Accurate measurement of oil in water is a critical element of oil and gas operations today because of the increase in produced water as a by-product of production.
With 70% of the world's production coming from mature fields, and with a renewed focus on enhanced recovery techniques such as Produced Water Re-Injection (PWRI) and water flooding, the oil and gas industry today is producing more water than oil.
The total discharge of produced water on the Norwegian continental shelf during 2011, for example, was 131 MMcm (4.6 tcf). The Norwegian Petroleum Directorate predicts this number will increase until 2015. Worldwide in 2010, produced water rates exceeded hydrocarbon globally by as much as 50%. On the UK continental shelf, this figure exceeded 130% between 2011 and 2012.
The economic case
From an economic perspective, accurate information on the size and amount of sand and oil in produced water – whether it is reinjection, discharged, or processed water – means more optimal operations, particularly during water and oil separation and treatment.
The greater the details on specific components, concentrations, and size distributions, the more likely there will be optimum production using enhanced design and separators and filters throughout the separation process.
Accurate information and control of water/oil content also improves oil recovery and oil quality as well as ensuring reliable oil production estimates and the long-term economic future of the field. This may negate revenues lost by produced water discharge.
There remains a danger to production if produced water is not carefully monitored, and not only just during the separation phase but also throughout production. Typical problems can include the plugging of disposal wells by solid particles and suspended oil droplets, and the plugging of lines, pumps, and valves due to inorganic scales.
Information on sand and oil size distributions and concentrations can help minimize plugging and a decline in formation permeability that can negatively impact field economics, reduce reservoir pressure and injectivity in water flooding operations, and negate PWRI's effectiveness as a secondary recovery technique.
The environmental case
The environmental case for oil in water monitoring is also strong. A number of regulators and bodies promulgate oil discharge limits. These include the OSPAR (Oslo/Paris) Convention, the main legal instrument overseeing international cooperation on the protection of the marine environment of the northeast Atlantic, to federal regulations in the Gulf of Mexico, the Norwegian State Pollution Control Authority (SFT), and the US Environmental Protection Agency (EPA).
OSPAR has set a target of "zero harmful discharge" from produced water by 2020. The current OSPAR limit is 30 mg of dispersed oil per liter (≈30 ppm) of produced water. In the United States, the EPA says produced water discharges must not exceed an oil daily maximum limitation of 35 mg/l of produced water (40 CFR 435.52(b)).
In addition, OSPAR recently set out a new risk-based approach for operators as a way to integrate the assessments of all substances in produced water and to identify those posing the greatest risk.
According to OSPAR, "where unacceptable environmental risks have been identified, contracting parties should review management options, evaluate measures, and develop and implement site-specific actions to reduce the risks to an acceptable level." This will make oil in water monitoring more important.
From international regulations to a company's reputation and the importance of protecting the marine environment, the environmental argument for oil in water monitoring and for reducing discharges is every bit as strong as the economic one.
Oil in water monitoring today
Traditionally, oil in water monitoring was a manual operation, with samples taken from the produced water discharge, acidified to a low pH, and then extracted through chemicals with oil content determined by the infrared absorbance of the sample extract and the total methylene (CH2) present.
This has limitations. As well as being labor intensive, results can be inconsistent. For example, spot samples, rather than continuous samples taken over time, are "snapshots" and may not represent the full real-time picture.
There is also potential confusion as to what constitutes "dissolved" and "dispersed" oil when both are extracted by the solvent. The result is that dissolved oil is often included in the dispersed oil content, making it more difficult to meet environmental requirements.
The recent emergence of online, inline oil in water monitoring technologies negates many of these limitations.
Inline monitoring that is online can provide direct measurements of the dispersed and suspended phase, and can generate more detailed information on the size distribution and concentration of oil and sand in water. The fact that the monitoring can be done in real time provides an effective early warning system for some production threats.
Real-time monitoring also optimizes the on-going separation process. In case of deviation, steps can be taken so that production can continue. Separators, hydrocyclones, and the type and regularity of chemical injection all can benefit production.
Even these technologies have drawbacks – particularly in remote offshore environments where often there are complex mixtures of produced water plus the dangers of scaling and possible contamination.
Rise of acoustic-based monitors
To address these issues, Mirmorax has developed an oil in water monitoring solution based on an ultrasonic measurement technique to analyze individual acoustic echoes from both solids and oil droplets.
This uses advanced signal processing to provide accurate data on size distribution and concentration. The amplitude of the scattered signals from each passing particle is used to characterize the produced water.
A highly focused ultrasonic transducer is inserted directly into the produced water flow, enabling direct measurements of the suspended particles and dispersed oil phase. In the transducer focus, particles passing through the measurement volume will scatter the transducer beam and generate reflected waves or acoustic echoes. These acoustic signatures contain particle specific information. A number of measurements are taken to generate a distribution and from the distribution of these amplitudes, the particle size distribution, and particle concentration can be calculated.
Furthermore, by being "inline," the monitor can provide direct measurements at the dispersed and suspended phase with more detailed information on the size distribution and concentration of oil and sand in water. The monitor caters to concentrations of 0 – 1000 + ppm and can provide size distributions from 2 to 3 um.
Scaling and contamination
Many monitors struggle with accuracy when there is scaling. To counteract this, the acoustic monitor can penetrate material that optical-based monitoring cannot. If there is oil film or scaling, ultrasonic technology can work because the ultrasonic energy can penetrate the layer and still transmit a signal into the produced water flow.
In addition, there is a new auto calibration to compensate for scale build-up that allows for several millimeters of scale and oil to grow on the ultrasound transducer without affecting accuracy.
Other recent developments include self-cleaning mechanisms that prevent the danger of thick oil clogging the system, and enhanced salinity detection, so salinity can be compensated for in the final measurements. Salinity is a valuable measurement when determining the origin of water from multiple wells. Information about the salinity and density mix also verifies how much of separation capacity is being used for each field.
It is through developments such as these that the monitor can cover a wider measurement range and compensate for layers of scaling, possible contamination, and salinity – issues that are particularly prevalent in offshore fields.
North Sea installations
Three Mirmorax oil in water monitoring units are used on Statoil's Kristin platform, where the ultrasonic pulse echo technology provides oil ppm measurements allowing monitoring and optimizing of produced water treatment.
In another North Sea oil field, where three oil in water monitors were installed, the monitors helped reduce the overall oil content in discharge water by more than 30%.
With increased water production and discharges in the North Sea, the growth in brownfields, and increasingly strict environmental regulations, detailed information on the size and amount of sand and oil in produced water is an absolute "must" for operators today.