Experimental investigation on the breakdown pressure and fracture propagation of radial borehole fracturing

Abstract Radial borehole fracturing is a technology combing hydraulic fracturing and slender boreholes which are radially drilled from the main wellbore by water jets. This paper aims to investigate how the radial boreholes influence the fracture extension when they are pressurized by the fracturing fluids. Concrete blocks containing radial boreholes were cast, the mechanical properties of such blocks were tested, and these samples were hydraulically fractured through a laboratory tri-axial fracturing apparatus. Experimental results show that increasing the azimuths of the radial boreholes enhances the breakdown pressures, but it will reduce the fracture extension distances (deviation distances) paralleling to the radial borehole axes. Similar trends appear when the horizontal stress differences diminish and the distances between adjacent layers of radial borehole axes increase. When changing the numbers of the radial boreholes, the variations of the breakdown pressures depend on the layouts of the radial boreholes. Besides, increasing the pump rates lifts the breakdown pressures and enhances the deviation distances. Compared to perforation fracturing, radial borehole fracturing has higher breakdown pressures and stronger control to fracture propagation. It is concluded that the extrusion forces exist in the horizontal zones between the radial boreholes. A conceptual model is proposed which can explain the variations of the deviation distances. When the injection time of the fracturing fluids is identical, the higher breakdown pressures will decrease the deviation distances because of the extrusion forces. There is a balance between the lower breakdown pressures and the higher pore pressures both caused by the more penetration of the fracturing fluid, which can either increase the deviation distances or reduce the deviation distances.