Increasingly, legislative and even non-governmental organization guidelines have to be factored into a responsible reservoir management strategy, reducing the risk of damage to both reservoir infrastructure and nearby marine plant and animal life. Flexible, modern improved oil recovery (IOR) systems can be an effective tool to this end, leveraging ocean-floor monitoring infrastructure as a significant tool to assist meeting such requirements.

Monitoring the environmental impact of IOR

The open-access WAN architecture of Octio’s REM system makes beam-forming for national security possible. (Image courtesy of Octio)

Protecting plant and marine life is no longer optional for responsible hydrocarbon exploitation; however, the long-term trend-monitoring and event-alerting required for this is not supported by high-cost and time-constrained streamer survey operations.

By contrast, a permanent seafloor sensor network investment for IOR can also be leveraged to meet these requirements, introducing advanced monitoring methods for environmental benefits and increasing the return on investment.

A seismic 4-D system is a phased sensor array of immense sensitivity, providing continuous, live data streams which can be processed to determine the temporal and spatial location of events and objects in the ocean environment. Permanently installed systems provide seafloor coupling of the highest fidelity data, allow repeatable or continuous surveys for both IOR and environmental purposes, and can easily be customized for specific reservoir geometries or requirements.

Additional sensors can be provided for specialist purposes such as pH, temperature, salinity, and other general measurements that may be required. Other specific uses might be, for example, listening for whale migration, significant in some ocean areas, or vibration monitoring, critical during seismic activity in close proximity to coral reefs.

Many different sensor types can easily be added to a well-designed infrastructure, likely a key leverage point for customers who want to collect regular quantifiable and documented data, always useful in combating the unknown.

Safety and hazard reduction, caprock integrity monitoring

In the Tordis field, produced water was re-injected and subsequently migrated back to the surface, creating a 131-ft (40-m) diameter crater. (Image courtesy of Statoil)

Surface leakage from reservoirs and subsurface storage locations is being reported more frequently, with seven such incidents reported in just the Norwegian sector of the North Sea over the past 10 years. A large number of unreported incidents of this type are suspected to have occurred worldwide, and these can represent a major risk to both offshore installation stability and the environment.

As well as the injection of fluid, ordinary oil or gas production will, in most cases, change the geomechanical stresses on the overburden, and where a large number of adjacent fields are entering into their tail-end production phase, the combined depletion can cause subsidence due to a marked pressure decrease.

Lack of control in an injection phase can cause the opposite effect, but in both cases a flow path leading fluids and associated gases to the surface can be created, stirring up surface sediments in the area and creating craters on the seafloor.

In the Tordis field, produced water was reinjected and subsequently migrated back to the surface, creating a 131-ft (40-m) diameter crater. By pure luck no installations were affected by this 23-ft (7-m) deep sink-hole.

The safe solution to this is close monitoring of risk areas using permanent seismic systems capable of passive microseismic monitoring. These reveal the activity in near-surface fault zones and facilitate frequent active seismic shooting to show the fluid and gas pathways opening towards the surface.

Critically, only repeat measurement techniques give early enough warning to control pressures and avoid seepage to the surface. Conventional 4-D towed techniques do not give frequent enough snapshots to avoid accidents since warning signs may be typically visible only a few weeks before a dangerous event.

CO2 sequestration

Nodes can function as a tool for not only time-lapse reservoir monitoring but also an infrastructure investment tool that can be leveraged for many other purposes.(Image courtesy of Octio)

With underground CO2 storage currently the most popular available solution for emission compensation, carbon capture and storage projects are being initiated on a worldwide basis at an ever-increasing rate. CO2 and other gases have been injected into geological formations for various other purposes, but long-term storage in underground formations is a new concept, carrying with it a number of key uncertainties.

One known issue is the vertical migration of CO2 across geological layers, and monitoring is vital during both the injection phase and the lifetime of the storage region to verify the migration paths of the injected CO2. The integrity of a storage reservoir over time can be monitored using active seismic and controlled-source electromagnetic (EM) surveys. In all cases, a permanent sensor array supporting seismic and EM sensors allows mapping of the changes in both reservoir structure and in CO2 extension and saturation.

Government legislation and environmental policies

Legislative measures now play a significant role in ensuring responsible and environmentally safe exploitation of hydrocarbon resources in fragile marine environments, and sustainable development policies increasingly dictate that operators demonstrate sound and justifiable management of the natural resources surrounding their systems with solid tools in place for the protection of vital environmental resources.

Norway has an overall national goal for the offshore petroleum industry of zero discharge for environmentally harmful substances. Fiscal incentives, specific emission and discharge controls, and compulsory monitoring of the environment have already been put in place to ensure this detailed goal is met.

All of these directly affect operational costs, and any approach which integrates environmental monitoring with an IOR-based reservoir monitoring investment will provide a clear reduction in these extra costs.

Offshore installation surveillance and monitoring

With significant operational activity surrounding most offshore installations, improved monitoring of marine activity such as vessel movements will surely contribute to safe operations, giving the permanent monitoring system a much wider role than just the repeat surveys needed for oil- and gas-related work and providing opportunities to contribute to advance tsunami warning systems, for example, and microseismic monitoring.

Beam-forming from the continuously available data could also make a welcome contribution to national security, and the open-access WAN architecture of systems — such as Octio’s REM system — makes this possible with little or no disturbance to the primary monitoring functions. It is entirely transparent to platform operations.

Flexible architecture

A thorough understanding of both reservoir and seabed characteristics is important for geological incident prevention and minimization of environmental impact in all depths where the offshore industry operates. Systems should be fit for all these purposes, with technology flexible enough to meet differing regional conditions and legislation.

In practice, feasibility studies are always required for new permanent monitoring system installations, and such studies increasingly need to take account of all environmental monitoring requirements that may apply in the system’s lifetime.

There is perhaps no substitute for real data from the field at an early stage to verify the results of a feasibility study. A compact scalable system that can be easily moved or expanded later gives advance knowledge and confidence in the benefits of large-scale permanent monitoring for a particular field.

These nodes can include additional sensor types including EM, high-frequency acoustic, flow, pH, and gas detection, They can function as a tool for not only time-lapse reservoir monitoring, but also an infrastructure investment tool that can be leveraged for many other purposes.