The hydraulic fracturing stimulation of unconventional shale reservoirs requires the identification of sections along the well with good reservoir and completion quality. Completion quality depends on the poromechanical properties of the reservoir. Shales are known to exhibit anisotropic elastic properties. The elastic anisotropic of shales is of first order as it affects four key geomechanical steps: the in-situ stress field, the stress concentration around the borehole, the failure properties both in tension and compression and the geometry of the hydraulic fracture. Consequently, the initiation pressure of hydraulic fractures at the wellbore predicted by data and models taking into account the anisotropy can be lower or higher by up to 100% as compared to isotropic conditions.

We performed a thorough sensitivity analysis of in-situ stress, material anisotropy and well orientation conditions for different scenarios for far field in-situ stress and near field borehole stress, under isotropic and anisotropic conditions. We consider three scenarios. Scenario-1 where both the far field in-situ stress and the near field borehole stress are computed under isotropic assumption. Scenario-2 where only the far field in-situ stress is computed using anisotropic assumption while keeping the isotropic solution for stress concentration is kept. Scenario-3 where both the far field in-situ stress and the stress concentration take into account the material anisotropy. In addition, for each scenario, we considered three different σHh cases. This study shows that the full anisotropic solution or scenario 3 is always the most appropriate one for predicting correct initiation pressures and the pressure is always lower for the anisotropic case. It is clear that the observed effects are magnified by the degree of anisotropy of the rock, i.e. each rock or formation will influence differently the initiation pressure. We conclude that the material anisotropy is affecting the in-situ stress model as well as the stress concentration around the borehole. Both effects need to be considered in order to account for the anisotropy. If the effect of the anisotropy is taken into account, for both the far and the near field, it will be possible to better predict the initiation pressure and better stimulate unconventional reservoirs.

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