A Method to Quantifiably Relate Unconfined Compressive Strength (UCS) between the Surface and the Subsurface for Mechanical Stratigraphic Applications

Enderlin, Milt ¹; Zahm, Chris ²; Naylor, Dennis ³ (1) Gearhart & Texas Christian University Energy Institute, Fort Worth, TX. (2) Bureau of Economic Geology, The University of Texas at Austin, Austin, TX. (3) Gearhart, Fort Worth, TX.

In recent publications it has been observed that many researchers describe the mechanical part of “mechanical stratigraphy” with rock attributes such as weak, strong, ductile, rigid, brittle, competent, shear modulus, rock bulk modulus, Young’s modulus, yield strength, cohesive strength, and fracture strength. Experience has shown that the translation of rock attributes at the surface (e.g. outcrop, office, and laboratory) and the subsurface is much easier if the transferrable attribute in question is reasonably well defined and can be quantified. Examples of these attributes include porosity, bulk density, and mineralogy. With regard to the mechanical part of “mechanical stratigraphy” rock strength provides workable criteria for evaluation since it is reasonably well defined and easily quantifiable at the surface and in the subsurface.

The goal of this presentation is to demonstrate that the mechanical attribute of Unconfined Compressive Strength (UCS), indexed in pounds per square inch (psi), can easily and inexpensively be estimated on the outcrop, in the core shed, and with the use of well log data, in the subsurface. Our experience has shown that outcrop and core can be systematically quantified using a mini indenter or penetrometer and micro rebound hammers. All tools are light weight (weighing less than a pound), hand-held, and easy to use on both the outcrop and core. For the subsurface, several methodologies will be presented for estimating UCS (psi) from well log data. We feel that the tools presented provide a versatile way to systematically quantify UCS, allows for significant numbers of samples to be analyzed, and provides a transferable mechanical attribute that can be directly applied in the subsurface without an intervening attribute conversion factor.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009