Global water production is on the increase — so much so that today we are producing more water than oil.

Whereas current oil production is 80 million b/d approximately, current estimates of global water production are 250 million b/d — a three-to-one ratio.

The increase in produced water is being seen on the Norwegian Continental Shelf where water/oil ratios have increased from 0.93 in 2005 to 1.13 in 2005 and annual emissions of oil into the sea are estimated at 21,990 bbl (3,000 tons) of oil, according to the Oljedirektoratet in Norway.

In the illustration, the y-axis is standard cubic meters in millions with the graph showing the current and estimated amount of produced water being discharged into the sea. With the current average oil-in-water content for all the installations on the Norwegian Continental Shelf, this translates into approximately 2,990 bbl (3,000 tons) of oil.

A closer look
The increase in produced water, whether it is re-injection, discharged or processed water,

Water/oil ratios have increased from 0.93 in 2005 to 1.13 in 2005 on the Norwegian Continental Shelf. (Graphic courtesy of Roxar)
has also led to growing need from the operator for a better monitoring of produced water.
With an increased focus on optimizing reservoir performance and on technologies which will turn marginal fields into economically feasible fields, the growth in produced water from the more mature fields, and the growing number of environmental requirements on water discharge, oil-in-water monitoring technologies are coming to the fore.

Optimizing production
There are a number of means by which increased oil-in-water monitoring can optimize offshore production.

There is the increase in revenue by separating the oil from the produced water. According to energy industry analysts Douglas-Westwood, 2.1 million bbl of oil are lost every day due to oil being lost through produced water discharge.

There are also other potential problems during the production phase that can be alleviated through produced water monitoring.

This can include the plugging of disposal wells by solid particles and suspended oil droplets; the plugging of lines, pumps and valves due to inorganic scales; and corrosion due to the electrochemical reactions of the water with piping walls. Careful monitoring and quick preventative action can save millions of dollars.

Finally, greater detail on the specific components of produced water — sand or oil as well as size distributions and concentration — will help the operator optimize the separation process and ensure that all separation equipment is designed to and working within its operating range with respect to particle size.

Seperators, filters, hydro-cyclones and chemical injection can all be accurately monitored to ensure that separation processes are performing optimally.

Produced water reinjection
The growth in brownfields — today more than 70% of the world’s oil and gas production comes from fields that are over 30 years old, according to the World Energy Organization — has inevitably led to a growth in enhanced oil recovery techniques such as water reinjection to ensure pressures are sustained. In these cases, water is injected into many oil fields to improve production, and often water from an oil- or gas- bearing formation flows to the surface during production.

It is essential that all oil and solid particles in the produced water re-injection (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.

Information on sand and oil size distributions and concentration will also minimize effects such as plugging and decline in formation permeability that can reduce reservoir pressure and injectivity in water flooding operations.

The environmental challenge
Another key market driver in the development of reliable and accurate oil-in-water monitoring is the tightening requirements on produced water discharge.

Today, oil in produced water accounts for about 90% of the total amount of oil discharged into the North Sea by the oil and gas industry, according to Statoil, and a number of
Statoil will use an oil-in-water monitor to measure oil discharge from its Sleipner complex of fields. (Photo courtesy of Statoil)
environmental regulations have emerged over the last few years to ensure the accurate measurement of oil in produced water. Leading this is the 2000/2001 Oslo/Paris Convention (OSPAR) — also known as the Convention for the Protection of the Marine Environment of the Northeast Atlantic.

OSPAR covers all the oil-producing coastal states of Western Europe with its goal being to “…prevent and eliminate pollution by oil and other substances caused by discharges of produced water into the sea.” The key requirement is that “no individual offshore installation should exceed a performance standard for dispersed oil of 30 mg/l for produced water discharged into the sea.”

OSPAR means that operators must now demonstrate to regulators and government the effective monitoring of oil in water. As well as avoiding any financial penalties, accurate monitoring can also open up opportunities for participating in emission trading schemes.

Monitoring
So, with a clear demand from operators, are today’s oil-in-water monitoring technologies rising to the challenge in offshore production?

If this question had been posed a few years ago, the answer would have been “No.” That was when manual sampling was the predominant tool for oil in water monitoring.
Manual sampling consists of taking at least 16 one-liter samples each month (according to OSPAR requirements for installations that discharge more than two tonnes of dispersed oil per year) from the produced water discharge, acidifying to a low PH and then extracting with certain chemicals.

