Abstract
Distributed acoustic sensors (DAS) in combination with high-repeatability seismic sources enable vertical seismic profiling (VSP) of the fracture evolution away from the observation boreholes. We discovered clear signatures of the seismic scattering on activated fractures during nine days of continuous seismic monitoring of the fracturing stages at the Austin Chalk/Eagle Ford Laboratory (ACEFL). The present study applies a novel approach to quantitative analysis of the scattering events in terms of the evolution of the geometry and elastic stiffness of individual fractures. Our characterization strategy sequentially refines the fracture models: from a stack of 1D soft layers to 3D rectangular inclusions. First, we estimated the number of fractures locations and reflectivity using a modified sparse-spike deconvolution of the stacked VSP traces. The fracture set consists of five fractures spaced by 15-30 m with reflectivity around 1%. Then, we developed a scattering integral method to refine these estimates along with an inversion of the fracture top and bottom for each monitoring vintage. We found that initially, some of the fractures were located above the monitoring fiber with the height above the monitoring fiber around 100 m. Then, we integrated the seismic interpretation with the low-frequency DAS and pressure and microseismic monitoring to reconstruct the activation process of the fractures. Most likely, some of the natural fractures slowly grew downwards to the monitoring fiber as a result of fluid injections in the stimulated well. This led to bright strain anomalies but did not trigger seismicity. The top of the fractures remained almost constant and limited by a lithological boundary/stress barrier. To our knowledge, this is the first time VSP data enabled tracking of the fracture evolution with such a high spatial and temporal resolution, which was previously only available for cross-well surveys and at a much smaller scale
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