Abstract
Hydraulic fracturing can improve the permeability of composite thin coal seam. Recently, characterizing hydraulic fracture (HF) propagation inside the coal seam and evaluating the permeability enhancement with HF extension remain challenging and crucial. In this work, based on the geological characteristics of the coal seam in a coal mine of the southwest China, the RFPA2D-Flow software is employed to simulate the HF propagation and its permeability-increasing effect in the composite thin coal seam, and a couple of outcomes were obtained. (1) Continuous propagation of the hydraulic microcrack-band is the prominent characteristic of HF propagation. With the increment of the injection-water pressure, HF generation in the composite thin coal seam can be divided into three stages: stress accumulation, stable fracture propagation, and unstable fracture propagation. (2) The hydraulic microcrack-band propagates continuously driven by the fluid-injection pressure. The microcrack-band not only cracks the coal seam but also fractures the gangue sandwiched between the coal seams. (3) The permeability in the composite thin coal seam increases significantly with the propagation of hydraulic microcrack-band. The permeability increases by 1~2 magnitudes after hydraulic fracturing. This study provides references to the field applications of hydraulic fracturing in the composite thin coal seam, such as optimizing hydraulic fracturing parameters, improving gas drainage, and safe-efficient mining.
Highlights
The composite thin coal seam consisting of the layers of coal and gangue is widely distributed in southwest China
With the increment of the injection-water pressure, hydraulic fracture (HF) generation in the composite thin coal seam can be divided into three stages: stress accumulation, stable fracture propagation, and unstable fracture propagation
This study provides references to the field applications of hydraulic fracturing in the composite thin coal seam, such as optimizing hydraulic fracturing parameters, improving gas drainage, and safe-efficient mining
Summary
The HF propagation law depends on many factors, such as the in situ stresses (confining pressures), rock properties, the distribution of weak plane ahead of the fracture extension path, and fluid-injection parameters. In RFPA, the damage mechanics model between elements can be applied to simulate the microcrack zone development surrounding HF, i.e., the real fracturing characteristics of HF extension; further, the permeability evolution with HF propagation in the thin coal seam can be better delineated. The research results can guide the field construction of hydraulic fracturing in the mining process of coal seam with multilayer gangue in southwest China and even around the world and provide new ideas and methods for the study of fracture propagation rule through the bedding plane
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