Large-scale Hydraulic Experiments Research Articles

Morphological characteristics of microscale fractures in gas shale and its pressure-dependent permeability

After hydraulic fracture, the brittleness of shale rocks has led to a network of fractures with different scales and orientations. So far, the flow characteristic investigations have been mostly focused on the matrix (nanoscale) and the macrofractures (wider than millimeter scale) with proppants. Between the nano- and macroscales, those microscale fractures that could not be artificially propped were not studied adequately, although they are essential for gas flow due to the extremely low permeability of the original matrix. To simulate the hydraulic-induced microfractures in the laboratory, we have successfully established a new method on the basis of the Brazilian test to produce microscale fractures in cores. X-ray microtomography exhibited the morphology and aperture scale ([Formula: see text]) of the inner fractures. The variety of the fractures morphology was consistent with the previous results of the large-scale hydraulic experiments. The microfractures (aperture [Formula: see text]) enhanced the core permeability by 2–6 orders of magnitude. We found that the pressure-dependent permeability could be expressed by power and exponential functions, whereas the porosity was not applicable to be included in the function. Except for mechanical properties, the fracture permeability and its pressure dependency were intensely influenced by the fracture aperture, tortuosity, and roughness. Furthermore, we suggested that the greater the proportion of natural fractures in the fracture network, the greater the permeability decline with the pressure increase. This knowledge would be essential in practice to estimate the production and to optimize the hydraulic fractures.

REPRODUCIBILITY OF SMALL SCALE HYDRAULIC EXPERIMENT ON CROSS-LEVEE BREACH BY OVERFLOW

River bank failure causes a heavy disaster. Many experiments had been conducted. Full-scale hydraulic experiment is very few, because of it has a limit to conduct in terms of space and budget. Therefore, many experiments were conducted in small scale and they need to be compared with the actual phenomenon. The large scale hydraulic experiment on river bank failure was conducted at Chiyoda experiment flume in 2008. We conducted small scale experiments on river bank failure and examined the reproducibility of the small scale experiments by comparison with the Chiyoda experiment. Fine-grained soil strongly influences the widening process of bank failure. In order to reproduce the results of experiment at Chiyoda experimental flume on dike failure, it is necessary to remove the fine material from the material of scaled dikes. And it is found that the time scale of widening process of bank failure follows the Froude similarity law.

STUDY ON COUNTERMEASURES OF GRAVELS AMONG BED PROTECTION WORKS IN DOWNSTREAM OF APRON FOR PIPING UNDER FALLING WORKS

Bed protection works are constructed for protection of local scour and as energy absorber in downstream of falling works. If hydraulic gradient in bed material were over standard value, seepage control seat would be designed for piping under the falling works. The hydraulic gradient is estimated by use of difference of water levels in upstream and downstream of the river crossing structures. But the hydraulic gradient may increase by bed deformation in down stream of the river crossing structures after the construction. And in the case of low density of concrete blocks of bed protection works, pressure would come down by separate flow in the downstream end of apron.In this paper, it is examined that causes of piping under falling works and countermeasures of gravels among bed protection works in downstream of apron by large-scale hydraulic experiment.