The Scratch Test: An Attractive Technique for Determining Strength and Elastic Properties of Sedimentary Rocks

Abstract

The Scratch Test is a relatively new technique for determination of mechanical properties of rocks. In a Scratch Test, the surface of the rock is scratched at constant depth (typically less than 1 mm) by a sharp cutter, while the applied forces are being monitored. It is found that these forces are closely related to the mechanical properties of the rock. The Scratch Test thus represents a direct measure on the core material, and provides continuous coverage of data for the entire length of available core material. The work reported here is a detailed study of the Scratch Test as a technique for determining strength and elastic properties of sedimentary rocks. The work is based on extensive laboratory testing of many sedimentary rocks with different mechanical properties. The results of the study show that parameters obtained in a Scratch Test, in particular the Specific Energy, correlate very well with the Uniaxial Compressive Strength (UCS). The accuracy of the Scratch Test for rock strength determination is seen to be at least comparable to the accuracy of the UCS Test, while the resolution is even better. It is also found that the Scratch Test may be used to determine the elastic modulus of rocks with good precision. The Scratch Test only requires access to a free surface of the rock. Hence, it may be run on most available core material. Provided that the core is in a reasonably good shape, no special preparation is required for the test, which is thus both quick and cheap. Unlike the UCS Test, the Scratch Test is almost non-destructive, and provides continuous data coverage. The Scratch Test is therefore a very attractive method for determination of stiffness and strength of core materials when addressing issues like reservoir compaction, hydraulic fracturing, borehole stability and sand production, offering a better resolution and data coverage than any other technique available today. Rock mechanical parameters derived from wire-line log data are continuous but have the disadvantage of being derived indirectly from other measurements, such as sonic velocity, density and porosity. Introduction Rock mechanical parameters of underground formations are required when addressing issues involving reservoir compaction, hydraulic fracturing, borehole stability and sand production. These parameters are primarily obtained along reservoir sections, even though data from the overburden also are needed in many applications. Laboratory measurements of field cores provide a direct determination of these parameters, but they yield only information at a limited number of locations along the wellbore, since the test methods require a significant amount of material. Rock mechanical parameters derived from wire-line log data are, on the other hand, continuous, but have the disadvantage of being derived indirectly from other measurements, such as sonic velocity, density and porosity. The scratch test may solve some of the problems related to laboratory measurements on field cores and wire-line logging tools: It is quick, cheap and continuous, requires significantly less rock material than ordinary laboratory testing for rock characterisation, and represents a direct measurement of rock mechanical parameters. Laboratory scratch measurements on field cores have the potential of increasing the amount of rock mechanical data from cores, since the test technique is continuous.

Relations between static and dynamic moduli of sedimentary rocks

“…Rock stiffness may vary a lot from one location to another, on all length scales. Even within the few centimetres covered by a core plug from a uniform formation, there may be large variations in stiffness (Schei et al 2000). Within 100 m range corresponding to the wavelength of a seismic wave, even larger variations will normally occur due to the presence of different rock types.…”

An Experimental Study to Reduce the Fracture Pressure of High Strength Rocks Using a Novel Thermochemical Fracturing Approach

“…The word intrinsic emphasizes that this energy is strictly used to cut the rock. A strong correlation between intrinsic specific energy and uniaxial compressive strength for various rocks is well established [48,49].…”

“…The principle of the scratch test is to control and monitor the continuous shearing action induced by the motion of a 3 Geofluids diamond (PDC) cutting tool on the surface of a rock core sample [47,48]. The continuous high-resolution profile of rock strength (UCS) along the core sample is derived from the force acting on the cutter.…”

A new approach to evaluate rock drillability of polycrystalline diamond compact bits using scratch test data

“…Richard et al (1998) further demonstrated that the friction coefficient of a scratch test is equal to the intrinsic friction coefficient of the tested rock. Schei et al (2000) analyzed the force acting on a scratch cutter and found that there is a strong correlation between the hardness of rock and scratch measurements. Based on this finding, Schei et al presented a preliminary model for rock hardness evaluation using scratch test results; however, the accuracy of the evaluation model is not very satisfactory.…”