Experimental Observations of Hydraulic Fracture Propagation Through Coal Blocks

Abstract

Abstract 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. Introduction In the last few years, tremendous research efforts have been devoted to the area of methane recovery from coal seams. Methane recovery from mineable coal seams is of interest to the petroleum and coal mining industries. The need to remove methane gas from a coal seam prior to the mining operation to reduce the risk of explosions has been recognized by the coal mining industry for many years. On the other hand, coalbed methane is considered a high quality energy resource which has attracted operators in the petroleum industry. The overall medium of a coal seam is characterized by three interacted systems; coal grains, micropores, and cleats. The primary porosity (micropores) and secondary porosity (macropores or cleats) characterizes the integrated porous medium of a coal matrix. Gas is stored by sorption on the large surface area of coal grains. In general, the cleat system is water saturated. To begin the desorption process, it is necessary to recover this water until the reservoir pressure is lower than the desorption pressure. Coal seams usually have low permeability; therefore, coalbed methane wells are stimulated to achieve commercially acceptable gas production rates. P. 239^