TABLE OF CONTENTS Knowledge of the mechanical properties and the in situ stresses of the subsurface formations is essential in connection with wellbore stability problems, fracturing operations, subsidence problems and sand production evaluation.
In this chapter we will look at methods for estimating mechanical properties and in situ stresses from field tests and field data. The methods can be grouped into three main categories: measurements while drilling, wireline logging methods and well tests.
We start by recalling the mechanical parameters and in situ stresses of a formation:
Elastic parameters. For an isotropic medium, there are two independent elastic moduli. However, these two parameters have a real and an imaginary part, that both will vary with frequency and with stress level, such that even in the isotropic case the use of two parameters is clearly a simplification.
Strength parameters. We have seen that the strength of a material is dependent on the stress level, and that failure criteria that describe actual data normally have at least 2 3 adjustable parameters.
In situ stresses. The in situ stresses are given by the three principal stresses and three parameters giving the orientation of the principal stresses. The stress level determines whether a rock is critically loaded or not. Additionally, the in situ stresses influence both the elastic parameters and the strength parameters.
The most important method for estimation of elastic parameters is acoustic logging, and in particular acoustic wireline logs.
