Mechanical properties of rocks: Pore pressure and scale effects

Pore pressure plays a major role when considering rocks mechanical properti es. rn that field, the concept of effective pressure is a key one to deal w ith fluids mechanical effects. However, its frequent use has been the sourc e of frequent confusing statements. Because of the various meanings which h ave been attached to that concept, an attempt is made in this paper to clar ify it and examine the validity of its various uses relative to rock mechan ical behaviour or rock properties, At a macroscopic scale, thermodynamics p rovides a powerful tool to investigate this. Reversible or irreversible the rmodynamics provide general relationships of great interest. But because re al rocks are non homogeneous systems, a microscopic approach is also requir ed in order to analyze the mechanical properties from a description of the small scale processes. The microscopic approach is complementary of the mac roscopic thermodynamic one as it leads to the calculation of the effective properties of the medium. In this last approach, effective medium theory is a powerful tool. The effective properties as derived from the microscale c an be nicely combined to thermodynamic relations to interpret pore fluid pr essure effects and scale effects. The example of elastic properties of poro us rocks is more specifically emphasized to illustrate this because of both its intrinsic interest and importance as far as applications are concerned .