• Rob Jeffrey

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  • Monitoring and Measuring Hydraulic Fracturing Growth During Preconditioning of a Roof Rock over a Coal Longwall Panel - Rob Jeffrey - Ken Mills

    Narrabri Coal Operations is longwall mining coal directly below a 15 to 20 m thick conglomerate sequence expected to be capable of producing a windblast upon first caving at longwall startup and producing periodic weighting during regular mining. Site characterisation and field trials were undertaken to evaluate hydraulic fracturing as a method to precondition the conglomerate strata sufficiently to promote normal caving behaviour at longwall startup and reduce the severity of periodic weighting. This paper presents the results of the trials and illustrates the effectiveness of hydraulic fracturing as a preconditioning technique.

    Initial work was directed at determining if hydraulic fractures were able to be grown with a horizontal orientation, which would allow efficient preconditioning of the rock mass by placing a number of fractures at different depths through the conglomerate from vertical boreholes drilled from the surface. The measurements and trials were designed to determine the in situ principal stresses, the hydraulic fracture orientation and growth rate, and whether the fractures could be extended as essentially parallel fractures to a radius of at least 30 m. Overcore stress measurements were used to determine the orientation and magnitude of the in situ principal stresses, a surface tiltmeter array was used to determine the hydraulic fracture orientation, and stress change monitoring, pressure monitoring and temperature logging in offset boreholes were used to establish the fracture growth rate, lateral extent, and that the fractures maintained their initial spacing to a radial distance of greater than 30 metres. The measurements and trials demonstrated that horizontal fractures could be extended parallel to one another to a distance of 30 to 50 m by injection of 5,000 to 15,000 litres of water at a rate of 400 to 500 L/min. Results from the trial allowed a preconditioning plan to be developed and successfully implemented. Monitoring-and-Measuring-Hydraulic-Fracturing-Growth-During-Preconditioning-of-a-Roof-Rock-over-a-Coal-Longwall-Panel-R.Jeffrey-K.Mills-2018.pdf1.8 MB
  • In-Situ Stress Measurements and Stress Change Monitoring to Monitor Overburden Caving Behaviour and Hydraulic Fracture Pre-Conditioning - Jesse Puller, Ken Mills, Rob Jeffrey

    A coal mine in New South Wales is longwall mining 300 m wide panels at a depth of 160–180 m directly below a 16–20 m thick conglomerate strata. As part of a strategy to use hydraulic fracturing to manage
    potential windblast and periodic caving hazards associated with these conglomerate strata, the in-situ stresses in the conglomerate were measured using ANZI strain cells and the overcoring method of stress relief. Changes in stress associated with abutment loading and placement of hydraulic fractures were also measured using ANZI strain cells installed from the surface and from underground. Overcore stress measurements have indicated that the vertical stress is the lowest principal stress so that hydraulic fractures
    placed ahead of mining form horizontally and so provide effective pre-conditioning to promote caving of the conglomerate strata. Monitoring of stress changes in the overburden strata during longwall retreat was undertaken at two different locations at the mine. The monitoring indicated stress changes were evident 150 m ahead of the longwall face and abutment loading reached a maximum increase of about 7.5 MPa. The stresses ahead of mining change gradually with distance to the approaching longwall and in a direction consistent with the horizontal in-situ stresses. There was no evidence in the stress change monitoring results to indicate significant cyclical forward abutment loading ahead of the face. The forward abutment load determined from the stress change monitoring is consistent with the weight of overburden
    strata overhanging the goaf indicated by subsidence monitoring. In-Situ-Stress-Measurements-and-Stress-Change-Monitoring-to-Monitor-Overburden-Caving-Behaviour-and-Hydraulic-Fracture-Pre-Conditioning-Jesse-Puller-Ken-Mills-Rob-Jeffrey-2015.pdf1.8 MB
  • Caving Induced by Hydraulic Fracturing at North Parkes Mine - Rob Jeffrey - Published 2000

    This paper describes the first use of hydraulic fracturing for cave inducement in a block caving mine. As of September 1999, several hundred hydraulic fracture treatments have been performed at Northparkes and are attributed with inducing about 7 million tonnes of ore to cave. Caving-Induced-by-Hydraulic-Fracturing-at-North-Parkes-Mine-R.Jeffrey.pdf267 KB
  • Remote High Resolution Stress Change Monitoring of Hydraulic Fractures - Ken Mills - Rob Jeffrey

    This paper describes the use of strain gauge based borehole instruments to monitor stress changes associated with the creation and extension of hydraulic fractures in massive rock strata at Northparkes Mine in Australia and Salvador Mine in Chile. This work was conducted as part of the International Caving Study ICSII.

