For all the technological developments in reservoir monitoring and subsea information gathering over the last few years, the fact remains that, according to a 2004 report by Statoil, Norsk Hydro and the Norwegian Petroleum Directorate, subsea wells have a 15 to 20% lower hydrocarbon recovery rate than their topside counterparts.

The report cited that the main reasons for this discrepancy were accessibility and lack of data for reservoir optimization. Moreover, the rise in multilateral and multizone completions has increased the need for a better monitoring of the contribution from each producing zone.

Accurate right-time data gives operators an opportunity to head off production-chain problems at the formation. (Graphic courtesy of Roxar)
Operators need to know more about their subsea fields on a minute-by-minute basis and have the access to the same instrumentation that is available topside. It is only through such information on the production of oil, water, gas and sand from the well bore that subsea gathering systems can work effectively in controlling the flow of oil and gas from wells to storage facilities, pipelines or processing units.

Operators also need the ability to measure and proactively intervene in the production process, through smart wells, for example, which while costly up front, can pay off when aligned with effective measurement systems. Finally, operators need access to continually updated models of their reservoirs to ensure production is optimized, recovery increased and subsea gathering systems operate effectively.

The need
It is not as if the technology to more effectively collect subsea data is not available. The last few years have seen a rapid increase in the accuracy, sensitivity and resilience of multiphase and wet gas metering, the development of highly accurate and resilient high pressure and high temperature (HP/HT) gauges and complete sensor systems, which provide real-time downhole production surveillance.

The software side has seen similar advances through both static and dynamic reservoir modeling, advanced simulation and the latest in uncertainty management.
The result of these hardware and software advances is an embracing of the smart well and digital oil field concepts and their three main components — total asset awareness, right-time analysis and decision-making and timely execution.

However, what is still missing is the ability to tie the information together — a means of integrating well planning and design with modeling, simulation, monitoring, optimization and management to provide a complete picture of the reservoir and ensure the subsea gathering systems are performing optimally.

In the ideal world, a combination of instrumentation and software should allow operators to monitor production continuously, change production profiles quickly and cost-effectively without the need for intervention, observe and control fields from remote locations, process large volumes of vital reservoir data quickly and use the most recent, up to date, information from the field when forecasting future reservoir performance.

Let’s take a look at a typical new subsea field development on the Norwegian Continental Shelf and examine subsea information gathering at its best.

The field
The typical new subsea development in the North Sea is 62 miles (100 km) offshore with a subsea tieback to an existing production platform.

Production comes from a series of deviated or horizontal wells with perhaps several branches radiating from the main borehole. Production data is required not only from each wellhead but also from each producing zone along the well path.

The key drivers for the operator are to increase recoverable assets and optimize production, guarantee flow assurance, avoid inefficiencies in the gathering systems and ensure that decisions are both improved and made earlier.

Going downhole
Such a field requires a fully integrated solution using a combination of downhole measurement tools, subsea measurement technology and reservoir management software.
Looking downhole, the field would benefit from the installation of downhole gauges to permanently monitor pressure and temperature in the wells. The only way to continuously monitor the production performance parameters of each individually perforated zone of a multilayer well would be by installing downhole sensors in each production zone.

One intelligent downhole network will allow operators to install up to 32 instruments on a single cable with no interdependence between the measurements. The downhole sensors are placed between each production zone and are used not only to monitor temperature and pressure, but also will soon be able to measure water cut, gas fraction, sand rate and flow velocity.

Data acquisition can be handled through a downhole network card (DHNC), which acts as an interface between topside communication and the downhole network.

Subsea metering
Subsea metering would be central to flow assurance in the field with multiphase or wet gas meters deployed on all oil producers and all gas wells.

Subsea multiphase meters would be able to provide accurate and continuous online monitoring of the flow rates of oil, water and gas in the subsea well streams, and subsea wet gas meters would provide extremely sensitive and accurate water production profiles in gas wells. Through the meters measuring the early onset of formation-water production in real time, the field operator can then take preventative or remedial action and ensure that bottlenecks in the well bore don’t translate into defficiencies in the gathering system.

An acoustic subsea sand monitor would also play a vital role in calculating sand production in pipeline flows and monitoring the integrity of the gravel packs located in the gas wells.

Rapid model updating
Playing a crucial role alongside this instrumentation will be the real-time monitoring software.
Working from a shared earth model, the models will integrate all available data including seismic, well log and other geological data and attempt to quantify all structural and reservoir property uncertainties in real-time.

Whereas current modeling technology only offers “what-if type” analysis — allowing for multiple realizations of what would happen if a well was choked back, for example — companies are looking to close the loop and use right-time data to rapidly update the reservoir model.

Using modeling, simulation and history matching software alongside each other, the operator will gain a rapid understanding of production behavior and be able to create robust forecasts from a shared earth model.

Drilling and well planning can also be incorporated into the rapid model updating with a closing of the loop between the analysis and integration of real-time drilling data and the updating of real-time geological models.

Tying it all together
There is no doubt that this typical field has highly effective reservoir monitoring and subsea information gathering systems in place.

Yet, do we have a complete picture of the reservoir and the behavior of each well — behavior that can have a sizeable impact on the effectiveness of gathering systems as well? Roxar has developed real-time monitoring and analysis software to provide that all-important data overview.

The company’s Fieldwatch, which will be based on the typical field’s offshore platform, will be used to store, monitor and manage the measurement data from all meters and gauges within the field and will establish an important link between real-time production optimization, right time reservoir characterization and production forecasting.

Fieldmanager will be based at the field’s onshore control center to provide a suite of analysis and interpretation tools, local storage for the data from the flow measurement instruments over the lifetime of the field and to receive the most recent data from the platform monitoring equipment.

The two systems will integrate instrument data such as downhole temperature, pressure and flow rates from a variety of field instruments into a common desktop for visualization, field monitoring, analysis and interpretation.

A broad range of powerful data interpretation and analysis capabilities are available, including the ability to calculate quantities from measured data such as the dynamic calculation of flow stream hydrocarbon composition. The rapid retrieval and display capabilities of both applications also give the user the ability to quickly visualize the data and identify trends, patterns or areas of interest for further analysis.

The majority of systems such as DCS and SCADA typically have limited storage capabilities. New full-life-of-field storage allows the operator to perform reservoir analyses on short- to mid-term historical data as well as the perform look-back analyses on the complete history of the well or reservoir.

The result is a complete solution from the field instruments to the end user’s desktop.
Reservoir monitoring has come a long way in better characterizing the subsurface, closing the subsea information gap, increasing reservoir performance and optimizing subsea gathering.

Operators today demand an integrated, simulation-based, asset management system, where the economic impact and associated risk of reservoir management and development decisions can be fully assessed and measured against a company’s risk tolerance.

With all the necessary tools and technologies on the market today and with such software-based solutions becoming available, closing the subsea information gap is only a matter of time.