Time-lapse seismic monitoring of individual hydraulic frac stages using a downhole DAS array

A fiber-optic distributed acoustic sensing system (DAS) was recently installed in an Apache-operated horizontal shale well and used to acquire downhole seismic data before, during, and after the well was treated in order to better understand the time-lapse seismic response to hydraulically stimulated fractures. In addition to a 3D/4D DAS vertical seismic profiling survey acquired before and after the well was treated, data were recorded from two fixed shot locations approximately 1 mile off each end of the lateral part of the well. These shots were repeated after each of the 78 frac stages used to treat the well in an attempt to achieve more continuous seismic monitoring of the hydraulic-stimulation process. Despite the noisy environment associated with ongoing frac operations, these data proved to be valuable in understanding the fundamental seismic response to each frac stage. The dominant time-lapse effect observed in the stage-by-stage data was a subtle time delay of the P-wave direct arrival when compared to data recorded prior to stimulation. This time-lapse effect was only present over a period of a few days once the stage was completed and would have been completely missed using the conventional approach of having a single monitor survey acquired after the well was treated. It was possible to model the kinematics of the P- and S-wave arrivals and invert for frac effectiveness at each stage based on the observed time delay in the seismic. Amplitude differences were also observed in the time-lapse shot records in which new waveforms appeared along the hydraulically stimulated portions of the well.