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Consulting

Fracture monitoring with passive seismic methods

Seismic exploration methods are the most successful methods for exploring hydrocarbon deposits, but also in mining and engineering exploration. Usually seismic works with artificial sources as explosions or vibrators. Recorded data are rearranged to produce a 2 or 3-dimensional image of the subsurface. To image time dependent effects, as fluid flow in reservoirs, seismic surveys have to be repeated what is usually named 4D- seismic or time laps seismic. It will deliver images in longer time intervals. Surveys, continuous in time have been developed but are still rarely used.

 

Passive seismic methods, methods using natural sources of seismic energy, are historically older and are used mainly in earthquake seismology. Observation of smaller seismological events, possibly men made, are also observed routinely for instance in the mining industry. Methods developed for earthquakes have been modified and used in this field successfully. Principally with passive seismic methods everything producing seismic energy in the subsurface can be studied. Observing small mining produced seismic event has been called seismic monitoring. Observing still smaller (usually high frequent) events in non destructive testing was also named acoustic emission (AE).

 

In the hydrocarbon business seismic events in the reservoir are produced during any kind of production. Events produced from fluid flow but also from internal subsidence have been successfully recorded and used to study fluid flow in time and space.

 

Much larger events in oil/gas reservoirs are generated during stimulation with artificial hydro-fracs. Monitoring the development of those fracs is usually called fracture monitoring. It is actually the only routinely available method to follow the development of a fracture in time and space. Online processing and interpretation allows feedback to the fracturing team.

 

There are possibilities to record the fracture generated events in the same well, but usually the noise generated within the well is a showstopper. Normally for the recording nearby observation wells are used. A typical distance is several hundreds of meters, but also in more than 1000m observations have been successful. Principally a singe 3-component geophone allows locating the event. The direction is derived from a hodogram-analysis of the P-wave arrival and the distance from the P-S travel time difference. Using more than one geophone gives redundancy and thus better results. Using a geophone array, i.e. several 3C-geophones on different levels allows location of the events from P-wave arrivals only and avoids the often problematic and inexact reading of S-wave onsets. Drill hole and array geometry have to be modelled in advance to estimate the location accuracies.

 

Observing production induced events in a reservoir needs continuous recording over a longer time period. Permanent installations are the best way to record continuously. They furthermore have superior coupling compared to temporary installations. There are techniques available to install geophone arrays behind the casing or on tubing in still used wells but also techniques to install geophones permanently in abandoned wells.

 

For frac-monitoring recording is only needed for several hours or days. Service companies offer multi-level instrumentation on wireline for this type of surveys. The instrumentation is close to those used for VSP measurements, but should allow continuous recording.

 

As a feedback to the fracture team is needed, the instrument should allow an online real time processing and interpretation. The software should pick events from the continuous data flow. This part of the processing is usually called triggering. A frequently used method is a shorttime/longtime quotient that can be used on a single channel or multi channel basis, where the multi channel version may be called coincidence trigger. A second processing step, after cutting out the events is the event location. This can be done using hodogram-analysis and P-S differences but also by triangulation of P-arrivals or by a combination of both. Assuming a homogeneous isotropic background is often insufficient and priory knowledge of a 3D-velocity background has to be imported.

 

Using adequate online software the event locations can be observer real time on the monitor. This gives the reservoir engineers an on line control of the fracture development.

 

After the survey is finished further seismological interpretation of the data is possible. This includes the estimation of parameters as: fracture length, fracture area, seismic moment, stress drop, stress directions and more. Those parameters are essential for the reservoir engineering and extremely helpful for planning future fracture activities in the same field.

 

 

What does HarbourDom offer?

 

HarbourDom is a technology transfer company. It participated in the development of passive seismic methods for decades. HarbourDomís international contact to instrument/software manufacturers and to service companies allows offering oil-companies help by introducing novel technologies as fracture monitoring. We can help with permanent and temporary installation. We choose for you the best available instrumentation and the best service suppliers. HarbourDom will supervise all activities and make sure to gain optimal results.

 

 

 

Downhole Geophone array

(Geospace)

 

 

 

 

 

Six channel recording unit for continuous registration

(ISSI)

 

 

 

Tri-axial borehole geophone

(ISSI)

 

 

 

 

Visualisation of event location and intensity (ISSI)

 

 

 

 

Geophone installations behind casing and on tubing (Bathellier&Czernichow 1997)

 

 

 

Hodogram analysis (Hardage)

 

 

 

 

Seismic events during injection in a geothermal well (Fehler)

 

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