Creating a Microseismic-based DFN

A key advanced processing technique that has emerged within the microseismic industry and is driving much of the recent innovation in understanding reservoir behaviour is Seismic Moment Tensor Inversion (SMTI).  This method connects seismic observations of a discrete event to the physical processes at the source that are causing the event, such as the event failure mechanism, principle strain axes and potential failure plane orientations.  Evaluating event failure mechanisms is a key aspect to understanding how the treatment programs will improve the drainage characteristics of the reservoir. 

Developing a microseismic DFNEach microseismic event can be viewed as failure in shear, tensile opening/closing or some combination thereof.  The failure occurs on a fracture plane (strike and dip) of a certain size that is itself, part of a discrete fracture network (DFN) of new or pre-existing fractures.  Therefore, microseismic event distributions can be used to reconstruct the DFN that is activating in response to the stimulation program.  Coupled with the dimensions of the failure planes, fracture orientations inferred from the moment tensor can generate an activated DFN model.

ESG's DFN models are developed using information about event locations, the orientation of the fracture plane as determined by SMTI analysis, and the source radius of the event.  Each microseismic event is modeled as a penny-shaped crack, and is visible in the DFN as a coloured 2D sphere.  The colour of the sphere may represent the type of failure mode which generated the microseismic event (i.e. tensile opening or closing, or shear slipping).  Features of interest in a microseismic DFN model include the complexity with which various fracture planes intersect each other, the size of the fracture planes which provides an indication of the overall stimulated surface area, and the degree to which fractures exhibit opening components as they migrate outwards from the wellbore.

During the generation of this DFN, it is imperative that values such as fracture length (determined by the source radius) be accurately characterized, particularly for larger magnitude events.  In faulted formations, it is not uncommon to observe some events that measure above zero on the magnitude scale.  Naturally, larger events release more energy and will be related to failure along a longer fracture surface.  As it has become well known in the industry that typical microseismic equipment, namely 15 Hz geophones, may underestimate these values; therefore the incorporation of 4.5 Hz geophones and force-balanced accelerometers (FBAs) that are tuned for the low frequency characteristics of larger magnitude events in ESG’s patented Hybrid approach may offer increased accuracy in characterizing fracture networks across a range of scales.

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