Hydraulic fracturing experiments were conducted on coal blocks in the laboratory to investigate the effect of stress variation on hydraulic fracture propagation through a coal formation.

The first part of this study presents the measurements of mechanical properties of coal samples, which include Young's modulus, Poisson's ratio, tensile strength, compressive strength, shear strength, and fracture toughness. Mohr envelopes were determined to obtain the failure characteristics for the coal used in this study. A relationship between Young's modulus and confining pressure was derived. The physical explanation of this relationship is given.

The second part of this study presents the experimental results of hydraulic fracture propagation through coal blocks. Two groups of experiments are presented; unconfined and confined tests. This paper discusses observations and results which ranged from multiple parallel fracture systems when the blocks were unconfined, to single fractures created when a confining stress was applied. Laboratory conditions were designed to investigate the creation of T-shaped fractures in the coal, and to study the treating pressure response during the creation of these unconventional fracture shapes.

Treatment pressure response during these tests was significantly different from those observed when fracturing sandstone blocks in the laboratory. The factors which govern this different type of pressure response (roughness, fracture tortuosity, cleat system, spalling, etc.) will be described in detail. Also, a comparison is drawn which relates the observations made during this laboratory testing to actual field reports made during mineback experiments.

New fracture width, height, and initiation criteria related to coal seam fracturing, are presented in this paper. These criteria are based on close examination of the rock properties of coal under different stress-loading conditions.

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