Terje Baustad
Kjell Wold
Emerson Process Management
Think of some of the biggest threats or obstacles to offshore production today and sand, corrosion and the dangers of high pressures within subsea production wells are all likely to come high up the list.
There are variants of risk from all three threats. In worst case scenarios, the problems of high-pressure gas migration may be more immediate than the slower forms of damage from corrosion or sand erosion, although such damage can occur quickly.
Many preventative technologies are available today. These include, to name just a few, intrusive and non-intrusive sand probes and monitors, online non-intrusive corrosion monitoring based on the electric field signature technology, and permanent downhole monitoring equipment that can provide online, downhole well pressure and temperature measurements, often in the most extreme conditions.
Wireless technology is helping operators monitoring equipment to parts of the reservoir previously considered unreachable.
Measuring pressure
For example, one of the most inaccessible areas of a subsea oil well is the annular space.
The annulus is the space between two concentric objects, such as between the wellbore and casing or between casing and tubing, where fluid can flow. The annulus provides a path to circulate fluid in the well, with a completed well normally consisting of at least two annuli. The “A” annulus is the space between the production tubing and the smallest casing string. The “B” annulus is between different casing strings.
Corrosion monitoring reports from Roxar Fieldwatch system.
For conventional land wells and dry wellhead offshore wells, operators can access the B annulus simply via valves to check and, if necessary, adjust pressure. The picture is radically different, however, in offshore subsea wells, where, after the sealing and cementing of the casing (due to required specifications so that the cementing can act as barrier against high pressures encountered deeper in the well), there is no access to the B annulus.
This leaves the operator without access to pressure measurements in the annular space of a subsea well following completion. If pressure exceeds the capacity of the casing (the casing heats due to the production flow, resulting in increased pressure in the trapped B annulus), for example, burst or collapsed casing strings are possible.
A loss of cement seal integrity can lead to oil or gas migrating vertically towards the surface along the outside of the casing, creating potentially hazardous situations, especially during workover operations. In the worst case scenarios, gas escape at the surface.
Furthermore, the fact that operators are “flying without instruments” so to speak can result in taking an overly cautious approach that leads to excessive and expensive over dimensioning of casings to compensate for worst case scenarios and the shutting down of wells prematurely, due to their inability to verify barrier integrity. This can cost the operator millions of dollars.
Regular pressure monitoring, on the other hand, can confirm the seals’ integrity and ensure that it remains that way by regulating well flow. Furthermore, subsequent completions also may benefit from more accurate model predictions.
This area which was previously off-limits now can be opened by using wireless technology. Emerson plans to unveil a new wireless-based sensor system that was developed over the last three years. The new solution will be attached to the same cable as the reservoir monitoring gauges and will directly measure pressure behind the casing string.
The sensor is based around an Integrated Downhole Network (IDN) system to carry signals from the wellbore to the customer monitoring system with a Downhole Network Controller Card (DHNC) placed in the subsea structure and connected to a ¼-in. electrical cable and a series of up to 32 sensors distributed throughout the completion string. A wireless PT transponder and antennae monitor activity in the B Annulus.
The result is an effective instrument for protecting well integrity that can detect variations in pressure behind the casing, provide an early warning of these conditions, and allow intervention or other remedial actions to be planned and implemented in a timely manner.
For offshore operators planning subsea production or injection wells and looking for improved well integrity monitoring, and for government regulatory agencies overseeing safety and environmental protection, the new solution is a step forward in well integrity and opens an element of the subsea production system previously inaccessible.
Wireless monitoring
As mentioned, while arguably on a different scale of importance to high pressures in the well casing, sand and corrosion also pose significant challenges to offshore operations.
The growth in deepwater and wet gas fields, the prevalence of brownfields (where water cuts are increasing in volume), and the popularity of saving cost by using carbon steel piping – a material vulnerable to saline formation water – does increase the likelihood of corrosion. Corrosion can lead to production losses, metal losses, which reduces the life of production and storage equipment, or safety and environmental set-backs, due to the corrosion of key infrastructure.
The same is the case for sand. With 70% of the world’s oil and gas reserves contained in sand reservoirs, sand clogged production equipment and obstacles to wellbore access can be common.
Many of the solutions to combat such erosion and corrosion challenges, however, are wireline-based, meaning they are restricted as to the areas they can monitor. They also are encumbered by the need for electrical power and cables used to position the sensors, as well as maintenance and data analysis implications.
In the next few months, Emerson plans to bring new corrosion and sand wireless transmitters to market – transmitters that can be used both upstream in the reservoir and downstream in refineries.
The transmitters, based on intrusive sensors installed into pipes or vessels through an access fittings system, will reduce installation costs compared to wired online systems, will allow for monitoring in previously inaccessible areas, and will ensure that vital measurement data can be collected and acted upon in real-time.
Other advantages of wireless include the saving of weight, space, and cost. A recent study by Emerson of an actual offshore platform with about 4,000 I/O found that installing wireless along with other technologies in the process control system can save up to 7%, or more than $1 million due to the weight savings in wiring and associated infrastructure.
In this case, the Smart Wireless transmitters will be just one element in a complete asset management system for offshore operations that include Emerson’s field monitoring system, Roxar Fieldwatch.
Emerson’s Smart Wireless solutions use the IEC 62591 (WirelessHart) standard and a portfolio of wireless products to safely and reliably provide greater insight on the platform regarding operations. The IEC 6291 wireless system, for example, is used today on a variety of offshore platforms to remotely monitor wellheads and heat exchangers, and to generate production-based information. Examples include Statoil’s Grane and Gullfaks’ platforms in the North Sea.
Roxar Fieldwatch, the latest version of which is scheduled for launch at OTC this month, will see a range of reservoir monitoring instrumentation incorporated into the field monitoring and data analysis system to give operators a more complete picture of the reservoir and of offshore production activities.
The result will be a complete Smart Wireless integrity management solution for protecting operators’ downstream and upstream assets and the successful and economic flow of hydrocarbons from offshore reservoir to refinery.
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