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From Wellbore to Reservoir Geomechanical Characterization of Naturally Fractured Reservoirs
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In naturally fractured reservoirs, the performance of fractured wells is closely related to in-situ stress state and natural fracture distributions. The anisotropic geomechanical behavior of naturally fractured rock makes it difficult to appropriately evaluate the stress and geomechanical properties of the field. In this study, a wellbore-based, integrated geomechanics-seismic model is proposed aiming to improve stress field prediction and characterization of naturally fractured reservoirs. The integrated approach, starting from a finite element based geomechanical model, which adopts anisotropic nonlinear elasticity to best capture the physical behavior of fractured rock. It is developed to estimate the current stress field at reservoir-scale. The apertures of natural fractures in the reservoir as well as the stiffness of the reservoir rocks are updated during the simulation. The constraints of the geomechanical model are the wellbore stress conditions and failure in the near wellbore region. To verify the geomechanical model results, the failure predicted by a borehole stability model under the simulated stress condition is compared to the measurement borehole breakouts based on well log interpretations. Comparing to conventional stability model, in which rock mass is assumed to be isotropic, the borehole stability model used in this study considers the elastic anisotropy to provide more reliable local stresses. Seismic anisotropy caused by open fractures is then calculated and serves as another calibration method to improve identifying open natural fractures. In this work, a field study is presented. Given the estimated fracture spacing and aperture, the wellbore-based, integrated geomechanics-seismic model estimates a more homogenous maximum and minimum horizontal stress magnitude variations throughout the field comparing to an isotropic linear elastic geomechanical model. This also results in a narrower range for the horizontal stress ratio. The different results in the magnitudes of horizontal stresses will also cause difference in predicted fracture apertures, which results in changes in fracture permeability and porosity in coupled flow-geomechanics reservoir simulation. Eventually, for this field study, the seismic velocity anisotropy is predicted based on the simulated stress condition.
Zhang, Wenyu (2018). From Wellbore to Reservoir Geomechanical Characterization of Naturally Fractured Reservoirs. Doctoral dissertation, Texas A & M University. Available electronically from