Numerical Investigation of Injection-Induced Fracture Propagation in Brittle Rocks with Two Injection Wells by a Modified Fluid-Mechanical Coupling Model

Song Wang,Luqing Zhang,Jian Zhou,Zhenhua Han

doi:10.3390/en13184718

Abstract

Hydraulic fracturing is a key technical means for stimulating tight and low permeability reservoirs to improve the production, which is widely employed in the development of unconventional energy resources, including shale gas, shale oil, gas hydrate, and dry hot rock. Although significant progress has been made in the simulation of fracturing a single well using two-dimensional Particle Flow Code (PFC2D), the understanding of the multi-well hydraulic fracturing characteristics is still limited. Exploring the mechanisms of fluid-driven fracture initiation, propagation and interaction under multi-well fracturing conditions is of great theoretical significance for creating complex fracture networks in the reservoir. In this study, a series of two-well fracturing simulations by a modified fluid-mechanical coupling algorithm were conducted to systematically investigate the effects of injection sequence and well spacing on breakdown pressure, fracture propagation and stress shadow. The results show that both injection sequence and well spacing make little difference on breakdown pressure but have huge impacts on fracture propagation pressure. Especially under hydrostatic pressure conditions, simultaneous injection and small well spacing increase the pore pressure between two injection wells and reduce the effective stress of rock to achieve lower fracture propagation pressure. The injection sequence can change the propagation direction of hydraulic fractures. When the in-situ stress is hydrostatic pressure, simultaneous injection compels the fractures to deflect and tend to propagate horizontally, which promotes the formation of complex fracture networks between two injection wells. When the maximum in-situ stress is in the horizontal direction, asynchronous injection is more conducive to the parallel propagation of multiple hydraulic fractures. Nevertheless, excessively small or large well spacing reduces the number of fracture branches in fracture networks. In addition, the stress shadow effect is found to be sensitive to both injection sequence and well spacing.

Highlights

With the rapid growth of the global economy, the conventional oil and gas resources with decreasing production cannot meet the need for energy
Plentiful models of contact bonds built into PFC including the Parallel-Bond Model (PBM), the Smooth-Joint Model (SJM), the Flat-Joint Model (FJM) and the Hertz Contact Model (HCM)
After constructing the fluid networks and adding the fluid parameters in the PBM, it was still vital to verify whether the fracturing simulation could describe the realistic fracturing behaviors, including the breakdown pressure and the propagation of hydraulic fractures

Summary

Introduction

With the rapid growth of the global economy, the conventional oil and gas resources with decreasing production cannot meet the need for energy. Unconventional energy resources such as shale gas, shale oil, gas hydrate, and dry hot rock geothermal energy are quite abundant, and their economically and technically feasible exploitation is an effective solution to the problem of future. Energies 2020, 13, 4718 energy shortages [1,2,3,4]. Mastering the mechanism of hydraulic fracturing technology is crucial to building a complex fracture network system and forming a high-yield unconventional energy reservoir

Methods

Results

Conclusion