Radial drilling-fracturing, combining radial drilling with hydraulic fracturing, is an innovative approach to develop unconventional reservoirs. Understanding the fracturing mechanism is essential for the successful design and operation of this technique. In this study, we conduct a series of fracturing experiments to investigate its unique fracture initiation and propagation characteristics. The cube made with cement mortar is used as the test specimen, in which the main well with radial laterals is pre-fabricated. Then, fracturing experiment on the specimen is performed in the tri-axial hydraulic fracturing test system. Two crucial factors are considered, including lateral number and lateral length. We also make a comparison between radial drilling-fracturing and conventional perforation fracturing. The fracture initiation pressure is recorded and the finial fracture geometry is analyzed. Results show that the fracture initiation pressure of radial drilling-fracturing is much lower than that of perforation fracturing; and with the lateral number increasing, fracture initiation pressure decreases monotonously. In addition, the radial laterals drilled from the main wellbore can guide the propagation of multi fractures, forming planar fractures, and the fracture reorientation controlled by in-situ stress is reduced. Therefore, the radial laterals drilled from the main wellbore can guide the propagation of fractures, and enlarge stimulated reservoir volume.
Hydraulic fracturing is the key technology for developing unconventional reservoirs (Vengosh et al. 2014, Ren et al. 2016, Tian et al. 2016, Liu et al. 2018a). There are two challenges in the field experience using this approach. One challenge is that the fracture initiating pressure is extremely high, enlarging the operation difficulty (Vengosh et al. 2014, Tang et al. 2017, Rui et al. 2018). The other challenge is that the geometry of hydraulic fracture is complex (Dahi-Taleghani and Olson 2011, Yang et al. 2016, Yang et al. 2017), which plays a significant negative influences on well performance (Xu et al. 2017a, Xu et al. 2017b, Ren et al. 2017). Radial drilling-fracturing, combining radial drilling and hydraulic fracturing, is an innovative approach to remedy the above problems (Liu et al. 2017a, Guo et al. 2018). As shown in Fig. 1, the radial borehole is drilled from the main wellbore; fracturing fluid is injected through the radial borehole to the formation; and fracture initiates and propagates along the radial borehole. There have been several practical applications in low permeability (Li et al. 2000), aged (Cinelli and Kamel 2013) and coalbed methane reservoir (Megorden et al. 2013), with good stimulation effectiveness. As the length of radial borehole is much longer than that of perforation, multiple fractures initiate from the radial borehole simultaneously during the fracturing process. Therefore, it is very challenging to analyze the fracture initiation and propagation mechanism, which is essential for its successful design and operation (Wu and Olson 2015, Feng and Gray 2018).