Measure of Rock Mechanical Properties from Scratching Test
Coudyzer, Christophe1, Edmond Poyol2, Fabrice Dagrain3 (1) Epslog SA, Belgium (2) Total, Pau, France (3) Faculté Polytechnique de Mons, Mons, Belgium
The present paper presents a portable apparatus, the Material
Strength Logger (MSL), and methodology to assess strength of rock from ”partially-destructive” scratching tests.
The MSL prototype measures vertical and horizontal components of
force applied to a cutter while making a groove at constant depth on the
surface of a rock specimen with sharp or blunt tool.
Experiments indicate that rock cutting is associated with a
“ductile” or a “brittle” mode of failure depending on depth of cut. The ductile
mode takes place at shallow depth of cut and is associated with plastic flow,
while brittle mode occurs above a threshold depth of cut and is characterized
by tensile crack propagation.
In ductile mode, forces mobilized on the cutting face are found to
be proportional to the cross-sectional area of the cut. Experiments on several
sedimentary rocks show that the coefficient of proportionality, referred to as
the intrinsic specific energy Â, is well correlated to material uniaxial compressive strength, q.
Furthermore, the friction coefficient Ì mobilized along the wear
flat of blunt cutter is found to be well correlated to the internal friction
angle of the rock.
The high spatial resolution and precision of the force
measurement provides fine logging of the two rock properties (Â, Ì) and thus
permit to isolate weak or damaged intervals, and capture heterogeneities or
anisotropies.
The paper
presents a particular petroleum application on core analysis where understanding
the vertical variability of rock properties along a reservoir tickness is of primordial importance for sanding prediction
analysis.