Ausama Giwelli | CSIRO - Academia.edu (original) (raw)

Papers by Ausama Giwelli

Journal of the Japan Petroleum Institute, 2013

$Research paper thumbnail of Estimation of closure of a fracture under normal stress based on aperture data$

International Journal of Rock Mechanics and Mining Sciences, 2008

The closure of 41-mm hydraulic fractures under normal stress in both loading and elastic closure ... more The closure of 41-mm hydraulic fractures under normal stress in both loading and elastic closure (unloading) was estimated according to the formula proposed by Brown and Scholz and based on data measured for the initial aperture. By introducing the concept of an effective/ineffective initial aperture and by assuming Gaussian and χ2 probability density functions (PDFs) of the initial aperture, the normal stress versus closure curve was determined from the standard deviation (SD) and the spectral moments of the initial aperture and the ratio of the mean effective initial aperture to the SD of the initial aperture. The results showed that the non-linearity in the normal stress versus closure curve at large normal stresses was reproduced better by the χ2 PDF of the initial aperture than the Gaussian PDF for both loading and elastic closure. Furthermore, based on the ratio of the mean effective initial aperture to the SD of the initial aperture determined for the hydraulic fractures, the effect of size on the normal stress versus closure curve was estimated for fracture areas in a tensile fracture of 1m. The results showed that closure increases with the size of the fracture area, and that the effect of size on the closure of the fracture is governed by the SD of the initial aperture.

$Research paper thumbnail of Experimental study of the effect of fracture size on closure behavior of a tensile fracture under normal stress$

International Journal of Rock Mechanics and Mining Sciences, 2009

ABSTRACT Tensile fractures that measured from 37.5 mm x 37.5 mm to 260 mm x 260 mm were created i... more ABSTRACT Tensile fractures that measured from 37.5 mm x 37.5 mm to 260 mm x 260 mm were created in sandstone perpendicular to the bedding plane by intending steel wedges, and closure of these fractures under normal stresses of upto 10 MPa was measured in the laboratory to investigate the effect of fracture size on closure behavior. Prior to the tests, the aperture distributions were determined by measuring the topography of the upper and lower surfaces using a non-contact surface profile measurement system with a laser profilometer, and the power spectral densities (PSDs) of the initial aperture and the surface heights were calculated by using a standard fast Fourier transform (FFT). The experimental results showed that at a given normal stress, closure significantly increases with an increase infracture size. However, the relation between normal stress and closure, normalized by the standard deviation (SD) of the initial aperture, is almost independent of the fracture size, since the SD of the initial apertureal so increases with fracture size. Thus, the size effect on the closure of a fracture under normal stress is governed by that on the SD of the initial aperture.

$Research paper thumbnail of Effect of shear displacement on the hydraulic conductivity of a fracture$

International Journal of Rock Mechanics and Mining Sciences, 2010

The effect of shear displacement inclined relative to macroscopic water flow on the hydraulic con... more The effect of shear displacement inclined relative to macroscopic water flow on the hydraulic conductivity of a rock fracture was estimated, using synthetic fractures that reproduce a tensile fracture in granite. The results showed that the hydraulic aperture normalized by the mean aperture increased with the angle between the directions of shear displacement and macroscopic water flow, according to a sinusoidal function of twice the angle. Formulae were established to estimate the hydraulic aperture of the fracture as a function of the mean aperture, the standard deviation of the initial aperture, the shear displacement, and the angle between the shear displacement and macroscopic water flow, based on results obtained in both this work and previous work, but neglecting scale effects. By assuming the mechanical properties of the fracture based on experimental results for granite, but neglecting scale effects, the hydraulic conductivity of the fracture with an arbitrary direction under a given state of stress (s 1 = 29 MPa, s 2 =25 MPa and s 3 =13.5 MPa) was estimated for macroscopic water flow in the directions of both s 1 and s 2 . When the contour map of the transmissivity of the fracture is plotted on a stereonet of the normal direction of the fracture in the principal axes of stress, there is a ridge (line of the local maximum) of transmissivity in the circumferential direction, and the inclination angle of the ridge from the s 3 -axis decreases with shear displacement, since shear dilation increases with both a decrease in normal stress and an increase in shear displacement. Furthermore, for the condition of stress given in this study, the transmissivity for macroscopic water flow in the direction of s 1 is maximum for a fracture with a normal direction within the s 2 -s 3 plane, while that in the direction of s 2 is maximum for a fracture with a normal direction within the s 1 -s 3 plane.

$Research paper thumbnail of Effects of non-uniform traction and specimen height in the direct shear test on stress and deformation in a rock fracture$

International Journal for Numerical and Analytical Methods in Geomechanics, 2012

ABSTRACT In the direct shear test (DST), an internal moment is distributed within the rock specim... more ABSTRACT In the direct shear test (DST), an internal moment is distributed within the rock specimen by non‐coaxial shear loads applied to the specimen, which cause non‐uniform distributions of both the traction on the loading planes and the stress and deformation in the specimen. To examine the validity of the DST for a rock fracture and to clarify the effect of specimen height, both the stress and deformation in a fracture in the DST were analyzed for specimens with three different heights using a three‐dimensional finite element method with quadratic joint elements for a fracture model. The constitutive law of the fracture considers the dependence of the non‐linear behavior of closure on shear displacement and that of shear stiffness on normal stress and was implemented in simulation code to give a conceptional fracture with uniform mechanical properties to extract only the effect of non‐uniform traction on the stress and deformation in the fracture. The results showed that both normal and shear stresses are concentrated near the end edges of the fracture, and these stress concentrations decrease with a decrease in the specimen height according to the magnitude of the moment produced by the non‐coaxial shear loads. Furthermore, although closure is greater near the end edges of the fracture, where normal stress is concentrated, this concentration of closure is not so significant within the range of this study because of the non‐linear behavior of closure, that is, closure does not significantly increase with an increase in normal stress at large normal stresses. Copyright © 2012 John Wiley & Sons, Ltd.

$Research paper thumbnail of Shear behaviour of fractured rock as a function of size and shear displacement$

Geomechanics and Geoengineering, 2014

This paper presents laboratory results regarding the shear behaviour of an artificial tensile fra... more This paper presents laboratory results regarding the shear behaviour of an artificial tensile fracture generated in granite. We used a direct shear rig to test fractures of different sizes (from 100 mm to 200 mm) under various shear displacements up to 20 mm and cyclic shear stresses with constant normal stress of 10 MPa. To determine the evolution of surface damage and aperture during shear, cyclic loading was performed at designated shear displacements. These changes in the surfaces topography were measured with a laser profilometer 'non-contact surface profile measurement system'. In addition, changes were also measured directly by using pressure-sensitive film.

American Journal of Orthodontics and Dentofacial Orthopedics, 2001

ABSTRACT Tensile fractures with various sizes of from 37.5 mm to 260 mm were created in a sandsto... more ABSTRACT Tensile fractures with various sizes of from 37.5 mm to 260 mm were created in a sandstone block perpendicular to the bedding plane by indenting wedges, and scale effect on the closure of the fracture under normal stress was investigate. The surface topography of the fractures was measured to determine the initial aperture distribution, which showed that the standard deviation (SD) of the initial aperture increases with fracture size. The closure of the fracture was measured in cyclic loading for normal stresses up to 10 MPa. The experimental results have shown that the closure of the fracture significantly increases with an increase in fracture size. However, the closure curves become approximately identical by normalizing the closure by the SD of the initial aperture and are almost independent of fracture size. Thus, the scale effect on the closure of the fracture under normal stress is governed by that on the SD of the initial aperture.