Experimental study on the effects of pre-cracks, fracturing fluid, and rock mechanical characteristics on directional hydraulic fracturing with axial pre-cracks

Published on May 1, 2021
· DOI :10.1007/S40948-021-00225-W
Liu Zhenghe1
Estimated H-index: 1
(TUT: Taiyuan University of Technology),
Shaohua Wang (TUT: Taiyuan University of Technology)+ 4 AuthorsYang Dong (TUT: Taiyuan University of Technology)
Source
Abstract
Directional hydraulic fracturing by prefabricating longitudinal (axial) cracks on the sides of boreholes is an effective technique to control hard roof and ensure mining safety. In the present work, a true triaxial hydraulic fracturing experimental device was used to study the influence of prefabricated crack (pre-crack) parameters (including inclination angle and crack length), fracturing fluid viscosity, and mechanical characteristics of rocks on directional hydraulic fracturing with axial pre-cracks. The results show that as the pre-crack inclination angle increases, the fracture initiation pressure gradually increases, and the fracture extension length increases firstly and then decreases. With the increase of the pre-crack length, the fracture initiation pressure declines and the crack propagates over a shorter distance. As the fracturing fluid viscosity increases, the fracture initiation pressure grows. When the viscosity reaches 40 mPa·s, the crack extension length increases remarkably. Moreover, larger fracture toughness results in a higher fracture initiation pressure and shorter fracture extension length. Our research results can help optimize parameters of angle and length of pre-cracks and the viscosity of fracturing fluids based on the characteristics of the surrounding rocks for coal seam roof control.
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Abstract The present paper investigates the effects of CO2-saturation and confining pressures on the fracture toughness of carbon dioxide saturated bituminous coals. Three-point bending experiments were performed with semi-circular coal specimens (SCB) after they were immersed in different CO2 pressures. The experimental results show that the pure modes I and II fracture toughnesses of coal samples gradually decrease with the growth of CO2-saturation pressure. Additionally, the fracture toughnes...
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Abstract The present work develops cohesive zone models (CZM), i.e. cohesion-separation laws, for mode I fractures in different rank coals, including weakly caking coals, gas coals, fat coals, meager-lean coals and anthracite, through disk-shaped compact tension tests. Firstly, the experiments show that with the coal rank rising, the critical crack separation displacements and the degrees of the nonlinearity of the softening function decline gradually. By fitting the experimental data with the f...
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