Monitoring - Microseismic Monitoring

The injector plus an offset. A string of geophones is placed in the offset. You need to be fairly certain of what direction the fracture will grow in for this to be an effective method. Ideally, the measurement well is perpendicular to the plane of the propagating fracture.
The injector and multiple offsets. With geophones in more than one offset, triangulation is possible to determine the exact position of the seismic source.

Back to Key Issues Previous More Details? Monitoring Best Practices

Formation Velocity: This can be determined from techniques such as using an "advanced" sonic log (P and S Waves) or from a crosswell survey.
Receiver Orientation: This can be determined from signals acquired when the injector is perforated, or with an air gun or other source (full interval scan).
Surveys: Well-to-well location needs to be established from surface surveys and deviation surveys.

Back to Key Issues Previous More Details? Monitoring Best Practices

In most published situations, with the notable exception of some PDO work, much of the microseismic monitoring has been for stimulation-related hydraulic fracturing. Nevertheless, the same concepts apply to longer-term injection situations.
Instrumentation: Receivers are installed in one or more offsets.
Calibration: The receivers are oriented with check shots (e.g., air gun or perforations). Background monitoring is carried out.
"Frac" Day: For stimulation treatments, monitoring is carried out before, during and after the treatment. Many of the events may occur after shut-down. Real-time or near-real-time processing is available.

Where are the Receivers Placed? The microseisms that are generated as a result of injecting into a hydraulic fracture are quite weak. Receivers placed at the surface are, in most cases, too far away to reliably detect these signals. On the other hand, if sensors are placed close to the injection zone in the well that is being injected into, there is too much noise from the pumping operations themselves.
An offset well is required. It is a difficult to carry out monitoring in the treatment well itself. The spacing between the treatment and monitoring well cannot be excessive.
More than one monitor well is preferable, but ... The distance to, and the elevation of the originating slippage event are determined from the arrival times measured at the receivers. The direction to the event is discriminated from the P-wave particle motion. Remember that these microseisms are small amplitude and high frequency and that the receiver distance will be a typical interwell spacing. High quality receivers are essential.
Quiet Environment: The environment in the offset must be relatively quiet - i.e., no gas bubbling through perforations.
Reservoir Difficulties: Interpretation and acquisition are complicated if there is layering, excessive attenuation and cultural noise.
Receiver Orientation: Best interpretation will occur if the fracture propagates normal to the monitoring well.
Receiver Limitations: The physical limitations of the receivers include their size and tolerance of temperature and pressure, etc.

Back to Key Issues Previous More Details? Monitoring Best Practices

Microseisms originate in an envelope around a hydraulically induced fracture. This envelope provides indications of the height, length, azimuth and indicates any asymmetrical growth characteristics (i.e., may grow more in one direction than another - up dip for example).
The events that you are dominantly seeing are fluid loss indiced slippage in fractures around the main treatment and not necessarily the main fracture itself.
The response of the formation around the fracture depends on the compressibility of the native fluid.

In a reservoir with a compressible fluid, the microseismic events will occur close to the main fracture - presuming short-term injection.
In a reservoir with a slightly compressible fluid (oil or injected water), the pressure will be altered farther away from the mother fracture and the envelope defining the fracture will extend deeper into the formation.

Back to Key Issues Previous More Details? Monitoring Best Practices

This type of monitoring expenditure will need to be justified by risk analysis. Technology and interpretation continue to improve, and presumably costs will decline. Nevertheless, the technology may have advantages particularly in situations where sweep is a problem.