Dynamic Strength of Oil Shale
Authors: Lankford J. 
Published: December 2007

In-situ retorting of oil shale requires explosive loading under overburden pressure to break up rock masses. Therefore, a study of the dynamic. confined failure strength under compressive loading was carried out for shale ranging in kerogen content from 11 to 45 gal/ton. It was found that the envelope of ultimate strength could be described by a first-order failure criterion that expands uniformly in principal stress space about the hydrostatic axis as the strain rate increases. The strength/log-strain-rate dependence was found to be non linear, with the strength doubling over seven orders of magnitude in strain rate. In these laboratory tests, considerable ductility at failure was encountered, with the strain at failure ranging from 2 to 35 percent. Failure strength was ordered consistently with respect to kerogen content, hut the strength-reducing presence of large calcite inclusions in the leanest grade of shale overcame the effect of lower kerogen content to the extent that specimens of intermediate richness exhibited the highest strength.

Introduction

Current processes under consideration for retorting oil shale in situ require fragmenting the oil-bearing shale bed. Explosive loading of boreholes is a means of achieving this end. To optimize the fragmentation process, knowledge of the dynamic strength and fracture characteristics of the oil shale is needed. Until recently, the strength properties of oil shale have been studied under rather limited conditions. Schmidt and Schuler and Sellers et al. established the unconfined, uniaxial compressive strength at slow strain rates for Anvil Point, Colo., oil shale. The principal conclusion of both studies is that material properties generally strongly depend on kerogen content, with fracture strength and Young's modulus decreasing and ductility increasing monotonically with increasing kerogen content. The strength of most rock is now known to strongly depend on strain rate, increasing with increasing rate of loading at constant temperature. However, oil-bearing shale as a rock type is not included in any of these studies. The principal goal of this study was to investigate the strain-rate dependence of strength and ductility for oil shale to predict the strength levels required for fracture under dynamic loading. The highest rates obtained under controlled testing (around 10(3)/second) are still lower than that of an explosive impulse.

EXPERIMENTAL PROCEDURES

ROCK DESCRIPTION AND PREPARATION The material used in this study was obtained from the U. S. Bureau of Mines test mine at Anvil Point, Colo. Anvil Point oil shale is a fine-grained Point, Colo. Anvil Point oil shale is a fine-grained (0.0004 to 0.0016 in.) sedimentary rock of variable kerogen content. Kerogen contents for the three blocks from which specimens were cored were 10.7 bbl/ton (lean), 32.0 bbl/ton (medium), and 45.7 bbl/ton (rich), as determined by Fischer Assay. The rich- and medium-grade shales were homogeneous in structure, although the layering of the Kerogen was clearly visible. The lean-grade shale also was layered, but contained numerous lens-shaped, calcite inclusions with typical dimensions of 0.04 × 0.4 in. These inclusions were oriented so that the lens lay parallel to the bedding plane. plane. The cylindrical test specimens were about 1/2 in. in diameter and 1 in. long. They were obtained by core drilling and grinding of the ends to obtain flatness and parallelism within 0.0002 in. Specimens from each boulder were taken from three orthogonal directions: normal to the bedding plane and two mutually perpendicular directions within the bedding plane. Tests with these specimens showed that, plane. Tests with these specimens showed that, while the susceptibility to failure was quite anisotropic, varying by as much as a factor of three for x vs y or z, the ultimate strength was fairly insensitive to orientation. Since ultimate strength was of primary interest, cores perpendicular to the bedding plane were used in subsequent testing. This also was the orientation used by other experimenters studying Anvil Point shale.

SPEJ

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