Passive seismic monitoring

In recent years, deformation of the reservoir host rocks has become a subject of great interest, prompted in part by the dramatic subsidence observed at Ekofisk platforms in the North Sea. One method of monitoring deformation is by passive seismic monitoring. It is called "passive" because the geopysicist does not activate a seismic source, but rather uses existing geophones to monitor ongoing changes in the rocks due to downhole conditions.

Identifying deformation

Deformation is an important aspect of reservoir production, even without a significant compaction drive in many cases. Previous studies have been published in the scientific and earthquake literature relating earthquakes to oil/gas production and to injection practices. The link between injection or production practices and seismicity, however, is complicated and not yet well-understood. The location and timing of microseismic events, or even large earthquakes, cannot easily be linked to a simple failure criterion in an otherwise static and nondeforming crust.

The overall deformation of the rock surrounding the producing reservoir (or zone of injection), as well as spatial variation in pore pressure, can alter the state of stress in the host rock; subsequent changes in either pore pressure or deformation-induced stresses can then cause seismic events, even though these may occur at conditions that would not have originally induced seismicity. Conversely, the history of production and injection may inhibit seismicity that would have occurred under similar conditions but with a different history. Thus, the evolution of stresses in and near a reservoir seems to be almost as important as the absolute values of those stresses, in determining whether or not seismicity will occur. Of course, not all rocks will fail suddenly, producing a seismic event, but may creep or flow, and this form of failure will not be detected by passive seismic monitoring.

The production or injection of fluids induces change in fluid pressure, stress on host reservoir rocks, and the occurrence of small (occasionally) large earthquake-like events, representing sudden shear failure along planes of weakness. These changes can occur at injection pressures well below the reservoir engineer’s “parting” pressure for tensile failure or during production at pressures below original reservoir pressure.

In some detailed studies, very small events indicate locations of fracture systems responsible for fluid migration (Fig. 1). In some other studies, the events identify faults that may be significant for reservoir management (Fig. 2), and seismicity may reveal reservoir behavior that aids in reservoir management. The migration of microseismic events, away from an injecting well, may also be used to determine permeability of the bulk rock, including fractures that serve as conduits of fluid flow.

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  Fig. 1—Passive seismic monitoring results showing fracture locations in a cross-sectional view of seismicity and intervals of gas production from Clinton County, Kentucky (after Phillips et al.).

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  Fig. 2—Passive monitoring for faults in Ekofisk: (a) Microearthquake locations at Ekofisk and (b) location of survey in the Ekofisk field, showing location of obscured reflection-seismic observations because of the gas cloud (after Maxwell and Urbancic)..

Benefits of passive seismic monitoring

There are multiple reasons to consider passive seismic monitoring, which include:

• Earthquake hazard evaluation (and subsequent mitigation)
• Deformation monitoring for reservoir management and optimization
• Monitoring of fluid leakage for environmental and economic considerations
• Providing additional time-lapse constraints for reservoir simulation

Earthquake monitoring has now become one standard way of monitoring reservoir and host-rock deformation. Passive seismic technology has gradually become more precise and accurate, even at low levels of seismicity, largely because of the placement of geophones downhole, which is away from surface noise and closer to the sources of seismic energy, and processing and analysis techniques developed for this purpose.

Because of the complexity, passive seismic monitoring is not currently used widely as a tool for reservoir management. Improved geomechanical reservoir modeling is likely to aid in interpretation of microseismic event observations for reservoir management purposes, and environmental monitoring considerations are likely to increase; given these enhanced applications for the technology, it is probable that microseismic passive monitoring will become more widespread in the near future.

Noteworthy papers in OnePetro

Cole, S. and Vanyan, L. Scattering Analysis of Passive Seismic Data. Presented at the 1990/1/1/.

de Ridder, S. and Biondi, B. Low Frequency Passive Seismic Interferometry For Land Data. Presented at the 2010/1/1/.

Draganov, D., Wapenaar, K., Artman, B. et al. Migration Methods For Passive Seismic Data. Presented at the 2004/1/1/.

Maxwell, S.C., Urbancic, T.I., Demerling, T. et al. Calibrating Borehole Seismic Attributes With Passive Seismic Data. Presented at the 2003/1/1/.

External links

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See also

PEH:Reservoir_Geophysics

Reservoir geophysics overview

Seismic imaging and inversion

Seismic time-lapse reservoir monitoring