Monitoring oil-in-water in offshore production
PRODUCED WATER REPORT
Monitoring oil-in-water in offshore production
Statoil’s use of an online oil-in-water monitor provides increased control over produced water quality.
Geir Aanensen, Roxar Flow Measurement; and Bjørn Tykkhelle, Statoil
In 2006, Statoil tested an ultrasonic, in-line oil-in-water monitor on Sleipner A platform. The company found that the online monitor minimizes errors that can result from traditional manual sampling, and it also provides early warning signs of potential hazards, thus allowing preemptive measures to correct possible problems.
The monitor’s real-time information on oil concentration can be used for more proactive operation and control, thus enabling better regulation of the produced water facility.
GLOBAL WATER PRODUCTION
The last few years have seen a dramatic increase in global water production. Globally, the average watercut is 75%, a 5% increase on watercuts 10 years ago.
Watercut is also increasing on the Norwegian Continental Shelf (NCS), where water-to-oil ratios have increased substantially. According to the Norwegian Petroleum Directorate (NPD), annual oil discharges to the sea total over 3,000 tons, with a water-to-oil ratio that increased to 1.2 in 2006 from 0.93 in 2004.
This ratio is expected to increase even further, despite the fact that the amount of water discharged into the sea in 2006 was lower than in 2005, due mainly to the reduction in water volume from the giant Statfjord field.
NCS’s Draugen oil field is a typical example of an older field producing more water. Discovered in 1984, its oil production peaked in 1999 at an average of 209,000 bpd, with no water production, according to the NPD. In 2005, production had dropped to an average of 104,000 bopd and water production had increased to 103,000 bpd.
In terms of oil-in-water concentration, the average concentration of dispersed oil from the NCS dropped to16.7 mg/L in 2006, from 19.5 mg/L in 2005. Figure 1 illustrates the rise in water production on the NCS and the last year’s decrease in average dispersed oil concentration.
As of 2007, with new environmental regulations coming into effect, it will become increasingly difficult to compare historical data, since a new analysis method for manual samples covers a different range of hydrocarbons than previous ones.
GREATER DETAIL NEEDED
New regulations and greater optimization of assets have driven operators to obtain detailed information on the size and amount of sand and oil in produced water. There are a number of reasons for this.
First, better oil-in-water monitoring can improve the overall efficiency of the water treatment facility, and operators might be able to increase revenue.
Potential obstacles during the production phase can also be tackled through greater monitoring of produced water. Examples include solid particles and suspended oil droplets plugging disposal wells, inorganic scales plugging lines, pumps and valves, and corrosion due to electrochemical reactions of water with piping walls. By carefully monitoring produced water and overcoming these challenges, operators can optimize production.
Greater detail on the specific components of produced water-sand or oil as well as size distributions and concentration-will help operators optimize the separation process, ensuring that all separation equipment is designed and working within its operating range.
Second, with the growing number of older fields, effective oil-in-water monitoring becomes even more important. According to Pipeline Magazine, more than 70% of the world’s oil and gas production comes from fields that are over 30 years old-fields that, like Draugen field, may have started off producing no water but are producing large volumes today.
The growth in brownfields has led to subsequent growth in enhanced oil recovery techniques, such as water reinjection to ensure pressures are sustained.
It is essential that oil and solid particles in produced water reinjection (PWRI) are detected to ensure higher recovery rates and longer lifetimes for existing oil fields. If not, surface sludge formation and oil saturation can cause significant problems. Effective monitoring and control over the reinjection process will optimize waterflooding of the reservoir and ensure maximum production performance.
Information on sand and oil size distributions and concentration will also minimize effects such as plugging and any decline in formation permeability, which can reduce reservoir pressure and injectivity in waterflooding operations.
Finally, there are the environmental implications of oil discharge. Currently, oil in produced water accounts for about 90% of the oil discharged into the North Sea by the oil and gas industry,* and a number of environmental regulations have emerged to ensure the accurate measurement of oil in produced water. Regulations include the 2000/2001 Oslo/Paris Convention (OSPAR), which requires that “no individual offshore installation should exceed dispersed oil of 30 mg/L for produced water discharged into the sea”; the Norwegian State Pollution Control Authority regulations, which call for zero harmful discharge into the sea; and in the UK, The Oil Pollution Prevention and Control (OPPC) Regulations 2005. It is, therefore, important that E&P operators demonstrate to regulators and governments effective oil-in-water monitoring.
IMPROVED MONITORING TECHNOLOGIES
Traditional oil-in-water monitoring consisted of manual sampling-taking at least 16 one-liter samples each month (according to OSPAR requirements) from the produced water discharge, acidifying to a low pH and then extracting with certain chemicals.
Once the solvent is extracted, infrared quantification would then take place with oil content determined by the infrared absorbance of the sample extract and the total methylene present.
The downside to this approach is that they are spot samples and, as the concentration of the oil in water often varies over time, operators are not getting the full picture. Figure 2 provides a good illustration of the dangers of manual sampling where the online and manual statistics are substantially different.
Online, in-line monitoring and its ability to provide direct measurements at the dispersed and suspended phase provide more accurate and detailed information on the size distribution and concentration of oil and sand.
In being an in-line monitor with no need for sidestreams or sample extractions, the monitor acts as a flow instrument providing direct measurements at the dispersed and suspended phase.
Monitoring in real time provides a highly effective early-warning system as opposed to manual sampling. With online monitoring, if something happens, such as the identification of a process upset, the operator knows about it and can react accordingly, thus reducing oil pollution.
Against this backdrop, Roxar and TNO Science and Industry developed an online, in-line oil-in-water monitor for oil and gas applications.
