Nondestructive Core-Strength Tester Uses Steel Ball To Evaluate Hardness
Authors: Wilson Adam 
Published: February 2015

This article, written by Editorial Manager Adam Wilson, contains highlights of paper SPE 158326, ’Nondestructive Strength-Index Testing Applications for Sand Failure Evaluation,’ by Gillian Daniels, SPE, Colin McPhee, SPE, Philip McCurdy, SPE, and Yelitza Sorrentino, SPE, Senergy, prepared for the 2012 SPE Asia Pacific Oil and Gas Conference and Exhibition, Perth, Australia, 22-24 October. The paper has not been peer reviewed.

This paper presents results from a nondestructive core-strength index tester that is less destructive than the Schmidt hammer and less intrusive, easier, faster, and less expensive than the core scratch tester. The portable hardness-index tester measures and compares the impact and rebound velocities of a small steel ball after its collision with a rock surface to determine rock hardness, which, in turn, reflects the relative strength of the rock.

Theory and Application of the Index Tester

The index tester (Fig. 1) is a hand-held, electronic, battery operated, spring-loaded device that provides an indirect method to predict rock strength. The Leeb hardness unit is calculated by comparing the impact (Vi) and rebound (Vr) velocities of a small steel ball after its collision with a rock surface (Fig. 2). The impact body rebounds faster from harder rocks than from softer ones, resulting in a higher energy quotient, which, in turn, reflects the relative strength of the rock. Features of the index tester include a liquid-crystal display of the results, internal data storage, and a connection for download of data for future analysis and reporting.

Advantages of the index tester include

It is portable, for use in the field or core store.

It can be set to compensate for angle of impact.

Repeatability is good.

Little damage occurs at the surface of the core because of the low impact energy.

It is, therefore, less destructive than the higher-impact Schmidt hammer and less intrusive, easier, faster, and less expensive to use than the core scratch tester. Fig. 3 compares the minimal impact left on the surface of the core by the index tester compared with the 10-mm channel cut by the scratch tester.

The index tester is run at regular intervals along the core surface, with the objective of using the data as an aid in sample selection and then to establish a simple equation for estimating rock strength from the Leeb hardness values calibrated to core-measured rock-strength data.

Practical Application of the Index Tester

Before viewing the core, it is recommended that all available log data and depth-shifted routine-core-analysis data be loaded to a petrophysical database to highlight the best-quality intervals with respect to porosity and permeability. There are a number of published and proprietary log/core strength correlations that can be used to develop a continuous rock-strength model in the absence of core data. Most models involve correlations between unconfined compressive strength (UCS) and logs that are sensitive to rock-strength variations—particularly sonic, density, and porosity. All available generic log-based models should be run over the reservoir intervals and displayed.