection (PWRI) and/or discharge, alongside normally unattended installations (NUI), have therefore all become increasingly considered by operators. Online OiW monitoring is an enabler for the implementation of the above.
Online OiW monitors 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 optimisation, such as chemical dosing. Online monitors also significantly reduce the number of samples taken for laboratory analyses, and therefore reduce the usage of solvents commonly required for laboratory OiW analyses.
Furthermore, deployment of online monitors can potentially lead to more accurate oil-in-produced-water discharge data, when compared to taking two samples and analysing them daily. Preliminarily studies have shown that uncertainty associated with OiW data obtained using sampling and laboratory analyses may be as high as ±50% (at 95% confidence level).
There are a significant number of technologies available on the market for online continuous OiW measurements. UV fluorescence-based technologies are probably the most commonly used. However, Laser Induced Fluorescence (LIF) technology is gaining acceptance and market share, due to the availability of 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 often found on a sensor’s optical window. However, 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 are popular for produced water re-injection operations as they offer the advantage of providing both concentration and size of oil droplets and solid particles. These monitors also allow for operators to be able to see oil droplets and solid particles on a screen. They are therefore increasingly used for process optimisation. Light scattering is a quick and robust measurement technique and is well used for ship bilge water treatment and discharge operations in the shipping industry. Its use for oil-in-produced water measurement applications has recently re-emerged.
Historically, online continuous OiW monitors are perceived to be unreliable and have poor performance. When it comes to reliable operations, like all online measurement devices, they require a proper calibration and regular maintenance.
However, one of the key issues related to reliable operations 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.
The perception of poor performance of online OiW monitors may also be linked to the way in which we assess their performance, by comparing results from the online monitors to those from using sampling and laboratory methods. As results obtained by sampling and laboratory methods have large uncertainties, it is entirely possible that the performance of the online monitors might have been mis-judged in the past.
Online OiW monitors have been predominately used for process trending and optimisation purposes for surface installations, but there is now an increasing demand by the oil and gas industry to utilise 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 subsea applications, without a reliable and accurate online OiW measurement instrument, discharge and / or re-injection of subsea separated produced water would be extremely difficult, if not impossible.
With the advancement of online OiW monitoring technologies (including fouling mitigations) and a more pragmatic approach in accepting online devices for discharge reporting, we are likely to see the following trends:
- Increasing use of online OiW monitors for process control and optimisation.
- Increasing use of online OiW monitors for reporting the discharge of produced water from manned installations.
- Emerging use of online OiW monitors for the management of produced water from unmanned installations.
- Ongoing efforts in developing online OiW monitors for subsea applications.
- Exploring new ways in defining OiW concentration and in assessing the performance of online OiW monitors.
Read the article online at: https://www.oilfieldtechnology.com/special-reports/06052020/benefits-of-online-oil-in-water-monitors-for-produced-water-management/
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