Quantitative Estimates of Nonlinear Flow Characteristics of Deformable Rough-Walled Rock Fractures with Various Lithologies

Qian Yin,Hongwen Jing,Lixin He,Dong Zhu

doi:10.3390/pr6090149

Abstract

The existence of surface roughness, various contact conditions and the occurrence of flow nonlinearity make the flow process in natural rock fractures more complicated. To evaluate the fluid flow regimes in deformable rough-walled rock fractures, a great many hydromechanical tests were conducted on nine real fractures artificially produced from a wide range of lithological diversity. For fractures with a certain JRC (fracture roughness coefficient) value, the confining pressure varied from 5 to 20 MPa, and the hydraulic pressure was increased from 0.4 to 6.0 MPa. The experimental results display that (i) regression analyses of the raw experimental data indicate that the Forchheimer’s law provides a perfect description for flow process through the fractures. The coefficients of viscous and inertial pressure drops undergo a growth of 2–3 orders of magnitude with an increase in the confining pressure; (ii) the hydraulic aperture decreases by approximately 87.41–92.81% as the confining pressure increases, and experiences a decrease of 1.52–2.96 times with the JRC values. A power-law function is used to evaluate the hydraulic aperture as a function of the nonlinear coefficient. The nonlinear coefficient decreases with increasing hydraulic aperture; (iii) using Forchheimer equation, the critical Reynolds number Rec was successfully assessed by choosing E percentage (generally 10%) of the nonlinear effect as the critical value between the linear and nonlinear flow regimes. The obtained Rec steadily increases with increasing confining pressure, while it diminishes with the JRC values; and (v) the transmissivity decreases as the pressure gradient increases. Additionally, transmissivity also exhibits a decreasing trend with both the confining pressures and JRC values due to fracture closure and tortuous and channeling flow paths in rougher fractures, and the rate of its decrease for a smaller confining pressure (5, 10 MPa) is more significant.

Highlights

Fluid flow through single rock fractures has been a matter of great concern over the past few decades in various engineering activities including oil and natural gas exploitation, geothermal extraction, CO2 geological sequestration, nuclear waste disposal, and underground coal gasification [1,2,3,4,5]
The nonlinear coefficient decreases with increasing hydraulic aperture; (iii) using Forchheimer equation, the critical Reynolds number Rec was successfully assessed by choosing E percentage of the nonlinear effect as the critical value between the linear and nonlinear flow regimes
The obtained Rec steadily increases with increasing confining pressure, while it diminishes with the JRC values; and (v) the transmissivity decreases as the pressure gradient increases

Summary

Introduction

Fluid flow through single rock fractures has been a matter of great concern over the past few decades in various engineering activities including oil and natural gas exploitation, geothermal extraction, CO2 geological sequestration, nuclear waste disposal, and underground coal gasification [1,2,3,4,5]. Fluid flowing through single fractures is approximately estimated using the cubic law, which is achieved from the analogy of laminar flow through two perfectly smooth parallel plates. Fluid flow experimental findings on real rock factures indicated that the cubic law generally overestimates the transmissivity of fractures [12,13]. Void space between the fracture surfaces provides the primary paths of fluid migration, bypassing the contact regions with twists and turns [14], which makes the flowing behaviors complicated and difficult to be accurately described

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Conclusion