Estimation of the state of stress
The stress polygon app allows the estimation of the stress state for a given set of observations from image logs, pore pressure, vertical stress, minimum horizontal stress Shmin, and mechanical properties. It also calculates the possible ranges of maximum horizontal stress, horizontal stress anisotropy, and relative stress magnitudes. The stress polygon can also be interpreted as a model based approach to estimate ranges of Shmax using first principles. The concept was originally introduced by Prof. Mark Zoback from Stanford University. For more details, read his 2007 book "Reservoir Geomechanics" (p.137).
Input data/observations:
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Vertical stress Sv (obtained from the integration of the density log, typically between 1.0 -1.1 psi/ft)
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Pore pressure (hydrostatic pressures are 0.433 psi/ft for fresh water and 0.465 psi/ft for brine)
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Effective UCS (Unconfined Compressive Strength) of your rock. Use empirical relations from the literature to convert your given logs (DT or porosity) into UCS.
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Coefficient of sliding friction (Mu, between 0.6 and 1, typically around 0.6). Lower for higher clay content
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Delta P (mud weight – pore pressure): Additional pore pressure from drilling mud weight.
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Observations from image logs regarding the presence (or absence) of tensile fractures and borehole breakouts (y -yes, n -no, NA - not available)
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Measured minimum horizontal stress (Shmin)
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Units: you can use any pressure or pressure gradient units, as long as you use the same for all parameters (use our Units Converter if needed), All units in the example are psi/ft.
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Depth: for conversion between pressure and pressure gradient units
Assumptions:
Rock tensile strength is zero; thermal stresses are zero; width of borehole breakouts is zero (setting this value to 0 means that no wellbore breakouts are observed).​
Results:
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Stress polygon for a given set of parameters
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Possible horizontal stress anisotropy (as long as input data is consistent) = (Shmax - Shmin)/Shmin * 100
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Minimum and maximum possible values for Shmax
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Minimum and maximum values of Aphi (relative stress magnitudes), a continuous parameters that describes the stress state from normal faulting (Aphi = 0.5), to strike slip faulting (Aphi = 1.5) to reverse faulting (Aphi = 2.5) (Zoback & Kohli, 2019).
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Meaning of different colored lines:
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Tensile failure line (if drilling induce tensile fractures are observed, the stress state is above this line)
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Compressive failure line (if borehole breakouts are observed, the stress state is above this line)
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Measured vertical stress (Sv)
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Measured minimum horizontal stress (Shmin)
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Possible stress state for given input parameters: Normal faulting (N), strike slip (SS), or reverse faulting (R) (or combinations, depending on where the green line falls relatively to the areas of the polygon)
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Not designed to work on mobile devices.
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