The Poisson?s ratio is a frequently used parameter in rock mechanics, often appearing in formulas derived from linear elasticity. Although soft rocks are not obeying linear elasticity, such formulas may still be used provided that a suitable value for the Poisson?s ratio can be obtained. For a soft rock there is however no unique value for this parameter: it is found to vary largely with stress level as well as amplitude and duration of the stress changes. A suitable value for the parameter can therefore only be found when the application of the parameter is specified in detail. This implies that data obtained in one context can not easily be utilized in another context, and it is thus relevant to raise the question whether the Poisson?s ratio is a useful parameter in rock mechanics.
The Poisson?s ratio is a key parameter in several applications of petroleum related rock mechanics. Many of the formulas in use are derived from the theory of linear elasticity where the Poisson?s ratio is a well defined parameter with a unique value, at least as far as the rocks can be considered to be isotropic. For weak rocks, which are frequently encountered in the petroleum industry, the assumption of linear elasticity ? or even elasticity ? is however often violated. This fact does not necessarily invalidate the use of formulas based on linear elasticity; however it implies that parameters like the Poisson?s ratio are not unique constants for a given material, but rather functions of amplitude, sign and duration of stress or strain increments as well as the current stress state and the stress history. Ideally, the relevant value for the Poisson?s ratio may be derived from laboratory tests on core material where the loading conditions in the field are recreated as closely as possible. In reality this is rarely possible, often due to lack of core material, and hence parameter values have to be derived from other tests.
Another question is whether the Poisson?s ratio is actually a useful parameter. Clearly, it is a handy tool to obtain simplicity in rock mechanical formulas, and to illustrate relations between different formulas. It is a problem however, that by using this parameter we may indicate the existence of relations that are not real. test