Study on influence of natural fractures on initiaition and propagation of hydraulic fracturing coal

Abstract

Hydraulic fracturing is one of the effective methods to improve the extraction efficiency and gas production of low-permeability coal seams with high gas content. Due to the existence of abundant natural fractures in the coal seam, the interaction between natural fractures and hydraulic fractures will make the initiation and propagation of hydraulic fractures show complexity and diversity, thereby affecting the effect of permeability enhancement of coal seam. In order to study the initiation and propagation of hydraulic fractured coal under the action of natural fractures, hydraulic fracturing tests on coal were carried out by using the self-developed multifunctional true triaxial fluid-solid coupling test system. Based on the theory of “pore wall stress concentration induced tensile fracture”, the fractures initiation law of hydraulic fractured coal was studied, and combined with fracture mechanics, the fracture propagation mechanism of hydraulic fractured coal was revealed from a mesoscopic perspective. The results shown that, the presence of natural fractures can induce hydraulic fractures to propagate along the direction of natural fractures, which greatly reduced the initiation pressure of coal. The required fracture initiation pressure for fracturing coal without obvious natural fractures was similar to the theoretical initiation pressure calculated according to the initiation criterion of tensile failure, which met the criterion of “tensile fracture induced by stress concentration in hole wall”. The required fracture initiation pressure for fracturing coal with obvious natural fractures was all less than the theoretical fracture initiation pressures calculated according to the initiation criterion of tensile failure, and the required fracture initiation pressure was smaller when the natural fracture was perpendicular to the direction of the minimum principal stress, which was 3.355 MPa. When the natural fracture was parallel to the minimum principal stress, the required initiation pressure was only greater than the minimum principal stress, which was 7.902 MPa. When the hydraulic fracture type was type I, the measured minimum and maximum propagation pressure values of coal without obvious natural fractures were greater than the theoretically calculated minimum and maximum propagation pressure values, and the difference ranges were 2.043−6.845 MPa and 3.951−8.576 MPa, respectively. When the coal contained obvious natural fractures and the direction of the natural fractures was parallel to the X direction, the existence of horizontal stress difference will cause the measured propagation pressure value of coal to be less than the theoretically calculated propagation pressure value. When the hydraulic fracture was type II. or type I-II., the pressure required for fracture propagation gradually decreased with the increase of the hydraulic fracture length.

Keywords

• natural fractures
• in-situ stress
• hydraulic fracturing
• cracks propagation
• nanoindentation