Feasibility of Radial Borehole Fracturing in Geothermal Exploitation: an Experimental Study

Enhanced geothermal system (EGS) is an important way of geothermal development, which takes advantage of the fractures serving as the channels of working fluid flow and heat transfer. But constrained by the geometries of hydraulic fractures formed through conventional fracturing technologies, the heat transfer areas are limited. Radial borehole fracturing combines hydraulic fracturing and radial boreholes which extend to the formation radially from 10 to 100 meters and have diameters of 20 to 50 millimeters. This paper aims to investigate whether radial borehole fracturing can increase the fracture areas in EGS system comparing with perforation fracturing. Nine cubic concretes (300*300*300mm) were cast after mixing sand, cement and water. Six of them contained radial boreholes and three had perforations. All cubic concretes were heated to 200℃ and fractured by a tri-axial fracturing test system with injection rates of 30ml/min and horizontal principal stress differences being 6 MPa. Then the fractures were scanned and the fracture areas were calculated. Three different angles between radial boreholes/perforations and maximum horizontal stress (0°, 45°, 90°) and two quantities of radial boreholes (2, 4) were studied. Experimental results show that radial borehole fracturing creates greater fracture areas then that of perforation fracturing if the orientations of radial boreholes and perforations do not consist with the direction of maximum horizontal stress. Because the fractures turn to the direction of maximum horizontal stress more quickly for perforation fracturing when perforations and radial boreholes have identical angles, namely radial boreholes guide the fractures better as they extend into the concretes. Besides, concretes with 4 radial boreholes have smoother fractures than concretes with 2 radial boreholes. In addition, the breakdown pressure of radial borehole fracturing is lower and increasing the quantities of radial boreholes reduces the breakdown pressure. This experimental investigation reveals that radial borehole fracturing can form larger fracture areas than perforations fracturing, which promotes the efficiency of heat extraction in EGS system.