Simultaneous Measurement of Pore and Elastic Properties of Rocks Under Triaxial Stress Conditions
Authors: Wilhelmi Bernhard , Somerton Wilbur H. 
Published: December 2007

Simultaneous measurement of pore and elastic properties of rocks under a wide range of triaxial stress conditions may be made with equipment and methods developed in this paper. Tests were run on three outcrop sandstones at several confining pressures and axial stresses up to approximately 80 percent of failure stress. The greater reduction in porosities and permeabilities occurred in the hydrostatic loading portion of the stress cycle, but substantial reductions in both properties did occur upon application of deviator (triaxial) stress. Triaxial elastic moduli were calculated from strain gauge measurements. Both Young's moduli and shear moduli were found to increase with increasing confining pressure. Poisson's ratios increased with axial stress but showed no consistent change with confining pressure. Only very general relations were observed between pore and elastic properties. The lesser decrease in porosity and permeability at higher stress values is related to increase in rigidity of rocks at higher stresses.

Introduction

Many investigators have studied the effects of hydrostatic stress on the properties and behavior of rocks. A few have studied the effects of triaxial stress fields on individual physical properties of rocks. To our knowledge no one has measured simultaneously pore and elastic properties of rocks under a wide range of triaxial stress conditions. This is the purpose of this work. Rocks exist in the subsurface under what Jaeger refers to as "polyaxial" stress conditions (all principal stresses unequal). When two of the three principal stresses are equal, this is referred to as triaxial loading. The triaxial method of loading is most commonly used in modern testing work because experimental difficulties in measuring physical properties of rocks are greatly reduced over polyaxial testing. Although triaxial loading may not reproduce subsurface conditions precisely, subsurface stress conditions are never known precisely anyway. Triaxial testing would seem to be more realistic than testing under hydrostatic loading (all principal stresses equal) for application of results to subsurface problems. The objectives of this work were threefold:

- to develop methods of measuring simultaneously pore and elastic properties of rocks under triaxial stress conditions,

- to determine whether changes in these properties are of sufficient magnitude to be considered in subsurface calculations and

- to attempt to relate changes in pore properties and elastic properties since they will have been measured at the same time under identical loading conditions.

REVIEW OF EARLIER WORK: PORE PROPERTIES

Several investigators have studied the effects of stress on the physical properties of rocks. Fatt and Davis showed the reduction of permeability of rock with hydrostatic loading. McLatchie et al. and Knutson and Bohor related permeability reduction to compressibility of reservoir rock, again under hydrostatic loading. Wyble, and later Redmond, evaluated the effects of applied radial pressure on conductivity, porosity and permeability of sandstones. In this latter work, no axial stress was applied. Dobrynin reviewed much of the previous work done on the effects of overburden pressure on physical properties of rocks. He concluded that changes in physical properties are controlled to a large extent by pore compressibility of the rock. Several sets of curves developed from empirical relations am presented. These curves permit estimation of change of properties with net overburden pressure, knowing the pore compressibility and porosity of the rock.