Experimental Study on Stress-Dependent Nonlinear Flow Behavior and Normalized Transmissivity of Real Rock Fracture Networks

Qian Yin,Richeng Liu,Haijian Su,Hongwen Jing,Liyuan Yu,Guowei Ma

doi:10.1155/2018/8217921

Abstract

The mechanism and quantitative descriptions of nonlinear fluid flow through rock fractures are difficult issues of high concern in underground engineering fields. In order to study the effects of fracture geometry and loading conditions on nonlinear flow properties and normalized transmissivity through fracture networks, stress-dependent fluid flow tests were conducted on real rock fracture networks with different number of intersections (1, 4, 7, and 12) and subjected to various applied boundary loads (7, 14, 21, 28, and 35 kN). For all cases, the inlet hydraulic pressures ranged from 0 to 0.6 MPa. The test results show that Forchheimer’s law provides an excellent description of the nonlinear fluid flow in fracture networks. The linear coefficient a and nonlinear coefficient b in Forchheimer’s law J=aQ+bQ2 generally decrease with the number of intersections but increase with the boundary load. The relationships between a and b can be well fitted with a power function. A nonlinear effect factor E=bQ2/(aQ+bQ2) was used to quantitatively characterize the nonlinear behaviors of fluid flow through fracture networks. By defining a critical value of E = 10%, the critical hydraulic gradient was calculated. The critical hydraulic gradient decreases with the number of intersections due to richer flowing paths but increases with the boundary load due to fracture closure. The transmissivity of fracture networks decreases with the hydraulic gradient, and the variation process can be estimated using an exponential function. A mathematical expression T/T0=1-exp⁡(-αJ-0.45) for decreased normalized transmissivity T/T0 against the hydraulic gradient J was established. When the hydraulic gradient is small, T/T0 holds a constant value of 1.0. With increasing hydraulic gradient, the reduction rate of T/T0 first increases and then decreases. The equivalent permeability of fracture networks decreases with the applied boundary load, and permeability changes at low load levels are more sensitive.

Highlights

Rock fracture networks constitute the main pathways of fluid flow and solute migration in deep underground projects, and during the past several decades, substantial efforts have been devoted to the estimation of fluid flow behavior and transmissivity of fractures in many geoengineering and geosciences such as underground tunneling [1,2,3], CO2 sequestration [4, 5], geothermal energy extraction [6,7,8], and hazardous wastes isolation [9,10,11]
We considered the average of joint roughness coefficient (JRC) values for a series of 2D profiles along a fracture surface in the length direction, which is a suggested method by the International Society for Rock Mechanics and Rock Engineering (ISRM) to calculate the JRC of a 3D single fracture [38]
The results indicate that JRC values of the fractures fluctuate within a very small range around 3.47

Summary

Introduction

Rock fracture networks constitute the main pathways of fluid flow and solute migration in deep underground projects, and during the past several decades, substantial efforts have been devoted to the estimation of fluid flow behavior and transmissivity of fractures in many geoengineering and geosciences such as underground tunneling [1,2,3], CO2 sequestration [4, 5], geothermal energy extraction [6,7,8], and hazardous wastes isolation [9,10,11]. The fluid flow in rock fractures is commonly assumed to follow the cubic law, in which the flow rate is linearly proportional to the pressure gradient [12,13,14]. When the flow rate/hydraulic head difference is large, deviation from the linear Darcy law may occur. In such case, the conductivity of the fractures calculated using the cubic law will be overestimated [15,16,17]. Natural rock fractures are often characteristic of rough walls, intersections, and asperity contacts, which make the fluid flow process even more complex and difficult to accurately describe [18,19,20,21,22].

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