While it is the new oil and gas developments in the early production phase which tend to get most of the attention, today more than 70% of the world's oil and gas production comes from fields that are more than 30 years old.
And with global energy consumption expected to triple over the next 50 years, it has become increasingly important to increase recovery rates from these brown fields - rates which currently tend to be between 35% and 40%.
Brown fields, however, come with unique challenges. As opposed to green fields, they tend to be characterized by a complex set of interdependencies between aging and new technologies and inevitably have far more environmental, health and safety implications than newer fields. The importance of minimizing disruption while upgrading technologies is also a key issue.

Cost implications
There are also cost and technology implications. Many brown fields still have the original technology in place that was utilized decades ago when the fields were in full production.
Such technologies are often outdated and unable to maximize the reservoir's performance. At the same time, however, exploration and production operators are unwilling to make the investments needed to dramatically improve recovery at a time when they are making massive investments in newer discoveries.
There are, however, solutions on the market today that can improve recovery rates, be incorporated seamlessly within existing facilities with minimum disruption to ongoing operations and alleviate operators' concerns of escalating capital spending.

A case history
One such solution is using multiphase meters to bolster well testing within brown fields. Many brown fields have very limited well-testing capabilities at a time when regulatory bodies are requiring operators to better monitor their wells and track well production.
Well testing also helps better determine the optimal production capacity of each well over the lifetime of the field, thereby avoiding the risk of overproducing the well while at the same time accelerating production and increasing recovery. Well testing will also discover wellbore damage and mechanical integrity issues at an early stage.
One way of increasing the number of well tests and reducing the amount of oil lost to well testing without the need for an expensive infrastructure overhaul is by installing multiphase meters on wellhead platforms which can determine oil, water and gas flow and provide additional characteristics like slugging tendency, watercuts, gas void fraction and gas-oil-ratio (GOR). This helps determine the correct timing of well-flow changes and monitors water or gas breakthroughs.
Multiphase meters will also make well testing easier and more frequent - especially with older brown fields, which have limited test separators and where not all platforms have test lines.
Roxar has installed its multiphase meters offshore for a leading Middle East oil and gas operator for this very purpose. The field in question is a mature offshore field discovered in the early 1960s with significant oil reserves. The field peaked at 50,000 b/d of oil in the early 1970s but declined to 20,000 b/d by the 1990s. Previously one centrally located test separator served the brown field with the platforms unmanned and having limited power.
And with not all platforms having test lines, some wells having to be shut in to be tested and long distances between the wellhead and test separators, there were inevitable production and accuracy costs.
The multiphase meters were installed on wellhead platforms - each of which had between one and nine wells. Most wells were also equipped with downhole pressure and temperature sensors through which a comprehensive field-wide model of the production system could be established. All well tests were then integrated and validated with the production model, providing a field-wide model of wellstream measurements.
By the nature of their design, multiphase meters are easy to install in tight spaces as opposed to the larger test separators. In this case, installation on the existing platform was seamless, no control room was required and the flow computer was installed in a field enclosure close to the meter (Figure 1).
The quarterly average number of well tests taken per month since the multiphase meters have been installed has increased from an average of 50 to 120. At the same time, the amount of lost oil due to well testing has been greatly reduced from a typical 12,000 bbl/m to between 20 bbl/m and 50 bbl/m. Assuming a conservative oil value of US $60 per barrel, the corresponding value of oil lost due to well testing has been reduced from almost $750,000 per month to a figure close to zero.
Over the last few years, through the implementation of new technologies and advanced drilling systems, the operator has successfully reversed nearly 25 years of steadily declining production. Today, the field is producing 138,000 b/d - nearly three times the previous peak.

The Safanya field
As was indicated in the previous case history, multiphase meters can also reduce the need for time-intensive barges and test separators.
Take the Safanya field, the largest offshore oil field in the world, which started production in 1957. For accurate reservoir management, each well is individually rate-tested to monitor well performance and to provide data for field allocation and planning purposes.
Previously, the wells were being tested by two barges, which are equipped with testing facilities but require extensive maintenance to maintain the current testing schedule. The barges are also prevented from testing wells approximately one-third of the year due to adverse weather conditions.
The solution to this has been to install one multiphase meter on each of the wellhead platforms. This allows for well testing on a regular basis and unrestricted by operational limitations due to weather constraints. In addition to more frequent and better data, significant savings in operating costs has been achieved.
A complete subsea separation system
Multiphase flow meters can also play an essential role in a complete subsea separation system in helping to increase recovery rates.
For example, Statoil is installing such a system for its existing subsea Tordis field, in production for 12 years.
The subsea separation system will remove water from the wellstream and re-
inject in a separate well to reduce back-pressure in the Tordis field and enable more hydrocarbons to be processed at the Gullfaks C platform.

Overseeing the separation process
There are times when the separation process needs to be scrutinized rigorously. This is particularly the case with the increase in brown fields and the growth in enhanced oil recovery techniques such as water reinjection supported by efficient water separation facilities.
To this end, Roxar is piloting its Oil-in-water monitor with Statoil on its North Sea Sleipner A platform. The monitor provides accurate, real-time information on the amount of sand and oil dispersed in water as well as monitoring the water volumes treated in process facilities.
By monitoring and controlling the solid particles present in re-injection water, the monitor will help to optimize water flooding of the reservoir and maximize production performance.
From an environmental standpoint, regularly an issue with brown fields, the monitor will also measure overboard water discharge from the platform and ensure that operators meet their environmental requirements for limited or zero oil emissions into seawater.

Not a complete solution
No one could claim that the technology described in this article is the complete solution to increasing recovery from brown fields. Multiphase metering, however, has proven to be one of the essential building blocks available for the upgrades of brown fields.
With brownfield projects notorious for exceeding budget and disrupting operations, there is much to be said for measurement and monitoring technologies that have minimal impact on existing operations, are able to smoothly fit into older infrastructures and, most importantly of all, play a crucial role in optimizing production and improving recovery rates.

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