Geomechanical properties calculated from well logs are called dynamical
properties, and not always correlate well with statical properties
measured on real lab physical core plug samples. However, they yield whole zone
continuous curves (instead of a collection of sampled points at core depths)
and provide useful insights to study a reservoir without using destructive rock
sampling tests (in the best case, the core may not be destroyed, but it is severely
altered, thus not really representing the insitu reservoir conditions).
Some common equations to estimate gemechanical dynamical properties from
well logs are:
GeolOil has 9 builtin function equations and correlations to compute
geomechanical properties from well logs:
 σ_{v} Total vertical stress through the integration of ρ_{b} bulk density log (a change in the slope may suggest gas presence)
 ν Poisson ratio from shear and compressional sonic logs
 G Shear modulus from shear travel time and bulk density logs
 E Young modulus from G and Poisson ratio ν
 K Bulk modulus from shear & compressional sonic logs, and bulk density log
 SPI Sand Production Index from G, and shear & compressional sonic logs
 UCS Unconfined Compressive Strength from E, K, and VSH empirical correlation
 UCS Unconfined Compressive Strength from empirical porosity correlation
 Biot's constant from empirical porosity correlation
The figure below shows a log geomechanics panel settings to calculate vertical stress
Since gas has a lighter density than oil or water, the computation of the
total vertical stress by integrating the bulk density curve can be helpful to detect
gas zones. A change on its gradient (a slight slope change in the log plot) suggests the
presence of gas (in gas state). In cases where there are curves for neutron porosity
(after matrix lithology corrections) and calculated density porosity, the gas crossover
usually corresponds with some minor change of slope or gradient on the total vertical
stress log computed curve:
The figure below shows the computation of Total Vertical Stress and its change of slope (gradient) around a gas zone
The next log shows a geomechanical study that computes if a reservoir cap rock is strong enough to withstand
the stresses of an injection process into a pay zone. The creation of fractures in the cap rock would produce
leaks and environmental problems. More details on this study, and the full workflow of the equations and
functions used are shown in the learnset.
The figure below shows a reservoir cap rock capability to resist the creation of fractures during an injection process
Another interesting geomechanical application is to compute the rock's
Poisson ratio ν and compare it to its fracture porosity.
As expected, the more brittle is a rock with a low Poisson's ratio ν,
the more prone to detect fractures:
The figure below shows in the rightmost track, a relationship between Poisson's ratio ν and fracture porosity

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