Global Geophysical has developed an approach for understanding unconventional production potential using ambient seismic recordings captured over days, weeks or months. Using proprietary processing techniques, ambient seismic reveals the extent and density of natural fractures; the size, orientation and shape of induced fractures; the volume of rock activated during stimulation; and the volume of rock active during production.

Ambient seismic can be collected during the acquisition of traditional 3-D seismic surveys and as a “monitoring” service for fracture stimulation, injection and production. The data can be recorded with a surface array or with a buried array. The detectability of the radiated seismic energy depends on the signal-to-noise ratio of the trace data after filtering has been applied to suppress the various types of noise. For good focusing, the surface wave noise must be removed. Trace processing for noise removal reveals and concentrates the real seismic energy. The optimum surface array design is a uniform hexagonal distribution of geophones covering the required aperture. This design samples noise at multiple azimuths so that surface noise is suppressed and not passed into the image.

Steps for computing the image volumes include data acquisition, velocity model building, trace processing for noise suppression, imaging and fracture image computation. Ambient seismic recordings have extreme value for the prediction of frack treatment performance before the actual treatment, focusing the seismic emissions during the frack treatment for estimating the volume of rock that is stimulated, mapping the time sequential activation of the fractures during treatment and measuring the active producing volume over time during production.

Increasingly, it is understood that the concentrated seismic energy revealed in trace processing is the result of long-duration signals (LDSs) rather than the microearthquakes (MEQs) that are recorded by other microseismic methods. LDSs are continuous seismic waveforms originating in the reservoir that last for seconds or minutes and that are episodic and pulsating in nature. It has been found that LDSs contribute most of the signal to natural fracture images. Traditional MEQ techniques fail to capture a significant amount, and sometimes the majority, of seismic energy. Recent experience by Global Geophysical and others also shows that natural fracture networks play a significant role in the effectiveness of hydraulic stimulation and in the long-term producibility of unconventional wells.

The fracture imaging method for this type of acquisition and data can be best described as a one-way travel time prestack depth migration. To image the ambient seismic signals, the time of investigation and the spatial volume for investigation must be chosen. Streaming the ambient signal through the depth-imaging algorithm images the depth volume for the entire time window of interest. The resulting volume contains the fracture surfaces that are extracted to compute volumetric fracture images. The integration of a large number of volumes from a significant period of recorded time combines all of the impulses at a single location to build up the signal and suppress the noise. The finished product is then an accurate representation of the acoustically active areas and volumes and can be used to understand fracture patterns, intensity and location as well as fracture propagation timing.

One of the most powerful applications of this technology is the ability to understand the potential effectiveness of fracture stimulation. Ambient seismic data collected before the frack job are used to image the natural seismicity from the reservoir, revealing acoustically active natural fractures. Mapping where natural fractures intersect a new or planned well helps predict the influence of the natural fractures on drilling and stimulation. Ambient seismic collected during stimulation allows for the characterization of induced fractures and of the overall impact of stimulation on the reservoir and on nearby wells. Finally, ambient seismic data captured while wells are in production helps define the active producing volume around the wellbore at that time in the well’s production history. Monitoring production volumes periodically over time allows for identification of missed pay, refracturing candidates and planning for infill drilling.

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