Knowledge of discrete transmissive features such as faults or fracture zones prior to drilling offers substantial benefits for all phases of operations from exploration to development planning and hydraulic fracture design. We describe a new application of a relatively new passive-seismic method. The product is images of seismic activity produced by discrete features such as faults and fractures in the reservoir prior to drilling. We will present data acquired with passive seismic listening arrays in two unconventional reservoirs and a fractured carbonate reservoir.
An extensive body of literature shows that fracture/fault zones with high resolved shear stress correlate positively with fracture transmissivity. In many cases, discontinuities with high resolved shear stress will be the most microseismically active. This method therefore allows mapping of many hydraulically transmissive zones prior to drilling or any other activity.
The passive seismic method (Tomographic Fracture Imaging™ or TFI) directly images seismically active hydraulic fractures and natural fractures as complex surfaces and networks. Until recently, TFI has been used primarily to image hydraulic fracture treatments. We report a new application: imaging ambient seismic activity both prior to frac monitoring and by quiet-time monitoring using 3D reflection grids, i.e. by recording on a reflection grid or dedicated passive seismic grid when shooting or vibrating is not in progress. The resulting images reveal seismically active fracture and fault zones that correlate well with features illuminated by fracing or that are imaged by 3D reflection seismic attributes. The presence and hydraulic transmissivity of some of these features have been validated by independent measures including chemical tracers and pressure monitoring.
This work also considers drivers of ambient seismic activity including earth tides and epeirogenic movements. Earth stress studies have established that the brittle crust is a self-organizing critical system in a state of frictional equilibrium and hence is constantly on the verge of movement. The earth's brittle crust is continually loaded by a variety of forces although tectonic movements, isostacy and isostacy-related flexure appear to dominate. It has been shown that stress or pressure changes of less than 0.01 atmospheres can stimulate seismicity. We present our work to date on quantitative correlations between earth tides and ambient seismic activity imaged on large grids. The results suggest that earth tides help stimulate release of stored elastic strain energy in the brittle crust.