Study on Seepage Characteristics of Radial Flow in Rock Mass Based on Radius Effect

Kaichen Liu,Yao Zhang,Junrui Chai,Ruiqing Li,Ying Tian,Yong Liu

doi:10.1155/2022/5368690

Abstract

The radial fluid flow in fractures is affected by the size of the inner diameter and the position of the outer diameter, but the influence of the two factors on the flow velocity remains unclear. This study reveals the relationship through the coupled shear flow experiments and numerical simulations. Experimental results show that the fracture aperture is decreased by 0.175 mm under the unit effective stress, with the increase of 0.902 MPa in the shear stress. COMSOL is used to simulate the seepage of fractures under different inner and outer diameters. Simulation results show that the transition from nonlinear to linear flow occurs in the radial direction when the hydraulic pressure is 0.2 and 0.4 MPa, and the positions of linear flow are 42 mm and 71 mm. The effect of the fracture surface results in a stratified flow velocity when the fluid flow enters the fracture aperture. Increasing the inner diameter raises the flow rate but decreases the maximum flow velocity. The maximum velocity difference can be as much as three times when the inner diameter difference is 6 mm. The seepage width of radial flow has a nonlinear relationship with the inner and outer diameters. The growth rate of the seepage width decreases as the ratio of the outer to the inner diameter increases. The modified cubic law considering the radius effect is proposed to improve the calculation accuracy of radial flow.

Highlights

Single fracture and fracture network are basic forms in rock masses, and the existence of fractures significantly affects the mechanical and hydraulic behaviors [1,2,3]. e permeability of fractures is generally several orders of magnitude larger than that of the matrix; the fluid flow mainly occurs in the fractures
Based on the shear flow tests and numerical simulations, the characteristics of radial flow under various hydraulic pressures and radii are comprehensively investigated, and the following conclusions can be obtained: (1) e shear process of relatively smooth fractures can be divided into three stages according to the stress characteristic points
(2) e flow regime in the radial flow is mainly nonlinear under high hydraulic pressures

Summary

Introduction

Single fracture and fracture network are basic forms in rock masses, and the existence of fractures significantly affects the mechanical and hydraulic behaviors [1,2,3]. e permeability of fractures is generally several orders of magnitude larger than that of the matrix; the fluid flow mainly occurs in the fractures. Zhang et al [33] investigated the velocity distribution along the radial distance in two-dimensional radial flow fractures Most of these studies focus on the permeability in macroscopic fractures, but the variation of the flow regime along the radial distance in three-dimensional fractures and the influence of the inner diameter on the fracture seepage are disregarded. E seepage width has no unified expression when the cubic law derived from the parallel flow is applied to radial flow, and a large error exists with the application. Erefore, this study first investigated aperture variation and fracture surface damage under different conditions through laboratory experiments and analyzed the shear stress during the complete shearing process. The modified cubic law in the radial flow regime is obtained with the identified relationship among seepage width, inner diameter, and outer diameter

Shear Test

Radial Flow Analysis

Conclusions