Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)

First Advisor

Robert Desbrandes


In this dissertation, a general method to determine the vertical effective stress in shales while drilling is developed. The concept is applied to the development of a model restricted to sodium smectitic shales, which are interpreted using Measurement-While-Drilling (MWD) resistivity data. Effective stress is introduced as the key parameter in the evaluation of petrophysical properties of shales, which provides a wide range of applications to the method. The model comprises two interpretation modules: an electrical module converts shale resistivity into porosity using a new formation factor relationship adapted from previous work, and a mechanical module relates porosity to void ratio using the one-dimensional compression theory. This approach eliminates the use of normal trends and allows a true real-time interpretation. Most of the advantages of the new model relative to conventional techniques result from the modular approach, which also leaves room for future improvements. In particular, the electrical module can be replaced by any other algorithm capable of providing shale porosity. Two applications are described. The first application allows the real-time evaluation of pore pressure, which is obtained from effective vertical stress using Terzaghi's relationship. The model lends itself particularly well to field implementation. It proved extremely versatile in a variety of drilling environments, including exploration drilling, and more accurate than conventional methods during field tests. The second application provides in situ shale permeability estimates using correlations between permeability and effective stress. A new experimental procedure is suggested to develop such correlations. The effective stress concept appeared to be a powerful interpretation tool in the study of shales, and it is suggested that generalized "effective stress logs" be developed and used routinely in the evaluation of saturated porous media.