From wellbore to seismic: An integrated approach for geomechanical characterizaiton of naturally fractured reservoirs
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Copyright 2018, Society of Petroleum Engineers. In unconventional plays, 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, we present a wellbore-based, integrated geomechanics-seismic model that aims to improve the current stress field prediction and characterization of naturally fractured reservoirs, and ultimately, to optimize reservoir development. The integrated model, starting from borehole-scale, uses a borehole stability model that considers the elastic anisotropy to provide the local stresses. Image log and other logging data are used to characterize natural fractures under local stress condition. A finite element based geomechanical model, which adopts anisotropic non-linear elasticity to best capture the physical behavior of fractured rock, is developed to estimate the current stress field at reservoir-scale, with wellbore stresses as constraints. The apertures of natural fractures in the reservoir are updated during the simulation. Seismic anisotropy caused by open fractures is then calculated and serves as another calibration method to improve identifying open natural fractures. In this paper, a case 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 geomechanics 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 case study, the seismic velocity anisotropy is predicted based on the simulated current stress condition. Comparing to the conventional method for characterizing naturally fractured reservoirs, this integrated approach utilizing a wellbore-based, geomechanics-seismic model can estimate the stress field and fracture properties more rigorously. The improved results can be beneficial in aspects of well planning, hydraulic fracture design, and coupled reservoir simulation in unconventional plays.
