Imaging a Hydraulically Fractured Bakken Shale Reservoir Using 4D-Tomography and Reverse VSP Techniques with Perforation Shot Sources

Hydrocarbon production from shale reservoirs varies considerably from one location to another in the same well. This variation can be due to geologic factors but also due to variabilities in the reservoir’ stimulation. Seismic images may be inconclusive in identifying areas where hydraulic fractures have penetrated and microseismic events can have uncertainties in their location and description. I propose the use of perforation shots to map regions where hydraulic stimulation was more effective using P- and SV-wave direct arrivals emitted by the shots. First, I introduce a method to calculate the moment when shots were triggered, the zero-time. For that, I use shot depths, velocity variance, and P- and SV-wave direct-arrival traveltime differences with an approximates a hyperbolic moveout solution. After testing this method with synthetic data, it was applied to real data acquired in the Bakken Formation of the Williston Basin (North Dakota). Using eighty shots triggered in two producer wells and recorded by six vertical wells, I calculated the P-wave fastest azimuth yielding approximately N70oE using a reverse VSP walkaround technique. Two high-frequency 2D seismic images of the overburden layers with a dominant frequency of roughly 300 Hz were obtained. Their dimensions are about 600 m high and 1200 m long. In stacked oil shale plays, the perforation shots can provide high-frequency images with a clear gain in seismic resolution and interpretability. A 3D grid-based anisotropic tomography procedure was conducted to estimate P-velocity variations. The variations point to a reduction of 3% in the P-velocity after fracturing and suggest that fracturing causes seismically observable changes. The region where the P-velocity was reduced is irregularly distributed but, apparently, the areas with the largest P-velocity reduction are more productive. Stages associated with natural fractures show almost no P-velocity reduction. Thus, perforation shots can provide compelling opportunities to seismically identify hydraulically fractured regions.