: Biot''s constant has a significant influence on in-situ stress calculation, which is an important input in many petroleum related rock mechanics applications. It is commonly assumed, or empirically derived. Stress-dependent nature of Biot''s constant is given much less attention.

An experimental program was undertaken to evaluate the dynamic as well as static Biot''s constant in Ekofisk chalk reservoir samples, as a function of confining pressure and differential stress in a triaxial cell. Results show that the Biot ’s constant for the chalk is slightly stress-sensitive, with its values decreasing with increasing deviatoric stress. Poisson ’s ratio was found to be more sensitive than the Biot ’s constant, with its values increasing with increasing differential stress. The static values of the Biot ’s constant were found to be greater than those obtained dynamically. From the experimental data, linear models for the variation of the above three parameters were obtained with depletion. Using these models, production induced horizontal stress changes and Ko were obtained for various field depletions.


Biot ’s constant is a poroelastic parameter which plays an important role in petroleum related rock mechanics. Along with Poisson ’s ratio, it directly affects the magnitude of in-situ stress estimates commonly made from sonic logs. The equation commonly used in the petroleum industry to estimate the total minimum in-situ principal stress, for a uniaxial strain boundary condition (no horizontal

movement), is:

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