Wollongong, Australia
Wollongong, Australia

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Zoorabadi M.,University of New South Wales | Zoorabadi M.,SCT Operations Pty Ltd | Saydam S.,University of New South Wales | Timms W.,University of New South Wales | Hebblewhite B.,University of New South Wales
Geomechanics and Geoengineering | Year: 2016

Roughness on rock joints produces a variable aperture across the joints and increases the flow path length. These conditions should be taken into account for a good approximation from cubic law. In this paper, the concept of local true aperture and tortuosity is applied to assumed joints where surfaces are matched to each other and correspond with standard Joint Roughness Coefficient (JRC) profiles. Furthermore, the hydraulic behaviour of JRC profiles is studied by a new laboratory experiment setup. The analytical approach provides new insights into the effects of roughness on hydraulic properties of rock joints. The results indicate that for a constant mechanical aperture, both the minimum local aperture and hydraulic aperture decrease with increasing JRC. Furthermore, tortuosity and standard deviation of local true aperture increase with JRC increment. The trend obtained between different parameters and JRC shows an obvious fluctuation for JRC lower than 10. On one hand, the results of this study along with a critical review of previous studies demonstrate that JRC profiles cannot present a precise roughness increment when JRC is less than 10. A new laboratory setup was designed to study the flow behaviour of JRC profiles. The results obtained from laboratory experiments under linear flow conditions validate the accuracy of the applied analytical method. © 2016 Taylor & Francis

Jeffrey R.G.,SCT Operations Pty. Ltd. | Chen Z.R.,CSIRO | Zhang X.,CSIRO | Bunger A.P.,University of Pittsburgh | Mills K.W.,SCT Operations Pty. Ltd.
Rock Mechanics and Rock Engineering | Year: 2015

Hydraulic fracture breakdown and reorientation data collected from two instrumented test borehole sites have been analyzed to assess the effect of the initiation type (axial or transverse) on the treating pressure. Vertical boreholes were drilled and fractures were placed in a conglomerate at depths of 140–180 m in a far-field stress field that favored horizontal fracture growth. Axial initiation resulted in high injection pressure, which was attributed to near-borehole tortuosity generated as the hydraulic fracture reoriented to align with the far-field stresses. Acoustic scanner logging of the boreholes after fracturing demonstrated that, in many cases, axial initiation occurred and when this was the case, treating pressures were high and consistent with near-borehole tortuous fracture paths. A fracture initiation analysis determined that initiation at abrasively cut circumferential slots should occur before axial initiation. Slots were cut to locate the initiation sites and to make transverse fracture initiation more likely. Transverse initiation from the vertical boreholes at pre-cut slots lowered the injection pressures during the fracture treatment by up to 12 MPa for water injected at approximately 500 L per minute. © 2015, Springer-Verlag Wien.

Wu B.,CSIRO | Zhang X.,CSIRO | Jeffrey R.G.,SCT Operations Pty Ltd | Bunger A.P.,University of Pittsburgh | Jia S.,Yangtze University
Applied Energy | Year: 2016

Multiple hydraulic fractures have been proposed for improving the performance of an enhanced geothermal system (EGS) by providing conductive flow pathways and increased contact area between flowing fluid and surrounding rock formation. Use of more fractures incurs a higher drilling and hydraulic fracturing cost, but the additional cost can be offset by improved operation performance of an EGS. In this paper, a model is presented for efficiently predicting the output temperature so as to optimize the number of fractures and fracture spacing to maximize the EGS lifetime under a constant circulation rate. This optimal spacing is shown to arise due to the interplay among number of fractures, fracture spacing, well depth, and the pre-existing geothermal gradient. Specifically, under a typical geothermal gradient associated with EGS for a 5 km total vertical depth of the well, the number of fractures N and the equal fracture spacing d have optimal values: 6 ⩽ N ⩽ 13 and 30 m ⩽ d ⩽ 90 m. In addition, the semi-analytical solution method presented is effective and efficient in computation and, for this reason, is useful for optimizing the design of a geothermal reservoir with multiple layers at equal or non-equal spacing. © 2016 Elsevier Ltd

Heritage Y.,SCT Operations Pty Ltd. | Stemp C.,SCT Operations Pty Ltd.
International Journal of Mining Science and Technology | Year: 2016

Traditional methods for assessing effective roof support can be difficult to apply to complex three-dimensional excavations. Through worked examples, the approach of combined two-dimensional and three-dimensional numerical modeling has been shown to be successful in understanding mechanisms of rock failure for unique excavation geometries and geotechnical properties and, in turn, provides adequate roof support recommendations for complex three-dimensional excavations in Australian coal mines. An interactive approach of monitoring and model review during the excavation process is an important part of model support recommendations to ensure rock failure and deformation in the model are representative of actual conditions, to provide effective and practical controls. © 2015 Published by Elsevier B.V. on behalf of China University of Mining & Technology.

Chen Z.,CSIRO | Jeffrey R.G.,SCT Operations Pty Ltd | Zhang X.,CSIRO | Kear J.,CSIRO
Society of Petroleum Engineers - SPE Asia Pacific Unconventional Resources Conference and Exhibition | Year: 2015

In this paper, the problem of a hydraulic fracture interacting with a pre-existing natural fracture has been investigated by using a cohesive zone finite element model. The model fully couples fluid flow, fracture propagation and elastic deformation, taking into account the friction between the contacting fracture surfaces and the interaction between the hydraulic fracture and the natural fracture. The effect of the field conditions, such as in-situ stresses, and rock and fracture mechanical and geometrical properties, intersection angle and the treatment parameters (fracturing fluid viscosity and injection rate) on the hydraulic fracture propagation behavior has been analyzed. The finite element modeling results provide detailed quantitative information on the development of various types of hydraulic fracture - natural fracture interaction, fracture geometry evolution and injection pressure history, and allow us to gain an in-depth understanding of the relative roles of various parameters. The value of a parameter calculated as the product of fracturing fluid viscosity and injection rate can be used as an indicator to gauge if crossing or diverting behavior is more likely. In addition, using a finite element approach allows the analysis to be extended to include the effects of fluid leakoff and poroelastic effect, and to study hydraulic fracture height growth through a system of nonhomogeneous layers and their bedding planes. Copyright 2015 Society of Petroleum Engineers.

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