Three-Dimensional Micro Flow-Stress-Damage (FSD) Model and Application in Hydraulic Fracturing in Brittle and Heterogeneous Rocks

Abstract

In order to investigate the hydraylic fracture development of the specimens and simulate the cracks drived by fluid flow in rocks, a flow-stress-damage (FSD) model, implemented with parallel Rock Failure Process Analysis code (parallel -RFPA3D), is presented. The numerical code is based on linear elastic damage mechanics on mesoscopic scale and FEM. For simulating the complete progressive 3D failure and macroscopic mechanical behaviors of rock materials, rock properties such as elastic constants, peak strength, and poisson ratio are randomly distributed to reflect the initial random distributed weakness in mesoscopic scale. The FSD model is used to represent the permeability variation at the two stages, that is stress-dependent permeability for pre-failure and deformation-dependent permeability for post-peak stage of rock at the elemental scale. The results of the simulation with 680,000-element cylindrical rock specimen coincide well with reported experimental results and the process of crack from initiation to the instability extensions is captured vividly. The results and the process indicate that the FSD model works well and parallel-RFPA3D incorporated with FSD model is a valid tool of understanding the physical essence of the evolution of fracture with large-scale elements and fluid flows in rocks.

Experimental Study on the Brittle-Ductile Response of a Heterogeneous Soft Coal Rock Mass under Multifactor Coupling

“…The ductility failure characteristic is reflected; i.e., the test curve has no apparent peak strain and residual strength, which presents a kind of similarity to the rheology of soft rock flow. The coal specimens in the loading process show internal factors (different degrees of heterogeneity m) that vary due to the degradation effect on the coal mass, from coal body crack initiation and propagation, overlapping with the process of heterogeneous coal mass destruction from shear compression to bulging expansion and even fracture failure (2) In the heterogeneous mechanical test curve of the soft coal specimen, there is an upwardly convex critical phase between the initial linear elastic compaction phase and the ductile failure phase. A key point can be determined at this phase which is called the critical inflection point H of the brittle-ductile failure transition.…”

“…A soft coal mass with a complex composition and a cementation structure is a typical inhomogeneous medium and has more complicated mechanical properties compared with other rock masses [1][2][3][4][5][6][7]. In addition, due to the high number of interformational pores, fractures, and microfractures caused by the occurrence of gas, this kind of gas-filled coal mass was found to be damaged to varying degrees [8][9][10][11].…”

Numerical Simulation of Hydraulic Fracturing in Heterogeneous Rock: The Effect of Perforation Angles and Bedding Plane on Hydraulic Fractures Evolutions

“…Flow-stressdamage (FSD) coupling model for heterogeneous rocks that takes into account the growth of existing fractures and the formation of new fractures is established herein. In [10][11][12][13][14][15][16], RFPA 2D 2.0-Flow bases on the following five basic assumptions:…”

Three-Dimensional Numerical Investigation of Coupled Flow-Stress-Damage Failure Process in Heterogeneous Poroelastic Rocks

“…The real stress state of rock mass is triaxial, and we need to confirm this as an irrefutable fact to evaluate the stability of rock engineering. Coupled flow-stress-damage mechanism under triaxial stress states is a vital issue [1,5,[24][25][26][27][28]. Moreover, fracture mechanics are difficult to use for establishing an analytical model and investigating the failure process variation of three-dimensional (3D) cracks, considering the heterogeneous anisotropy and nonlinear features of rocks and complicated constraint conditions [11].…”