Stress distribution properties and deformation–fracture mechanisms in hydraulic fracturing of coal

Abstract

The continuous injection of fracturing fluid results in a redistribution of stress in the area affected by fracturing, which promotes the deformation and failure of coal. To reveal the distribution law of water pressure in coal and the deformation properties of the coal matrix induced by fracturing. Based on triaxial compression experiments and hydraulic fracturing physics simulations, we have developed a fracture stress distribution model for cylindrical coal samples by combining the principles of elastic mechanics. The concept of the strain ratio (S) is proposed, and its physical significance is explained. Combining the strain, crack propagation and acoustic emission (AE) energy of the coal sample during fracturing, we have recovered the entire deformation and failure process of columnar coal samples. It was shown that S has a negative logarithmic dependence on the confining pressure and an exponential dependence on the stress difference (Δσ) and fracturing rate (vw). The deformation and failure process of hydraulic fracturing is dominated by injection pressure. At high Δσ or high vw, coal samples deform and fracture along the direction of the minimum principal stress as the water pressure increases in stage III. During the fracture, the injection pressure is linearly and positively correlated with the radial stress but negatively correlated with the axial stress. The stress concentration is more pronounced near the borehole, which is the starting point for damage to coal samples. Hydraulic fracturing increases the complexity and connectivity of fractures, which in turn tend to simplify under the control of the maximum principal stress as vw increases. The AE energy of hydraulic fracturing has a linear and positive correlation with Smax. After Δσ and vw are determined, the deformation and failure process of the cylindrical coal sample during fracture is characterized by the combination of strain, fracture propagation and AE energy.