ABSTRACT

1 INTRODUCTION

Two commonly used techniques to release trapped methane gas from coal and oil shales are hydraulic fracturing and dynamic fracturing. The first technique cracks open the coal seam by the injection of a pressurized fluid. The second technique makes use of a controlled explosion to generate and extend multiple fractures in an oil or gas formation. In order to control and optimize both of these fracturing processes, it is necessary to predict the onset of crack propagation in the coal seam. One concept commonly used to predict the onset of unstable crack propagation in isotropic materials is the plane strain fracture toughness (KIc). KTC is a material property, and thus is independent of specimen size and crack length. This concept has also been shown to be applicable to anistropic materials (Konish et al. 1972, and Guess et al. 1973). Therefore, the concept of plane strain fracture toughness which is so widely applied to predict the onset of fracture in engineering materials may also be useful in predicting the onset of fracture in geological materials such as coal and oil shales. This paper examines fracture toughness values for coal from three different coal seams. The effect of the anisotropic nature of coal on fracture toughness is also studied both experimentally and analytically.

2 EXPERIMENTS

For determining the plane strain fracture toughness, two different testing procedures were performed. The first procedure was a modified version of the ASTM standard E-399 (Schmidt 1976) and used a three point bend specimen (3PB). Because of difficulties in machining suitable (3PB) specimens an alternate testing procedure was required. The second procedure (Klepaczko et al. 1984) used a wedge to split a compact tension specimen (WLCT). Experimental observations of anisotropic effects on fracture toughness are compared with theoretical predictions based upon the strain energy density theory (Sih 1974) and the maximum normal stress criterion (Schmidt 1980). Because of the theoretical considerations above it is necessary to determine the elastic constants and the ultimate tensile strength of the coal tested.

2.1 Experimental procedure for fracture toughness testing

Fracture toughness specimens were machined from randomly selected lumps of coal approximately 0.30 cubic meters. A water cooled diamond tip circular saw was used to machine each specimen. As stated previously two specimen geometries were used (Figs. 1,2). The nominal size for the 3PB specimen was 150 x 50 x 25 mm with a 25 mm starter crack and the nominal size for the WLCT was a 50 mm cube with a 25 mm starter flaw. Aluminum inserts were placed between the wedge and the notch of the WLCT specimens. A jeweler's saw having a width of 0.46 mm was used to further sharpen the tip of the notch for both specimens. This produced a starter flaw with a root radius of 0.23 mm. The test specimens were placed in a small loading rig and were hand loaded at a rate of 178 N/min by a screw mechanism. The rate of loading was controlled by observing the voltage output of a linear variable differential transformer (LVDT) located across the notch mouth of each specimen. The voltage output of the LVDT was proportional to the notch opening displacement (NOD).

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