Understanding the Variation of Fracture Toughness With Sonic and Other Rock Mechanical Properties—Some Laboratory Scale Studies and Their Implications

Abstract

Abstract

Objectives/Scope:

Fracture toughness is an intrinsic material property expressing a material's resistance to crack propagation, and it is a measure of the energy required to create a new surface in a material. Considering the complicated laboratory test of fracture toughness, this study is an attempt at finding a satisfactory relationship between fracture toughness with acoustic and other rock mechanical properties which can estimated with available techniques.

Procedures

Variety of sandstone rock samples were collected, grain size ranging from fine to coarse grain. They were then subjected to different laboratory scale tests, both destructive and non-destructive. Laboratory tests involved determination of compressional slowness, density, unconfined compressive strength (UCS), tensile strength, brittleness, Sievers' J-value and drilling rate index (DRI). The DRI is assessed on the basis of two laboratory tests, the Brittleness Value S₂₀ and Sievers' J-value SJ. Brittleness Test gives a good measure for the ability of the rock to resist crushing by repeated impacts and Sievers' Miniature Drill Test gives a measure for the surface hardness (resistance to indentation) of the rock. The purpose was to understand and obtain an empirical relationship of fracture toughness with different rock properties.

Observations

Samples tested had a compressional slowness values ranging from 65us/ft to 110us/ft. Experiments on the same samples revealed fracture toughness values varies from 0.3 to 2 MPa-m^1/2. Out of all the laboratory parameters determined, fracture toughness exhibited decent relationship with tensile strength (R²=0.97), compressional slowness (R²=0.49), UCS (R²=0.54) and DRI (R²=0.46). All of these correlations obtained are non-linear. Lowest uncertainty relationship is obtained with tensile strength with coefficient of determination of 0.97.

Analysis proved that fast and stronger rock has larger fracture toughness.

Novel/Additive Information

This study will be helpful to estimation of fracture toughness of sandstones based on their sonic and rock mechanical parameters, which in turn can be utilized for classifying rocks into different categories, understand and model fracture propagation, drill energy requirements, drilling rate prediction, wear rate etc. The correlations developed here are confined to sandstone rock with similar rock properties.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016