Research on the fracture propagation mechanism of multi-directional radial boreholes fracturing

Radial boreholes fracturing is a kind of reservoir stimulation technique, which can guide fractures to extend directionally in multiple directions and realize multi-fracture stereoscopic reconstruction of reservoirs. Current research on radial boreholes fracturing mainly focuses on single, single row or double row radial boreholes, but investigations into the multi-directional radial borehole fracturing remain relatively sparse, its fracture propagation mechanism as well as stereoscopic reservoir stimulation effect are still unclear. This paper established a fracture propagation model of multi-directional radial boreholes fracturing based on the discrete lattice method, and the fracture propagation mechanism and the stereoscopic reservoir stimulation under effect of multiple parameters are analyzed. The results indicate that through multi-directional radial boreholes fracturing, multiple main fractures extending deep into the formation can be formed, which can realize multi-fracture stereoscopic reservoir stimulation. The radial borehole azimuth should be set at 45° and the inter-borehole angle at 90°, at which time the fracture area is the largest, the fracture propagation is the most uniform, and the effect of stereoscopic reservoir stimulation is the best. As the radius increases, the fracture area increases, the area difference coefficient decreases significantly, and the effect of radial boreholes to guide the directional propagation of the fracture increases; if the radial boreholes spacing is too large, the fracture may show a nonunion state and the effective fracture cannot be formed, so the spacing should be ≤ 1 m; the multi-directional radial boreholes fracturing is more suitable for the formation with low level stress difference, but it still has a certain degree of stimulation effect under high stress differential conditions. As elastic modulus increases, Poisson's ratio decreases, the degree of fracture uniformity decreases, the fracture complexity increases, and the radial boreholes have a weakened role in guiding fractures. Within the range allowed by the field operation, appropriately increasing the fracturing fluid injection rate and viscosity is conducive to enhancing the guiding effect of radial boreholes on fractures and improving the effect of stereoscopic reservoir stimulation.