Direct Imaging of Stimulation of Reservoir-scale Natural Fracture Networks in Unconventional Reservoirs
Alfred Lacazette, Jan Vermilye, and Charles Sicking
Global Geophysical Services, Inc., Denver, Colorado, USA
A new passive seismic imaging method - Tomographic Fracture Imaging™ (TFI) - provides direct images of reservoir-scale induced and natural fractures as complex 3D surfaces and networks. Images are obtained either during hydraulic fracturing and/or by imaging ambient (purely natural) seismic emissions (Geiser et al, 2012; Lacazette et al, 2013). TFIs provide detailed information on the interaction of hydraulic fractures with natural fracture systems (Figure 1). Furthermore, TFIs provide insights into the geomechanical consequences of different fracturing methods including hydraulic shear fracturing and suppression of natural fracture systems by stress increase due to fracturing. The existence of such effects are predicted by Discrete Element Modeling (e.g. Nagel et al, 2012; Nagel and Sanchez-Nagel, 2011) but have only now been verified by direct observations.
Case studies presented will include examples from hydraulic fracture treatments and ambient surveys. The results are validated with independent data including chemical and radioactive tracer studies, pressure monitoring, production logs, reflection seismic data, and well results. Examples will be presented from the U.S., South America, and (pending client permission) the presentation will show striking results from several recent projects in China.
Tomographic Fracture Imaging works because the earth’s crust is a pervasively fractured, self-organized critical system (e.g. Leary, 1997) in a state of frictional equilibrium (Zoback, 2007). Stress or pressure perturbations of <0.01 atm can induce measureable seismic activity (Ziv and Rubin, 2000). Cumulative activity volumes are generated by stacking the total activity from large numbers of single or multi-component receivers in surface or shallow buried arrays over extended periods of time. By summing total trace energy, the method captures much more energy than conventional passive seismic methods that look only for discrete microearthquakes. Energy is captured from large microearthquakes; small microearthquakes not resolved by conventional methods; Long-Period, Long-Duration (LPLD) activity (Das & Zoback, 2011); and perhaps other types of seismic phenomena that are as yet unclassified. Stacking over extended time periods (minutes, hours or even days) images discrete fractures and provides tremendous additional sensitivity. Fractures are imaged because activity occurs sporadically, not continuously, over the fracture surface. Also, imaging over time is required to eliminate random noise (which is self-canceling over time) and stack coherent, spatially stable signals above the noise level.
Finding the seismically active fracture surfaces reveals much of the fracture network that is likely to be hydraulically transmissive. Fractures with high resolved shear stress are likely to be seismically active, and an 18-year-old body of work (e.g. Barton et al, 1995) shows that high resolved shear stress on a fracture correlates positively with the fracture’s hydraulic transmissivity.
AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013