The monitor is based on an ultrasonic measurement principle. Through the insertion of an ultrasonic transducer directly into the produced water flow, ultrasonic technology takes individual acoustic pulse-echo measurements from solids, oil droplets and gas.
Each detected echo is analyzed and classified as coming from an oil droplet, a sand particle or a gas bubble. Concentration levels can be calculated based on size distribution.
The monitor is optimized for concentrations of 0-1,000 ppm, and by separating and analyzing individual acoustic pulse-echo measurements, it can provide complete size distributions from 2-3 µm and upward. Calculations can be made simultaneously for oil and sand.
MONITORING SLEIPNER A FIELD
The Roxar Oil-in-water monitor was installed at the Sleipner A platform in May 2006. Sleipner A is a fixed platform located in the North Sea in Block 15/9, about 150 mi (240 km) west of Stavanger, Norway.
The installation produces gas/condensate, and the expected concentration and oil droplet size range is relatively low.
Statoil is using the oil-in-water monitor to measure overboard water discharge and ensure that it meets environmental requirements. The monitor also acts as an early warning detection system in the water treatment facility and plays a vital role in helping Statoil efficiently monitor the separation process.
The monitor is remotely accessed, and consequently, important information can be distributed to both onshore and offshore personnel. For example, it is very useful for key personnel onshore to monitor the reflector amplitude, since it contains important information concerning equipment degradation, scaling, temperature and salinity changes. Furthermore, the maintenance program can be optimized based on this information, helping Statoil minimize other related costs.
Registering oil concentration. Immediately after installation, the monitor was interfaced with the onboard Plant Information and Process Control and Data Acquisition systems to evaluate trends and variations found in separators, degassing vessels, flowrate and, of course, measured oil concentration.
The water level of the degassing vessel was monitored to determine the need for, and frequency of, skimming operations. In addition, it was verified that the increase in water levels in various separator tanks was related to specific pumps being turned on, and that the on/off frequency of these pumps could be seen in the measured concentration levels.
After having the monitor in continuous operation for a period of time, measurements from September 2006 were extracted to verify that fluctuations in the measurements were related to the separation and water treatment process. This was carried out to establish confidence in the monitor with the staff involved.
A period with a distinctly higher concentration level is shown in Fig. 3, where concentration measurements are plotted against water level in the test separator. The increase and following rapid decrease in water level in the separator corresponds to an increase in the oil-in-water concentration downstream. This behavior can be seen constantly, and always with an increase in oil concentration as a result.
Reliability and robustness. One of the biggest issues with online oil-in-water monitoring has been reliability and robustness in produced water environments. Long term stability, even during upset conditions, is a prerequisite for a range of measurement devices used in the oil and gas industry. For oil-in-water monitoring, this has not always been straightforward to achieve.
On Sleipner A, the online measurements have been analyzed during periods of production instability. The use of an ultrasonic measurement principle allows for better robustness in produced water environments, simply because sound has the ability to ‘sound penetrate’ oil and grease settling on the insertion probe. In addition, the reflector design allows for continuous reference measurements that can be used to compensate for common changes in the medium, for example salinity, temperature and fouling.
To demonstrate that the monitor is also able to work properly during unstable production conditions, a three week period from Oct. 12, 2006, to Nov. 14, 2006, was selected for further investigation. Statoil confirmed that upset conditions were experienced twice during that period.
In order to evaluate if the changes seen in concentration readings were real, online measurements were compared to IR measurements (manual samples) for the same period. In Fig. 4, there is a sound relationship between online oil concentration measurements and IR results, the increase and decrease in oil concentration being found in both signals. The values presented are average values for each 24 hour period.
This simple analysis demonstrates how the monitor was able to perform accurate measurements even during and after production instability, and that the compensation feature allowed for reliable measurements over longer periods of time.
This is due to the combination of ultrasonic energy, to maintain measurements and reflector design, to compensate for reduction in reflector amplitude. During this period, Statoil could monitor the unit on a day-to-day basis, using remote access, in order to track how the unit performed when subject to changing conditions.
Monitoring and manual samples. The example above also highlights another well-known challenge when comparing manual laboratory samples with online measurements: Single spot measurements will almost certainly differ from online results.
Whether talking about online monitors or manual samples, it is a clear prerequisite that measured results are representative of actual oil concentration. If the online monitor does not ‘see’ a representative water sample, the measured result may be correct for that sample, but it will not represent true oil-in-water concentration.
The same goes for manual samples. Experiences on Sleipner A have shown that there are a range of issues with manual sampling that-even when extraordinary care is taken-are difficult to remove altogether. Oil settling and accumulating in piping, on nozzles, valves, etc., can be observed, but the effect on measured oil concentration is impossible to quantify.
In addition, the position of the manual sample system compared to an online system is crucial, especially in highly dynamic processes. The actual procedure by which samples are analyzed is also important. Some operators have reported that samples taken at the same time will give different results when analyzed at different labs, but using the same accredited analysis method.
With a true in-line monitor, these effects are minimized. Since there are no sample extraction mechanisms, pumps, or homogenization chambers of any kind, the produced water flows through the monitor without being influenced by the measurement equipment itself. However, with only a small fraction of the total water volume actually being measured, it is important that the flow is well mixed to avoid bias in the measurements.
Using an ultrasonic oil-in-water monitor from Roxar, Statoil tested processed and produced water discharge on the Sleipner A platform.
Results and experience from Sleipner A indicate that the monitor is reliable even during production instability. The monitor’s remote access allows for efficient diagnostics and operation and a well planned and cost effective service and maintenance program. Trends found in the online measurements show a sound relationship with manual samples.
For Statoil, the introduction of real-time information on oil concentration enables better control of the produced water facility, since the information can be used for more proactive operation and control, particularly in such a dynamic process.
The monitor is now installed on a permanent basis.
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