Introduction

Borehole stability technology is an effective method of finding desired mud weight. Experience has shown that by getting drilling fluid weight right first, gives greater flexibility to optimize the mud system to meet all objectives at least cost.  Stability technology though strictly desires composite impact on formation resulting due to variety of reasons, operating on Earth, e.g. mechanical, porous, chemical and thermal. But on practical consideration, mechanical phenomenon is given the maximum credence. This is because mechanical phenomenon appears most dominant, relatively easy to estimate, and to an extent well understood.

The constitutive behavior of sound propagation and its relation to mechanical properties of the formation has opened up wire-line logs utilities to work out effects on mechanical properties, over a range of stresses that would operate at different mud weights.

Mechanical properties of the formations are derived from longitudinal and shear slowness, in conjunction with formation density values. These are wire-line derived parameters. These elastic properties (often referred as Geo-mechanical properties) are empirically proportional to absolute values, when measured on core / cutting in the laboratory, and/or in the field employing engineering measurements.