Rock-Strength Data From Cores and Logs
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 121972, "Rock Strength From Cores and Logs: Where We Stand and Ways To Go," by A. Khaksar, P.G. Taylor, Z. Fang, T. Kayes, A. Salazar, and K. Rahman, SPE, Helix RDS, prepared for the 2009 SPE Europec/EAGE Annual Conference and Exhibition, Amsterdam, 8-11 June. The paper has not been peer reviewed.
Accurate rock-strength knowledge is essential for in-situ stress estimation, wellbore-stability analysis, sand-production prediction, and other geomechanical applications. When sufficient rock-strength core data exist, computing techniques, such as fuzzy logic and cluster-pattern recognition, coupled with sedimentary-facies analysis and diagenetic classification, can improve strength estimation. Semicontinuous impact-energy logs from portable nondestructive testing tools can be correlated with petrophysical logs to generate mechanical facies and improved sampling for conventional rock testing pattern.

Introduction
Rock mechanical properties are essential for accurate in-situ stress analysis and geomechanical evaluations including wellbore-stability analysis, sand-production prediction and management, hydraulic-fracturing design, fault-stability and -reactivation analysis, and other geomechanical applications. The rock mechanical (strength) parameters typically required to populate a geomechanical model that is based on the linear Mohr-Coulomb failure criterion are unconfined compressive strength (UCS), friction angle or coefficient of internal friction, and thick-wall- (hollow) cylinder (TWC) strength, which may be needed for sanding evaluation and calibration.
Laboratory-based rock-strength values typically are determined with triaxial tests on cylindrical samples that are cut from cores obtained at depths of interest. Continuous profiles of rock strength vs. depth can be estimated by use of well logs and empirical core/log relationships. In most cases, core-strength databases are limited, discontinuous, and biased toward stronger intervals. Quality core plugs of nonreservoir formations (e.g., mudstones and shales), where most hole-instability problems occur, are rarely available for testing. The full-length paper outlines good practices for obtaining quality rock-strength data from core tests then presents common empirical rock-strength equations and discusses ways to improve rock-strength estimates.

Rock Strength From Core Tests
Often, samples for rock-mechanics testing are taken only after the initial slab cut is made. This process reduces the core diameter and, hence, the plug length, usually making the length/diameter ratio unacceptable. It is even worse when no samples for rock-mechanics tests are cut and the core is allowed to dry out and, possibly, deteriorate, depending on mineralogy.
The rock-mechanics program must be planned well in advance of the core being cut because many interests must be accommodated and potential pitfalls overcome. Good communications between different departments will help streamline the process, maximize the benefits of geomechanics, and avoid repeated work.