The validity of the cubic law for laminar flow of fluids through open fractures consisting of parallel planar plates has been established by others over a wide range of conditions with apertures ranging down to a minimum of 0.2 µm. The law may be given in simplified form by Q/Δh = C(2b)3, where Q is the flow rate, Δh is the difference in hydraulic head, C is a constant that depends on the flow geometry and fluid properties, and 2b is the fracture aperture. The validity of this law for flow in a closed fracture where the surfaces are in contact and the aperture is being decreased under stress has been investigated at room temperature by using homogeneous samples of granite, basalt, and marble. Tension fractures were artificially induced, and the laboratory setup used radial as well as straight flow geometries. Apertures ranged from 250 down to 4µm, which was the minimum size that could be attained under a normal stress of 20 MPa. The cubic law was found to be valid whether the fracture surfaces were held open or were being closed under stress, and the results are not dependent on rock type. Permeability was uniquely defined by fracture aperture and was independent of the stress history used in these investigations. The effects of deviations from the ideal parallel plate concept only cause an apparent reduction in flow and may be incorporated into the cubic law by replacing C by C/ƒ. The factor ƒ varied from 1.04 to 1.65 in these investigations. The model of a fracture that is being closed under normal stress is visualized as being controlled by the strength of the asperities that are in contact. These contact areas are able to withstand significant stresses while maintaining space for fluids to continue to flow as the fracture aperture decreases. The controlling factor is the magnitude of the aperture, and since flow depends on (2b)3, a slight change in aperture evidently can easily dominate any other change in the geometry of the flow field. Thus one does not see any noticeable shift in the correlations of our experimental results in passing from a condition where the fracture surfaces were held open to one where the surfaces were being closed under stress.

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VALIDITY OF CUBIC LAW FOR FLUID FLOW IN A DEFORMABLE ROCK FRACTURE - eScholarship

A review of techniques, advances and outstanding issues in numerical modelling for rock mechanics and rock engineering

Some new Q-value correlations to assist in site characterisation and tunnel design

This paper provides a review of hydromechanical (HM) couplings in fractured rock, with special emphasis on HM interactions as a result of, or directly connected with human activities. In the early 1960s, the coupling between hydraulic and mechanical processes in fractured rock started to receive wide attention. A series of events including dam failures, landslides, and injection-induced earthquakes were believed to result from HM interaction. Moreover, the advent of the computer technology in the 1970s made possible the integration of nonlinear processes such as stress–permeability coupling and rock mass failure into coupled HM analysis. Coupled HM analysis is currently being applied to many geological engineering practices. One key parameter in such analyses is a good estimate of the relationship between stress and permeability. Based on available laboratory and field data, it was found that the permeability of fractured rock masses tends to be most sensitive to stress changes at shallow depth (low stress) and in areas of low in-situ permeability. In highly permeable, fractured rock sections, fluid flow may take place in clusters of connected fractures which are locked open as a result of previous shear dislocation or partial cementation of hard mineral filling. Such locked-open fractures tend to be relatively insensitive to stress and may therefore be conductive at great depths. Because of the great variability of HM properties in fractured rock, and the difficulties in using laboratory data for deriving in-situ material properties, the HM properties of fractured rock masses are best characterized in situ.

/pdf/the-role-of-hydromechanical-coupling-in-fractured-rock-2rn2brylk4.pdf

The role of hydromechanical coupling in fractured rock engineering

The permeability of deep (1000 m; 3300 ft) coal seams is commonly low. For deep coal seams, significant reservoir pressure drawdown is required to promote gas desorption because of the Langmuir-type isotherm that typifies coals. Hence, a large permeability decline may occur because of pressure drawdown and the resulting increase in effective stress, depending on coal properties and the stress field during production. However, the permeability decline can potentially be offset by the permeability enhancement caused by the matrix shrinkage associated with methane desorption. The predictability of varying permeability is critical for coalbed gas exploration and production-well management. We have investigated quantitatively the effects of reservoir pressure and sorption-induced volumetric strain on coal-seam permeability with constraints from the adsorption isotherm and associated volumetric strain measured on a Cretaceous Mesaverde Group coal (Piceance basin) and derived a stress-dependent permeability model. Our results suggest that the favorable coal properties that can result in less permeability reduction during earlier production and an earlier strong permeability rebound (increase in permeability caused by coal shrinkage) with methane desorption include (1) large bulk or Young's modulus; (2) large adsorption or Langmuir volume; (3) high Langmuir pressure; (4) high initial permeability and dense cleat spacing; and (5) low initial reservoir pressure and high in-situ gas content. Permeability variation with gas production is further dependent on the orientation of the coal seam, the reservoir stress field, and the cleat structure. Well completion with injection of N2 and displacement of CH4 only results in short-term enhancement of permeability and does not promote the overall gas production for the coal studied.

Volumetric strain associated with methane desorption and its impact on coalbed gas production from deep coal seams

Strength, deformation and conductivity coupling of rock joints

La construction de barrages, tunnels et talus dans des roches porteuses d'eau et fissurees cause des interactions complexes entre la deformation des fissures et la contrainte effective. La deformation de la fissure peut prendre la forme de fermeture normale, d'ouverture, de cisaillement et de dilatation. Resultats de nombreuses annees de recherche sur les proprietes des diaclases synthetisees dans un modele de comportement de diaclases couplees. Description des methodes de caracterisation des diaclases pour obtenir les donnees d'entree necessaires


 Applications of a novel 3-D mapping and visualization technology for mapping underground natural gas plastic/metallic pipelines and improving pipeline assets management beyond the capability of other known technologies are presented. The developed technology is based on combining Radar and Forced-Resonance-Imaging (FRI) principles referred to as the Bakhtar Buried Pipe Detector (BPD) with its original platform developed under US Air Force SBIR programs referred to as “BakhtarRadar”. This extremely low-power (≤ 10 dBm) detection and volumetric imaging technology defines a new rapid and cost-effective approach that uses the ability of casing and related embedded contents to absorption of RF energy in certain quantum emission frequency bands as a means of determining their presence. Furthermore, it uses the absorbed energy to differentiate between buried metallic and plastic pipes and reconstruct 3-D volumetric images from buried targets.
 The above-ground survey is conducted by scanning the application-specific sensor head (FR antennae) over the area of interest at controlled speed rate of 5–30 centimeters per seconds depending on the buried pipeline size, collecting sub-surface data, detecting the buried pipeline location followed by constructing 3-D mapping volumetric images in a rapid succession. The results provide uniquely defined high-resolution information on location and alignment, depth, type (metallic and/or plastic pipes), and dimensional details of buried pipelines. The field calibration and blind testing at PG&E’s service territory demonstrate BPD’s capability of collecting, screening, analyzing, managing, as well as integrating pipeline location data from multiple sources, such as utility operators, locating service providers, etc.

K. Bakhtar

Papers

Strength, deformation and conductivity coupling of rock joints

Strength, deformation and conductivity coupling of rock joints

Forced Resonance Imaging for 3-D Mapping of Underground Pipeline Assets