SEISMIC VELOCITY DETERMINATION BY USING STRATIGRAPHIC EMPHASIS TO PREDICT PORE PRESSURE DISTRIBUTION IN THE DEVONIAN DUVERNAY FORMATION IN THE FOX CREEK AREA, ALBERTA, CANADA

Many unconventional shale plays are characterized by overpressure that may contribute to the risk of fault reactivation and induced microseismicity during hydraulic fracturing operations. The Devonian Duvernay Formation is one of the foremost unconventional shale resource in the Alberta Basin. Well testing results show that the Duvernay Formation is characterized by overpressure gradient that reaches up to 18-20 kPa/m. The induced seismic events (up to 3.9 Mw) occurred in the study block of the Fox Creek area during hydraulic fracturing operations in May-June 2015. This study aims to refine interval velocities from 3D seismic data in the Fox Creek area to predict formation pressure in the Duvernay and overlying Ireton Formations between the wells using modified Eaton’s method. The application of this method for the consolidated Devonian sedimentary succession of the Alberta Basin is used with careful validation of predicted formation pressure by available pressure measurements in the wells of the study block.

The interval velocity field is improved by applying correction factor using interval velocity in wells obtained from time-depth relationship. The correction factor is propagated between the wells using the Gaussian random function with a trend estimated from the Sweetness and Relative Acoustic Impedance seismic attributes. The refined interval velocity field has good vertical and lateral resolution and it is consistent with the rock lithology. The interval velocities in the Duvernay Formation show two peaks in distribution, with higher values in the kerogen-lean carbonate-rich shales of the middle Duvernay and lower velocities in the kerogen-rich shales of the upper Duvernay. The obtained refined interval velocities in the Duvernay Formation are validated by the data of core ultrasonic velocity available in a calibration well.

The formation pressure gradient predicted from 3D seismic interval velocities in the study area ranges from 15 to 18 kPa/m, which corresponds well to the values of formation pressure gradient obtained from well testing data (from 15 to 20 kPa/m). The obtained cube of predicted formation pressure gradient will help to model lateral changes of horizontal stresses within the Duvernay and Ireton Formations across the study block.

We thank Pulse Seismic for donation of multicomponent seismic data, Schlumberger for consolidated license donation to the University of Alberta. This work was funded by NSERC through grants RGPIN 2019 04397 and DGECR 2019 00186.