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The hydraulic properties of rock fractures are of considerable interest in several areas of engineering applications such as geothermal energy utilization, radioactive waste management, CO2 sequestration, and enhanced oil recovery in naturally fractured reservoirs. Natural rock fractures exhibit the contact-area characteristic due to shearing or normal-compressing processes. This work develops a new theoretical model to quantify effects of contact area on hydraulic properties of rough-walled rock fractures for incompressible, single-phase laminar flow.
In this work, we present a critical review of the main eight current models for contact-area influence on hydraulic properties of rock fractures. All the current models have inherent limitations in accuracy and applications. All models, for example, cannot capture the nature physics of tortuous flow caused by contact area – ignoring local head loss due to tortuous flow, neglecting effect of contact-area location and area, and applying vertical aperture instead of flow-oriented aperture, which generally overshoot the hydraulic properties. Some models are restricted to specific applications. Models by Walsh [1981] and Zimmerman et al. [1992] are only applicable in parallel cases. Models like Zimmerman and Bodvarsson [1996], Wang et al. [2014] and Yeo [2001] works well for relatively smooth fractures with limited range of fractional contact area – 0.5, 0.42 respectively. Our proposed model is free from the limitations of the current models. The proposed model is based on (1) reflecting the tortuous-flow physics by constructing flowpaths based on percolation theory (2) considering the local head loss, (3) modifying the aperture field by orienting it with flow direction, (4) quantifying the effect of contact-area location by introducing a dynamic correction factor. Our proposed model is applicable to fractional contact area of 1 theoretically and more general fractures. To assess the performance of the proposed model, we compare it with direct numerical simulations by full-physics Navier-Stokes Equations (NSEs), previous corrected models, and experimental measurement data collecting from other published works.
The proposed model is fitted very well to NSEs simulation results and experimental data collecting from other works. Moreover, our model is more accurate than other current models. The results show that the hydraulic aperture decreases with the increase of fractional contact area almost linearly. The location of contact area has a big impact on hydraulic aperture even with same contact area. The modified correction term reflects contact-area influence dynamically and demonstrates a strong impact on fluid flow at high fractional contact area. As void spaces surrounded by contact region cannot contribute to fluid flow, they should be incorporated in the contact area. The purpose of this work is to propose a new model for engineering purpose by providing a physics-inspired and data-driven approach and it may be extended to study the hydraulic behaviors in multi-phase flow, complex fracture networks and other fractured-rock hydrology problems.
| References | Zimmerman, R.W. and Bodvarsson, G.S., 1996. Hydraulic conductivity of rock fractures. Transport in porous media, 23, pp.1-30. Walsh, J.B., 1981, October. Effect of pore pressure and confining pressure on fracture permeability. In International journal of rock mechanics and mining sciences & geomechanics abstracts (Vol. 18, No. 5, pp. 429-435). Pergamon. Zimmerman, R.W., Chen, D.W. and Cook, N.G., 1992. The effect of contact area on the permeability of fractures. Journal of Hydrology, 139(1-4), pp.79-96. Yeo, W., 2001. Effect of contact obstacles on fluid flow in rock fractures. Geosciences Journal, 5, pp.139-143. |
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| Country | Saudi Arabia |
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