Analysis of Geomechanical Data for Horn River Basin Gas Shales, NE British Columbia, Canada

Abstract Geomechanical data from the public domain for offset wells in the Horn River Basin, north–east British Columbia, Canada, were analyzed to obtain in–situ stress and rock mechanical properties in Devonian gas shale members that are under development. Static rock mechanical properties (e.g., Young’s modulus, unconfined compressive strength, peak cohesion) were calculated from wireline logs from offset wells for the shale members of interest. Derived static Young’s moduli for these shale members range from 18 to 31 GPa, and estimated unconfined compressive strength values range from 41 to 63 MPa. Assuming an average peak friction angle of 35°, the calculated peak cohesion values range from 11 to 16 MPa. From three offset wells, the average vertical stress gradient at the top of the Devonian shales was estimated to be 24.8 kPa/m. Borehole breakouts and anisotropy of the sonic log velocities from offset wells were analyzed to estimate the current orientations of the in–situ minimum and maximum horizontal stresses. The predicted orientations of the in–situ horizontal stress were compared to the well azimuths of drilled offset horizontal wells surrounding the property of interest. This regional data show that the dominant orientation of the in–situ minimum horizontal stress is NNW–SSE. Hence, hydraulic fractures will propagate perpendicular to this in an ENE–WSW orientation. The average orientation of the horizontal wells drilled by the other operators in the basin to date is NNW–SSE, with a wide scatter of other well orientations. There was insufficient data in the public domain (e.g., oriented caliper logs) to assess whether orientations or magnitudes of the horizontal stress vary over the basin, or whether they are affected by small or large faults. The average contrast of the magnitude of the horizontal in–situ stress between the shale members of interest was determined from the dynamic Poisson's ratio to understand the propensity for a hydraulic fracture to grow vertically out of zone. The result suggests relatively small contrasts may exist between the shale members. Recommendations were provided for obtaining site– specific in–situ stress data and rock mechanical properties.

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