Influence of Grain Size Heterogeneity and In-Situ Stress on the Hydraulic Fracturing Process by PFC2D Modeling

Abstract

A modified fluid-mechanically coupled algorithm in PFC 2D was adopted in this article to study the influence of grain size heterogeneity and in-situ stress on hydraulic fracturing behavior. Simulated results showed that the in-situ stress and grain size heterogeneity significantly affect the initiation, growth, and spatial distribution of the hydraulic fractures: (1) the initiation and breakdown pressure are gradually reduced with the increase of the grain size heterogeneity; (2) with increased in-situ stress, the initiation and breakdown pressure increase, and the reduction effect of grain size heterogeneity on the breakdown pressure becomes more obvious; (3) in grain size homogeneous rock, the initiation pressure decreases with increasing in-situ stress ratio, however, the initiation pressure of grain size heterogeneous rock is almost unaffected by the in-situ stress ratio; (4) The in-situ stress ratio and grain size heterogeneity affect the spatial distribution of hydraulic fractures simultaneously. When the in-situ stress ratio is larger than 1, the hydraulic fractures propagate substantially along the direction of the maximum principal stress. When the in-situ stress ratio is 1, the initiation position and extension direction of hydraulic fractures are random and complex fracture networks can easily develop in a grain size homogeneous model.

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

“…where e is the hydraulic aperture; P 2 − P 1 is the fluid pressure difference between the reservoir domains; L represents the length of the flow channel; µ is the fluid dynamic viscosity. Under the action of confining pressure and fluid pressure, the hydraulic aperture e changes with the normal stress σ of the bond, which is described as [53,54]:…”

Numerical Investigation of Mineral Grain Shape Effects on Strength and Fracture Behaviors of Rock Material

“…Rock is a natural heterogeneous material composed of a variety of minerals of different geometries, strengths, and deformation characteristics. Rock heterogeneity can be defined as the uneven changes of the mineral composition and microstructure in the spatial distribution influenced by the diagenesis and tectonics [1]. The macroscopic failure of rock is the gradual evolution of internal microcracks [2][3][4], so microheterogeneity significantly affects the macroscopic mechanical properties of rock [5][6][7][8].…”

Geomechanical key parameters of the process of hydraulic fracturing propagation in fractured medium

“…To simulate the HF process in naturally fractured formations while considering the effect of NFs and multiple fracture interactions in two spaces, numerical models including the Finite Element Method (FEM) (Feng et al, 2015), the extended-FEM (Dahi- Taleghani and Olson, 2011;Zhang et al, 2018a), Cohesive Zone Model (CZM) based on the XFEM (Wang et al, 2018a, b), mixed-FE and the Discontinuous Galerkin Methods (DGM) (Hoteit and Firoozabadi, 2006), Displacement Discontinuous Method (DDM) (Abdollahipour et al, 2015;Behnia et al, 2015;Cheng et al, 2017;Zhang and Jeffrey, 2012), Boundary Element Method (BEM) (Vu et al, 2015), BEM by coupling DDM (Jiang and Cheng, 2018), Displacement Discontinuity Analysis (DDA) (Morgan and Aral, 2015), Distinct Element Method (DEM) by using PFC2D software (Han et al, 2018;Zhou et al, 2017), and hybrid Discrete-FEM (DFEM) (Liu C. et al, 2018) have been proposed. With the development of computing power of computers, the interaction between natural fracture and hydraulic fracture in three spaces was studied.…”