Up to as recently as 10 to 15 years ago, most financial analysts had one key performance indicator by which they judged an oil company's performance - $/barrel discovered.
Yet, while exploration replacement rates remain important, in most cases this is insufficient for companies to reach their long-term targets of production replacement and growth. The reasons for this are two-fold: fewer big discoveries and more difficult access to oil rich zones.
As a result, we are seeing production optimization gaining credence - highly promising from both from an economic and resource preservation perspective.
The need to increase production and recovery rates is even more critical in subsea developments, as these are particularly costly. Furthermore, as subsea production increases, optimization in these fields will show up more visibly in companies' profit and loss and balance sheets.
It is no longer acceptable to live with lower recovery rates from subsea wells than for topside wells. The front runners are setting their sights for substantially higher recovery from their subsea fields and the safeguarding of their assets.
Subsea information gap
Because of the subsea information gap, operators are unable to measure and intervene proactively to optimize production. This information gap translates into a production gap with the production recovery factor for the same area with the same operator tending to be 15% lower in a subsea well than in a similar topside well.
While there are some noteworthy exceptions, too often subsea production monitoring has been based on approximations and guesswork. Take the attempts to detect sand in the production stream, for example, where the information on flow rate, water cut, pressure drops and temperature distributions were seen as the best way of measuring sand. With these inexact measurements, however, come inevitable risks - ignoring sand build-up could lead to the loss of the well or, just as serious, overreacting to the existence of sand and the subsequent unnecessary reduction or shutting down of production.
Other subsea economic optimization attempts have included trial shutting in of selected wells. Besides the revenue lost from deferred oil production and increased costs for subsea test lines, separators and completion equipment, is the danger that the well may never recover - often the practice is irreversible.
In short, operating a subsea field development without the latest in instrumentation and measurement devices is like flying at night without instruments.
And the results of 'flying blind' can be highly damaging - limited step outs with increased scaling, corrosion, and plugging; a continued assumption of the worst case scenario with increased capital and operating expenses; the opportunity to intervene only after something has gone wrong, and most importantly, lower oil recovery.
It's time to make technically recoverable oil and gas (currently 5 trillion to 10 trillion boe) economically recoverable as well. And this can only be achieved through technologies that can take control of subsea production systems and close the information gap.
Growth of multiphase meters
In order to understand their subsea systems and optimize production in the reservoir, oil and gas operators need access to the best available data on gas, oil and water flow rates. Today's multiphase meters provide this.
Through placement of the multiphase meter on the wellhead, jumper or in the manifold, it will provide critical, reliable and easy-to-use real-time information on a well's capabilities during production, such as water saturation and possible breakthrough, gas coning, permeability and flow characteristics.
The information can then be used to determine the optimal production capacity of each well over the field's lifetime, avoiding the risk of overproduction, while accelerating production and increasing recovery.
Multiphase meters and the data they provide will also allow the oil and gas operator to react to changes in the production flow before damage is done to the well or reservoir.
Multiphase meters will immediately detect a rising water cut, for example, through the change in the multiphase composition at the subsea wellhead. And by examining the real-time information from the downhole pressure and temperature gauges, the operator can gain an even better understanding of where the problem originates. This enables quicker implementation of remedial action and forms the basis for more precise planning on later expansion.
And with one of the biggest obstacles to extracting information from subsea wells being the costs and risks of intervention, today's multiphase meters are easy to install and easy to maintain, incorporating retrievable electronics. By proper analysis of the meter data, the operator can also get important clues or even answers to information sought from well logging.
Complete sensor network
Multiphase meters are just one element of an oil company's integrated reservoir production management system. A natural next step is to analyze the meter data in conjunction with permanent downhole pressure and temperature instrumentation tools to provide a more complete picture.
Reacting to this and in response to its belief that more information is required from the reservoir, Roxar has developed a new IWIS (Intelligent Well Interface Standardization) compliant Intelligent Downhole Network (IDN). The network will allow sensor instrumentation to be connected together to form a complete system for downhole production surveillance and control.
A problem detected by the multiphase meter, for example, can be diagnosed quicker if combined with downhole sensor measurements, such as reservoir pressure and temperature, flow rate, fluid fraction, sand detection and chemical properties from each producing reservoir zone. Figure 1 shows a multiphase production network in practice.
Wet gas meters
One of the biggest challenges to subsea production is unchecked water, particularly in condensate and gas wells.
Water and formation water in particular, can lead to scaling and corrosion of the pipelines and chokes leading to a significant reduction in well production. Formation water also often carries with it solids and particles which tend to plug the formation, thus reducing production efficiencies.
With natural gas becoming an increasingly important energy source, the number of subsea wet gas fields - at close to 100% gas void fraction (GVF) have multiplied.
Today's advanced wet gas meters provide accurate and sensitive water measurement using advanced microwave-based dielectric measurements as well as accurate gas and condensate flow rates based on standard delta pressure devices.
With the industry requiring a detection sensitivity of at least 120 liters of water per hour in a 10,000,000 SM3/day gas well (a level of detection not possible with the multiphase meter), today's wet gas meter is rising to this challenge.
Sand detection
Produced sand can clog production equipment, erode completion components, impede wellbore access, interfere with the operation of downhole equipment and even lead to wells being prematurely shut in or abandoned.
Today's acoustic subsea sand monitoring devices provide an "early warning" system - an immediate response when sand is present. Using new digital signal processing in subsea retrievable sand monitors and integrating this with the velocity measurement from subsea retrievable multiphase and wet gas meters, it is possible to determine the actual sand production rate more accurately - an example of how different subsea tools can be utilized together to yield more accurate information. Acceptable level of sand production can hence be established and controlled.
Integrated network
All these technologies are part of today's integrated subsea production network. And, to ensure this information is collated together, software tools allow operators to log all relevant production data and distribute to end users for analysis and decision making.
For example, engineers will be able to determine in real time the effects on downhole pressure or oil production in one well if an adjacent well is shut. The results are an increase in the company's reservoir management capabilities.
Technologies in practice
There is no better example of technology making a difference than the North Sea which has seen a dramatic increase in production over the last 30 years - much of this due to the embracing of new technologies (Figure 2).
In Statoil's Gullfaks field, a pilot project demonstrated that that it was not appropriate to apply the same production limit to all wells. Instead, it was determined that by analyzing the sand and other properties of each well, oil production could be increased by as much as 30,000 b/d. With a crude oil price of around $60/bbl, this would result in an increased value from the field of a huge $1.8 million per day.
Sand detection has also been central to Statoil's Statfjord field. In a tail production phase of the kind which Statfjord has now entered, measures which increase oil output and extend the field's producing life are essential for increasing value. The close monitoring of sand at the field is leading to longer production phases
And it is not just the North Sea. At the South Timbalier field, in the Gulf of Mexico run by Chevron, adopting a sand detector has resulted in increased production of 600 bbl of oil and 10 Mcf/d of gas.
The number of multiphase meter installations is increasing, too. The largest meter ever delivered to the oil and gas industry has been installed on a riser tie-in line carrying multiphase production from five separate flow lines from the Stær and Svale satellites. When production starts up, the meter will monitor the riser production rates for the purpose of allocation among field partners.
Conclusion
These new technological developments can have a dramatic effect on the industry, including fewer and fewer fixed production platforms; more efficient development of marginal, complex reservoirs; and perhaps most important of all, the preservation of resources for future generations.
And the combined effects of closing the subea information gap will be there for all to see - a better reservoir understanding and ultimately accelerated production.

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