An operator must measure water content to devise an effective flow maintenance plan.

A crucial component of flow assurance today is the need to predict and measure the water production profile in the well.

There are a number of reasons for this, by far the biggest being the potentially devastating effects unchecked water can have on flow assurance. Water, and especially saline water, can cause scaling, hydrates and corrosion in wells and pipelines, leading to the worst-case scenario of wells being shut down.

And the damage is not just limited to flow assurance upstream. Excessive formation water, as a result of water breakthrough, may also exceed the water treatment and glycol regeneration capacities of the downstream plant.

By measuring the early onset of formation-water production in real time, operators can take preventative or remedial action, such as adjusting the pH in the monoethylene glycol/water mixture, injecting the right amount of corrosion inhibitor or, more drastically, choking the well or instigating zonal isolation.

Gas fields

There are a number of other recent drivers in the industry that have increased the need for accurate water production profiles.

Firstly, with natural gas becoming an increasingly important energy source, the numbers of major subsea gas fields are multiplying worldwide. The United States Energy Information Administration estimates world proven natural gas reserves to be around 5,210.8 Tcf. And much of this gas is wet gas - defined as being around 98% to 100% gas void fraction (GVF).

For such developments with typically high pressure and high flow rates and where it is difficult to detect the water production profile in the wet-gas well, the ability to detect water is critical for optimizing production; preventing hydrate, scale and corrosion in the pipelines; and ensuring a reliability of supply.

Secondly, the last few years have seen an increase in subsea tiebacks as a means of managing production costs and avoiding expensive platforms.

With tiebacks of more than 62 miles (100 km) common and 310-mile (500-km) tiebacks in the planning stages, there is the risk of not detecting water breakthrough early enough to avoid potentially disastrous consequences. The key is to know exactly when and how much water is being produced, requiring very sensitive, accurate and reliable measurements of the water in the gas stream.

How accurate is accurate?

Against this background it is clear that the accurate and quick detection of water is essential to flow assurance. But to what degree of accuracy and sensitivity should we be aspiring to, and how early does the water need to be detected? The answer is highly accurate and very quickly.

Many fields have a desired water detection sensitivity of as little as 0.005% by volume, and it is important that any water is detected to this degree of accuracy.

The traditional multiphase meter, for all its effectiveness, is simply unable to measure to this degree of accuracy and sensitivity.

To fill this void, today's wet gas meter uses advanced microwave-based dielectric measurements and generates accurate gas and condensate flow rates based on standard delta pressure devices. The meter detects the resonant frequency in a microwave resonance cavity with the resonant frequency depending on the dielectric properties of the fluid mixture present in the cavity.

And with the permittivity of water (~60 to 200) much higher than that of gas (~1) or oil condensate (~2), the dielectric properties of the wet gas mixture are consequently very sensitive to the water content.

Performance tests on Roxar's Wet Gas Meter have shown the meter able to detect changes in the water production with sensitivity better than +0.005% volume, while the absolute accuracy was +0.1% volume in high GVF (greater than 98.5%) cases.

In the Carina Aries field in Argentina, for example, water injection testing took place where 31 b/d was initially injected per day and then reduced by 6.29 b/d at each test point - what worked out at 41 liters per hour. The measurements were incredibly precise and accurate with the company's wet gas meters detecting a 6.29-b/d change in a 188,700-b/d flow. This effectively amounts to 3 liters of water in a room of 100 cubic meters - highly sensitive!

In addition to the microwave technology, a pressure, volume, temperature software package is an integral part of the meter and is used to calculate the individual liquid (condensate) and gas densities and the actual gas/oil volume ratio (GOR) at meter conditions.

The calculated GOR is subsequently employed to discriminate between gas and oil and hence to deduce the oil fraction and gas fraction once the water fraction has been found through the meter. Condensed and saline formation water can also be distinguished.

And to address the need of quick detection, the wet gas meter is able to perform online and direct measurements so that water can be detected as soon as it starts to be produced.

Ormen Lange field

The Ormen Lange field is the largest natural gas field under development in the Norwegian Continental shelf and lies 62 miles (100 km) northwest of the Norwegian coast in water depths between 2,625 and 3,281 ft (800 and 1,100 m).

Proven gas reserves are 14 Tcf, and production is expected to be 706 Bcf of gas per year, with the gas likely to cover 20% of the UK's gas requirements for up to 40 years. The field will be going into production in 2007.

With a subsea tieback of more than 75 miles (120 km) to Nyhamna on Norway's west coast, the need for the field to be operated remotely with no offshore platforms and the importance of a high degree of accuracy and sensitivity in water detection, Ormen Lange is an excellent example of how wet gas meters are helping to improve flow assurance and are central to a field's development concept.

The instrument company is working with the operators - Hydro during the development stage and Shell after 2007 - to install eight subsea wet gas meters and implement remote management systems which help to optimize production.

The need for accuracy in water detection in the field is vital. Depending on the flow assurance philosophy selected, studies performed show that even small amounts of saline water can result in large and rapid scaling problems with scale prevention requiring a sensitivity that is significantly better than what is required for hydrate prevention.

And with the field not using conventional offshore platforms but instead connecting wellheads on the ocean floor directly by pipes to an onshore processing facility, scaling and corrosion in the pipelines is to be avoided at all costs.

Today, the desired water detection sensitivity for Ormen Lange field is 0.005% by volume - a detection accuracy of 9 gallons of water an hour in a 100 MMcf/d gas well.

The wet gas meters are achieving these levels and helping to ensure a future safe and reliable supply of gas production from this unique offshore field.

Snøhvit field

The importance of wet gas metering in fields where there are long tiebacks is also exemplified in the Snøhvit field wet gas field in the Barents Sea, where the untreated well stream is piped 100 miles (160 km) from the field to a gas liquefaction plant in Northern Norway.

The consequence of not using wet gas meters for such a development would be unacceptably high with an over-injection of chemicals (hydrate inhibitors and others), as well as a loss of control of the long distance multiphase pipeline system.

By using the wet gas meters in the field, remedial action can be taken as soon as the water is detected, whereas without the wet gas meters a lag of 3 days would ensue. The result would be a build-up of large deposits of salt in the pipeline.

Optimize wells

At a time when exploration replacement rates, although important, remain insufficient for companies to reach their long-term targets of production replacement and growth, there is a growing focus on enhanced recovery and increased production.

By continually measuring formation-water production, operators are being able to operate each well aggressively "at the limit" of its water production and are well on their way to increased flow assurance, accelerated production and maximum reservoir performance.

Comments