Abstract
The nonlinear flow regimes of a crossed fracture model consisting of two fractures have been investigated, in which the influences of hydraulic gradient, surface roughness, intersecting angle, and scale effect have been taken into account. However, in these attempts, the aperture of the two crossed fractures is the same and effects of aperture ratio have not been considered. This study aims to extend their works, characterizing nonlinear flow through a system of two intersecting fractures with different apertures. First, three experiment models with two fractures having different apertures were established and flow tests were carried out. Then, numerical simulations by solving the Navier-Stokes equations were performed and the results compared with the experiment results. Finally, the effects of fracture aperture on the critical pressure difference and the ratio of hydraulic aperture to mechanical aperture were systematically analyzed. The results show that the numerical simulation results agree well with those of the fluid flow tests, which indicates that the visualization techniques and the numerical simulation code are reliable. With the increment of flow rate, the pressure difference increases first linearly and then nonlinearly, which can be best fitted using Forchheimer’s law. The two coefficients in Forchheimer’s law decrease with the increasing number of outlets. When increasing fracture aperture from 3 mm to 5 mm, the critical pressure difference increases significantly. However, when continuously increasing fracture aperture from 5 mm to 7 mm, the critical pressure difference changes are negligibly small. The ratio of hydraulic aperture to mechanical aperture decreases more significantly for a fracture that has a larger aperture. Increasing fracture aperture from 5 mm to 7 mm, that has a negligibly small effect on the critical pressure difference will however significantly influence the ratio of hydraulic aperture to mechanical aperture.
Highlights
The assessment of hydraulic properties of deep fractured rock masses has attracted attention in many fields, such as CO2 sequestration [1,2], enhanced oil recovery [3,4], and groundwater use [5,6,7,8,9].In tight rocks, the permeability of fractures is much larger than that of the rock matrix, and as a result, the rock matrix is assumed to be impermeable [10,11,12,13]
This assumption is commonly adopted when calculating the permeability of highly fractured rock masses using modelling techniques such as discrete fracture network (DFN) models [14,15,16]
Processes 2018, 6, 94 the fluid flow through fractures is in the linear regime and follows the cubic law in which the flow rate is linearly proportional to the pressure difference [10,17,18,19,20]
Summary
In tight rocks (i.e., granite and basalt), the permeability of fractures is much larger than that of the rock matrix, and as a result, the rock matrix is assumed to be impermeable [10,11,12,13] This assumption is commonly adopted when calculating the permeability of highly fractured rock masses using modelling techniques such as discrete fracture network (DFN) models [14,15,16]. In the high-pressure packer tests and/or karst systems, the fluid flow is in the nonlinear regime where the cubic law is not suitable [21,22] In such a case, the permeability/conductivity of a fractured rock mass will be underestimated when still using the cubic law [21]. The nonlinear flow characteristics of fractured rock masses should be investigated
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