Multi-stage and multi-well fracturing and induced stress evaluation: An experiment study

Abstract

Multi-stage and multi-well fracturing of horizontal wells are indispensable techniques in developing shale oil and gas. Increasing the stimulated reservoir volume (SRV) is of vital importance for production. However, further research is still required due to the uncertainty of fracture propagation mechanism and the evolution of induced stress. To date, there are few true triaxial hydraulic fracturing physical simulations of large-natural shale samples. By using 13 cubic shale samples (30 × 30 × 30 cm), we investigated the effect of in-situ stress, treatment parameters, and different patterns of multi-well fracturing (staggered or tip-to-tip) on fracture propagation. The pumping pressure data is collected, and the evolution data of the induced stress is measured by a strain gauge which is glued on the rock surface. The fracture geometry is analyzed based on post-fracturing reconstruction, the fracturing data, and the induced stress evolution data. Finally, the suggestions for improving multi-stage and multi-well fracturing treatment are proposed. The results show that: A low overburden stress makes the bedding plane easier to be opened, and it also stops the main hydraulic fracture propagating to the deep reservoir. A high injection rate generates higher net pressure in fracture, and the hydraulic fracture can propagate and cross the bedding plane easily. While using a low injection rate, the liquid can only leak along the bedding plane near the wellbore, leading to a simple fracture geometry. The more fracturing stages, the more complex the fracture network is, and the induced stress data shows an obvious increase. The addition of temporary plugging agent causes a significant increase in pumping pressure and induced stress curves, and the fracture geometry is more complex. Compared with the experiment that has no temporary plugging agent, the induced stress has an increase of 0.7 MPa. In multi-well fracturing experiments, the type of multi-well fracturing (staggered or tip-to-tip) plays an important role on fracture propagation: The fracture complexity in tip-to-tip pattern is significantly lower than that in fracturing staggered pattern because of the “frac-hit”. The longer time interval between treating two wells, the lower the induced stress and the more regular fracture geometry. Further, the fractures that generated in the first well will be reopened by the fluid from the second well when the “frac-connection” occurs between the two wells, causing the induced stress to rise again. The results may provide a reference for the multi-stage and multi-well fracturing design.