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.