Simulation Research and Application of Complex Fracture Network for SRV

Abstract

Stimulated Reservoir Volume (SRV) is one of new emerging hydraulic fracturing techniques to develop shale gas, tight sandstone and other unconventional reservoirs. The favorable geological conditions and reasonable fracturing design are critical factors to form complex fracture network that is different from conventional bi-wing fracture. In the reservoir, conductivity and drainage space can be enhanced by the fracture network. At present, numerical simulation of complex fractures is still based on the pseudo-3D model and depend on huge amount of calculation to obtain the fracture network. Therefore, this method has distinct differences in actual propagation and need to be computed intensively. Applying the theory of mechanics of materials and fracture mechanics, the equations of expansion and propagation for natural fracture are derived and the equation of stress shadow is adopted to consider the additional normal stress induced by adjacent fractures. Based on the propagation pressure, the length of branching fracture can be obtained by establishing a novel fracture network model. The model can be solved explicitly through the net pressure. This method can reduce the iterations effectively when many natural fracture must be accounted for the realization of numerical calculation. In order to verify the accuracy of the results, the parameters applied in the treatment are adopted as input for simulation, and the data of microseismic mapping are also used for matching the fracture network. Introduction Hydraulic fracturing has become one of the most important technologies in the development tight oil resources. During the process of reservoir stimulation, how to create more fractures in tight sandstone reservoir becomes the key issue. However, some naturally fractured sand formations have geomechanical properties that allow hydraulically induced discrete fractures to initiate, propagate and lead to a complex fracture network. Many researchers have conducted a series of experiments and numerical simulations to investigate the mechanism of fracture propagation. Also, some key factors which affect the complex fracture network such as natural fracture, horizontal in situ stress difference, fracturing fluid viscosity, and injection rate [1,2,3,4] of fracturing fluid have been investigated. Blanton [5,6]discussed the relationship between induced fracture and natural fracture which displayed that hydraulic fractures cross the pre-existing fractures only under high differential stress conditions and high approach angle. In addition, the stress ratio of [7, 8]maximum principal horizontal stress to minimum principal horizontal stress below 1.5 demonstrated proportionally increasing branching and fracture multiplicity with proportionally decreasing stress orientation. In other words, the hydraulic fractures are more easily to extend along the natural fracture under the low horizontal stress difference [9, 10]. Chen mian and Zhou jian [11,12] used true triaxial hydraulic fracturing test to study the effect of natural fractures on hydraulic fracture propagation, such as stress difference, approach angle. The experimental results [13,14,15]showed that horizontal stress difference and approach angle are the main factors influencing the shear failure. Numerical simulation is another effective way to understand the mechanism of fracture propagation, many researchers have studied the fracture propagating mechanism by using 2D or 3D International Conference on Modeling, Analysis, Simulation Technologies and Applications (MASTA 2019) Copyright © 2019, the Authors. Published by Atlantis Press. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/). Advances in Intelligent Systems Research, volume 168