Geomechanics of Orthorhombic Media
- Mehdi E. Far (Halliburton) | John A. Quirein (Halliburton) | Natasa Mekic (Halliburton)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- Publication Date
- December 2016
- Document Type
- Journal Paper
- 588 - 596
- 2016. Society of Petrophysicists & Well Log Analysts
- 2 in the last 30 days
- 236 since 2007
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We investigate the geomechanics of orthorhombic media for modeling fractured shales, using theoretical data, and present simplified equations for computing anisotropic stresses in orthorhombic media that yield accurate estimates (compared to the values computed using the exact equations) of the anisotropic stresses in heavily fractured media when shear-wave splitting (SWS) is more than 10%. In terms of mechanical properties, i.e., Poisson’s ratios and Young’s moduli, the simplified equations can be defined using three mechanical properties. We used two theoretical models to create data for fractured shales with two sets of vertical fractures (VTI background + fractures). Different fracture densities were considered, which resulted in variable SWS values. These data were used to investigate the mechanical behavior of media with orthorhombic symmetry. Two possibilities were considered; Case 1 is a horizontal well drilled perpendicular to natural fractures or along the minimum horizontal stress, and Case 2 is a horizontal well drilled parallel to natural fractures or perpendicular to minimum horizontal stress. The values of Poisson’s ratios and Young’s moduli decrease with increasing fracture density, although a few moduli values will remain almost unchanged. For all fracture densities, the magnitude of stress anisotropy (δH - δh) can be accurately computed using the simplified orthorhombic model; for higher fracture densities, models converge to the exact values. For Case 1, δh can be accurately computed using the simpler vertically transverse isotropic (VTI) model, whereas the δH calculation using VTI or isotropic models will result in significant errors.
In unconventional reservoirs, it is important to know the stress variations in different intervals for various applications, such as hydraulic fracturing, wellbore stability etc. In order to understand the variations of maximum and minimum horizontal stresses, engineers use poroelastic Hooke’s law that can give an estimate of the horizontal stresses from moduli. Moduli are usually computed from sonic log measurements. However, sonic measurements provide the dynamic moduli, which must be corrected to obtain the static moduli (e.g., Tutuncu, 2010).
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