Once the solvent was extracted, infrared quantification would then take place with oil content determined by the infrared absorbance of the sample extract and the total methylene (CH2) that is present (as defined in the OSPAR Agreement 1997-16).
Manual samples, however, have a key weakness. As they are spot samples and as the concentration of the oil in water often vary over time, operators are not getting the full, accurate picture.

Furthermore, the labor-intensive nature of the process and the negative impact on staff productivity make it unpopular with operators.

Online, inline
Online, inline, real-time monitoring of oil in water, however, meets many of the requirements of today’s operator.

First, as an inline 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.

The benefits for operators are more detailed information on the size distribution and concentration of oil and sand in water. The Roxar oil-in-water monitor, for example, based on a patented solution with TNO Science and Industry, caters to concentrations of about 1,000 parts per million (ppm).

And by separating and analyzing individual acoustic pulse-echo measurements, the monitor can provide complete size distributions ranging from the extremely low 2 to 3 micrometers.
The fact that the monitoring is able to take place in real time also provides a highly effective early warning system. When the water sample analysis comes back from the laboratory showing that something is wrong, the damage may already be done. With online monitoring, if something happens such as the identification of a process upset, the operator knows about it and can react accordingly (as a result reducing oil pollution).

Real-time monitoring also optimizes the entire ongoing separation process. With any deviation, one can quickly step in so that production can continue and be optimized. Separators, hydrocyclones, and the type and regularity of chemical injection can all be run accordingly.

Ultrasonic pulse echo technology
Yet if online monitoring offers such clear benefits over manual analysis, why isn’t it more prevalent?

Previous obstacles to online monitoring have included doubts as to its ability to effectively characterize complex water mixtures through to concerns about the accuracy, maintenance and calibration and its robustness in harsh environments.

Today’s technologies and in particular ultrasonic pulse echo technology, however, are overcoming these concerns.

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. These are then separated and analyzed to provide accurate information to the operator on size distribution and concentration for oil and sand, as already mentioned.
An added benefit is that the technology can “sound-penetrate” material. If there is an issue of oil film or scaling, the ultrasonic technology can work just as effectively and accurately because the ultrasonic energy will penetrate the layer and still transmit a signal into the produced water flow. This is not the case with the majority of today’s oil-in-water monitors, which are reliant on optical technology.

With an increasing focus on oil-in-water monitoring at higher pressures, there is also a need for oil-in-water monitors to take calculations simultaneously.

Through the ultrasonic technology, simultaneous calculations can be made using the generalized scattering model where scattering curves for oil and sand, respectively, are implemented in the model and, using feature extraction and classification of echoes, the correct forward model is used for each individual response.

The net result is increased accuracy and a positive impact on both optimizing production and meeting environmental requirements.

Increased robustness
Yet can such online monitoring technologies operate in the harshest of offshore production conditions? The answer is yes, with the ultrasonic technology providing the necessary robustness.

By using advanced auto diagnostics functionality the monitor is also able to detect and overcome challenges such as equipment degradation, scaling and temperature or salinity changes.

Changing the way oil-in-water is monitored
Today’s technologies are changing the way oil in water is being monitored, providing the operator with greater detail and accuracy in water characterization information as well as greater reliability and robustness. With the need to optimize production, meet environmental requirements and maximize returns from brownfields, the timing couldn’t have been better.
Case study - Statoil
An oil-in-water monitor was installed at Statoil’s Sleipner A platform on May 16, 2006. Sleipner A is a fixed platform located in the North Sea in block 15/9, approximately 150 miles (240 km) west of Stavanger, Norway, and serving the Sleipner East, Sleipner West and Sigyn gas and condensate fields. The installation produces gas/condensate from different wells, and the concentration and oil droplet size range is expected to be relatively low.

Statoil will use the oil-in-water monitor to measure overboard water discharge from the platform and ensure that it meets environmental requirements for limited or zero oil emissions into seawater. The monitor will also act as an early warning detection system in the water treatment facility and will play a vital role in helping Statoil efficiently monitor the separation process.

Since installation, several tests have been completed to demonstrate the performance of the monitor. Data analyses clearly show that both in terms of accuracy and sensitivity, the monitor performs according to specifications. There is, as expected, a clear correlation between measured oil-in-water concentration and changes in the water level in the gravity separators.

The Statoil pilot to date has confirmed the oil-in-water monitor’s ability to provide accurate information to Statoil on the size distribution and concentration of oil and is already playing a key role in monitoring Statoil’s overboard discharge and separation process.