Geomechanics Column

Geomechanics Column is an interbedded microblog with short and easy-to-read articles on geomechanics. All the articles are also provided in a single-page PDF format.


 

Three Main Elements of Geomechanics

January 14, 2016  PDF Version: Three Main Elements of Geomechanics


 

Drilling
Some of the troubling issues that may happen during drilling (source: Aldred et al., 1999)

Have you ever tried to define something that you have spent your life on understanding and exploring it in just one short sentence? If you have, you can understand that it is not an easy job because you need to be abstract and complete simultaneously. Last time I tried to find a an inclusive while simple definition for geomechanics (and everything it deals with) I came up with this sentence :

“Geomechanics studies the mechanical response of rocks to the events that disturb their initial state.

The three main elements of this definition are:

(i) An ‘initial state’ defined by in-situ stresses, ambient pressure and temperature.

(ii) A ‘disturbing event’ that can be either natural (i.e., tectonic movement, magma movement, fluid flow, hydrocarbon generation, etc.) or human-induced (examples from petroleum industry are drilling, completion, fracturing, production and injection, thermal operations).

(iii) Rock’s response to the disturbance:  how it deforms (constitutive behaviour models) and how it fails (failure criteria).

Almost all the work done in all different geomechanical studies is about characterizing these three elements though sometimes it may seem difficult to identify them separately as they are usually intertwined.


Mohr-Coulomb System and Its Historic Legends

January 5, 2016  PDF Version: Mohr-Coulomb System


Coulomb-Mohr

Charles Augustin de Coulomb, a French scientist, probably has the oldest legacy in the history in geomechanics. This is not all, he is probably the most cited person in geomechanics history. His very simple equation to describe failure in soils and rocks is still the most popular failure criterion. Nevertheless, geomechanics was not the first time most of us heard his name. We mostly know him through our high school physics classes on static electricity. Christian Otto Mohr, a German Engineer, is not much less famous than Coulomb in geomechanics and solid mechanics, in general. His ingenious approach to demonstrate stresses by drawing circles in a special coordinate system allowed calculation of stresses after rotation with a simple graphical method. Even today with all the available advanced computational and graphic tools,  several experts still would rather using his circles to show stresses because of their simple power in helping them better understand the stress state and its variations. When combined with the Coulomb failure criterion, Mohr’s circles becomes a very powerful tool for demonstration of failure in soils and rocks, so we usually hear these two names together as Mohr-Coulomb though they have been developed based on two completely separate concepts.



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