Experimental study of hydraulic fracture propagation in multi-hole synchronous fracturing in horizontal wells in sandstone

Hydraulic fracture propagation in multi-hole synchronous fracturing plays a critical role in forming complex fracture networks in unconventional reservoirs. However, the propagation mechanism of multi-hole synchronous fracturing is still unclear, especially the effects of the fracturing borehole spacing and natural fracture. In this study, a series of experiments using a triaxial loading system were conducted to investigate the stress shadow effect in multi-hole hydraulic fractures propagation with different borehole spacings and the interaction between hydraulic fractures and natural fractures in specimens during hydraulic fracturing. The results indicated the following: 1) There is an obvious stress shadow effect in multiple fracture propagation, significantly influencing the fracture propagation path, direction and fracture initiation pressure. Hydraulic fracture propagation in multi-hole fracturing tests in specimens with different fracturing borehole spacings is significantly different, and increasing the fracturing borehole spacing can effectively change the fracture propagation path in the interior borehole and reduce the stress shadow effect. Compared to single cluster fracturing, hydraulic fractures in multi-hole fracturing show a single-wing form instead of double-wing propagation at the pre-existing artificial fractures, presenting asymmetrical and elliptical propagation of hydraulic fractures network. (2) The stress shadow effect effectively helps communicate adjacent fractures, resulting in a more complex fractures network. Under the stress conditions of 5/8/12 MPa and specimen size of 300 × 300 × 50 mm, when the fracturing hole spacing is small (less than 50 mm), hydraulic fractures perpendicular to the direction of the minimum horizontal principal stress generate from the external fracturing holes on both sides, and the hydraulic fractures parallel to the direction of the minimum horizontal principal stress generate from the middle fracturing hole. These hydraulic fractures propagate and interconnect, forming a complex fracture network. There is a critical spacing of 50 mm. (3) Under the influence of stress shadow, multiple fractures are more likely to penetrate artificial pre-existing fractures compared to hydraulic fractures in single fracturing hole, and can more effectively connect artificial pre-existing fractures, resulting in more complex fracture shapes.