In general, continuous containment monitoring is also of interest for underground nuclear waste storage, mining activity and geothermal energy generation.

Stress field changes generally induce or trigger microseismic events, which can be detected, located and interpreted if a sufficiently sensitive microseismic network is installed.

NORSAR is also frequently involved in research projects through the Research Council of Norway and directly through joint-industry projects.

Reservoir monitoring

Monitoring oil and gas fields

NORSAR is currently providing near-real time microseismic monitoring services for the Ekofisk field, and has been involved in monitoring projects at Grane and Oseberg all in the Norwegian North Sea. Such fields are usually installed with permanent ocean-floor cable systems, sometimes including additional downhole geophones. The microseismic analysis provides valuable information complementary to production history, injection schemes, subsidence measurements, general field development activity and interpretation of geomechanical parameters. The detectability threshold of microseismicity and the precision of microseismic event locations can be tested with for example perforation shots. NORSAR’s experience from Ekofisk has shown that anisotropy information for the 3D velocity model was vital to obtain the correct depth of perforation shots.

Noise cancellation techniques

Since offshore data is frequently disturbed with strong coherent noise, like seismic interferences from active seismic surveys at far-off fields, vessel traffic and platform/infrastructure noise, specially adapted noise cancellation techniques have been developed at NORSAR to obtain the best results.

Noise cancellation

The figure above shows an example of effectively removing three different types of noise from the seismogram sections without reducing the signal from a microseismic event. In this case the noise was identified to originate from seismic shooting originating from two different neighboring fields and platform noise. Moreover, the microseismic event was not visible on the data before noise removal, and migration-based stacking approaches could only identify an event detection after successful noise-removal.

Using shear-waves

NORSAR's experience in processing large sensor-layout offshore data also shows that significant amounts of events are only recognized by its S-wave energy, which is generally much stronger then the P-wave energy. Hence, rotation of the three-component data and dedicated 3C-processing, including detailed S-wave velocity models are required, and NORSAR applies routinely combined P- and S-wave processing.

Towards real-time warning systems

Monitoring of microseismicity is one of the few methods that is able to provide real-time information services on the rock mechanical activities within and around a reservoir.

Especially with respect to hazard detection and subsequent mitigation, permanent microseismic monitoring should be part of any real-time warning system.

Regulators in various countries have, and still are setting up their requirements for monitoring standards, and a proper design of such microseismic monitoring networks is therefore useful and a cost-saving measure (see e.g. NORSAR's software on microseismic network design: MDesign). Certainly, the final performance of a given monitoring network should also be evaluated under real noise condition. Comply with regulator's monitoring requirements.

NORSAR is also frequently involved in research projects through the Research Council of Norway and directly through joint-industry projects.

For more information:

  • NORSAR's work on providing near-real time microseismic monitoring services for the Ekofisk field (e.g. Oye et al., 2014, EAGE).
  • NORSAR's monitoring projects at Grane and Oseberg (Dando et al., 2016, SEG abstracts; Bussat et al., 2016, First Break).