Imaging Fracture Networks with Ambient Seismicity
Lacazette, Alfred; Fereja, Samuel; Sicking, Charles; Vermilye, Jan; Geiser, Peter; Thompson, Laird
Tomographic Fracture Imaging (TFI) is a relatively new but well-validated passive-seismic imaging method routinely used for hydraulic fracture monitoring (Geiser et al, 2012). The method directly images hydraulic fractures and natural fractures stimulated by fracture treatments as complex surfaces and networks rather than as clusters of hypocenters. This presentation reports a new application of the method: imaging ambient seismic activity both prior to frac monitoring and on 3-D reflection grids. The resulting images reveal seismically active fracture and fault zones that correlate well with features imaged by 3-D reflection seismic attributes and with independent measures of hydraulic transmissivity such as chemical tracers. Knowledge of discrete transmissive features prior to any drilling at all offers substantial benefits for all phases of operations from exploration to development planning and hydraulic fracture design. This method also appears to have great potential for geothermal, earthquake and other hazard studies.
TFI works by imaging cumulative seismic activity over time periods ranging from seconds to hours or even days. The method does not attempt to discriminate individual events, but simply images total activity. For this reason, TFI accumulates both microearthquake (MEQ) energy and Long-Period, Long-Duration (LPLD) energy (Zoback et al, 2012).
In general, seismically-active zones are expected to be hydraulically transmissive because the most active features are the ones under the highest resolved shear stress and an extensive body of literature extending back to 1992 shows that resolved shear stress correlates positively with fracture transmissivity.
Ambient activity does not require large driving stress. Earth stress studies have established that the brittle crust is in a state of frictional equilibrium and is hence constantly on the verge of movement. Also, it has been shown that stress or pressure changes of as little as 0.01. atm can stimulate seismicity. At the time of this writing we think that the primary drivers of the ambient activity imaged by TFI are earth tides and tectonic activity. Detailed studies are underway to correlate ambient activity with earth tides and other potential drivers. The results of these studies will be part of this talk.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013