Biot’s Coefficient: A Highly Undervalued Geomechanical Parameter
Recently, we reviewed about 90 geomechanical lab reports (from different labs, different formations, different producing companies) containing extensive measurements of all different sorts of mechanical rock properties and we could not find even a trace of Biot’s coefficient measurement. Due to its role in definition of effective stresses, we know that it is unlikely to solve any problem in geomechanics without direct use of poroelasticity theory. Nevertheless, in comparison to other geomechanical parameters (i.e., pore pressure, stresses, elastic and failure properties), Biot’s coefficient seems to be the most undervalued parameter in our studies as we leave it for our back-calculations or lucky guestimations. This does not seem scientifically just as the role of this parameter is not really less than many of the other parameters. So, why don’t we put more effort in measurement of this parameter? Is this because we have a great knowledge of this parameter and we know how to do accurate estimation? Very Unlikely! Or only because it is complicated and expensive to measure? Or maybe because it is not in the routine procedures of the commercial labs that we order our tests to? Is any or all of these reasons good enough to neglect this important parameter and its influence on the results? The answer is simply ‘No’.
Concept of Effective Stress: A very elementary but crucially important concept of rock mechanics initially introduced by Terzaghi and then developed by Biot. Total stresses acting on a fluid-filled are beard by the rock’s two main constituents: a part is taken by fluid leading to pore pressure and the rest by skeleton/grains resulting in effective stresses in the structure. In its general form, changes in one of these components can affect the other one.
Unfortunately, this is not our only issue with Biot’s coefficient. Probably a more fundamental issue is the eligibility of poroelastic models for modeling many of the rocks especially the unconventional plays. Can we really use this theory to model mechanical behaviour of these rocks? Shouldn’t we start investigating other practical theories and solutions?
Maurice Anthony Biot in New York, September 1964 (Source: www.olemiss.edu)