Study On Shear & Tensile Activation Of Hydraulic Fracturing In Natural Fractured Reservoirs
Authors: Zhang Yang , Qiao Yan , Zhou Fujian , Ren Dengfeng , Liu Yuzhang , Bai Jie 
Published: October 2023

The most fundamental purpose of reservoir stimulation is to maximize stimulation volume. For reservoirs developed with natural fractures, the maximization of fracture activation (tensile & shear activation) area should be pursued. Many scholars have studied the mechanical conditions of natural fractures activation, and used stress shadow to optimize cluster spacing. However, there is a lack of in-depth research on fracture propagation law, relationship between fracture activation and stress shadow, and factors affecting natural fracture activation.

In this paper, numerical simulation methods are used to establish a true three-dimensional hydraulic fracturing model for reservoirs with natural fractures, and the quantitative evaluation indicators of stress shadow area and activation degree of natural fractures are formed. By studying the effects of different natural fracture properties, mechanical and engineering parameters on fracture propagation, stress shadow and natural fracture activation, the main controlling factors affecting fracture propagation and turning, the relationship between fracture activation and stress shadow, and changing rules of fracture activation are clarified. Finally, the fracture turning and accuracy of model are verified by microseismic monitoring data.

Studies have shown that large-scale, high-density natural fractures, high net pressure, low in-situ stress difference, and moderate stress-fracture angle (30°~60°) can easily divert the propagation of hydraulic fractures instead of expanding in the horizontal maximum principal stress (Shmax) direction. Fracture tensile activation has a strong positive correlation with stress shadow area, that is, tensile activation leads to the increase of stress shadow area, while fracture shear activation has almost no correlation. Fracture tensile & shear activation area is in a competitive relationship, and shear fractures are dominant when the stressfracture angle is 45-60°, the in-situ stress difference and pore pressure are high. The main controlling factors affecting activated fractures total area are geological factors and mechanical parameters, which are positively correlated with natural fracture density, dip angle, in-situ stress difference, pumping volume, pore pressure, negatively correlated with friction angle and cohesion, and have optimal values with fracture size (60m), stress-fracture angle (45°) and net pressure. Micro-seismic data show that when natural fractures density is high, the hydraulic fractures propagation direction deflects and doesn't expand in the horizontal maximum principal stress direction, which further verifies the accuracy of the method and conclusions.

There is no obvious correlation between natural fracture activation area and stress shadow. It is not applicable to optimize cluster spacing of reservoirs with natural fractures only by stress shadow. The natural fracture activation area should be used as an indicator for measure optimization. In addition, this paper provides a reference for the selection of reservoir stimulation horizons, the formulation of working systems, and the realization of large hydraulic fracture diversion.