    These instruments proved very sensitive to the stress changes induced by the hydraulic fractures close to the fracture plane. Analysis of the stress changes observed allowed the fracture orientation and non-symmetric fracture growth to be constrained sufficiently that a clearer insight into fracture behaviour could be obtained at both sites, particularly when combined with other observations. Recognition of the elastic stress reorientation about an opening mode hydraulic fracture has proved to be an important element in the interpretation of stress change monitoring data.

    The nature of the stress reorientation is useful in discriminating between opening and shearing mode fracture growth. A technique of identifying a range of possible solutions of fracture orientation and non-symmetric fracture growth consistent with the stress changes observed on multiple instruments has been developed. Unique definition of fracture orientation from the stress change instruments is possible if the instruments are sufficiently distributed relative to the hydraulic fracture plane. Remote-High-Resolution-Stress-Change-Monitoring-of-Hydraulic-Fractures-K.Mills-R.Jeffrey.pdf1 MB
  • Hydraulic Fracturing to Induce Caving: Fracture Model Development and Comparison to Field Data - Rob Jeffrey - Ken Mills

    Hydraulic fracturing is used at Moonee Colliery to induce caving as part of the routine operation of this longwall mine. Measurements undertaken to successfully introduce hydraulic fracturing to Moonee and pressure records routinely obtained from each treatment provide a unique opportunity to develop and test a new model of hydraulic fracture growth near a free surface. This paper presents the results of the comparison for several fracture treatments, demonstrating that the model is able to match the treatment data. Hydraulic-Fracturing-to-Induce-Caving-Fracture-Model-Development-Comparison-to-Field-Data-R.Jeffrey-K.Mills.pdf395 KB
  • Sand Propped Hydraulic Fracture Stimulation of Horizontal In seam Gas Drainage Holes at Dartbrook Coal Mine - Rob Jeffrey

    Longwalls 107, 108 and 109 at Dartbrook Coal Mine contained coal with a high gas content and low permeability. Horizontal in-seam drain holes were found to have low gas production rates compared with drainage rates in previous panels. Hydraulic fracture stimulations, using water and sand, were therefore carried out in three boreholes in Longwalls 109 and 108 at Dartbrook to assess the effectiveness of sand propped fractures in stimulating gas drainage from in-seam boreholes. Boreholes 108-10-10 and 108 -7-1 were stimulated with 20 and 10 fractures respectively and, on average, 100 kg of sand was placed into each fracture. The fractures placed into LW 109 were to be mined and mapped, but operational constraints precluded mapping of these fractures.

    The stimulations produced a significant increase in gas drainage rates from the two boreholes. Hole 108-10-10, which ran perpendicular to the major joint system in the seam, increased its early gas rate by a factor of about 180 while hole 108-7-1, which was drilled parallel to the joint set, increased its rate by about 22 compared to pre-stimulation rates. The stimulated gas rates continuously increased for several weeks and the higher rates were sustained for the entire period the holes were monitored. Based on the higher stimulation effect achieved in hole 108-10-10 (drilled perpendicular to the jointing) compared with hole 108-7-1 (drilled parallel to the jointing), target drainage holes drilled perpendicular (northsouth) to the jointing are better stimulation candidates.

    Fracture modeling suggests the sand proppant bank may extend to 15m from the borehole. The unpropped portion of the fracture may extend to more than 40m. A purpose-built fracturing system was developed and used at Dartbrook to stimulate holes that covered most of LW109. This full-scale enhancement of gas drainage was successful and allowed efficient mining of that panel.

    [Coal Operators' Conference, University of Wollongong & the Australiasian Institute of Mining and Metallurgy, 2004] Sand-Propped-Hydraulic-Fracture-Stimulation-of-Horizontal-In-seam-Gas-Drainage-Holes-at-Dartbrook-Coal-Mine-R.Jeffrey-2004.pdf265 KB
  • Growth Analysis and Fracture Mechanics Based on Measured Stress Change near Full Size Hydraulic Fracture - Rob Jeffrey - Ken Mills

    This paper describes the successful measurement of stress changes induced in a crystalline rock mass adjacent to a full size hydraulic fracture. A hydraulic fracture was initially formed using water and subsequently 2-D numerical models. The full three dimensional stress changes were measured using four ANZI stresscells installed and tested in situ prior to the start of hydraulic fracturing. The instruments were installed in pairs in two boreholes located some 7-8mm laterally and 17-20m above the injection point. The in situ stressfield at the site was such that the hydraulic fracture passed within approximately 5m of the instruments allowing the stress changes associated wit the passage of the fracture tip to be monitored as well as the stressed induced in the rock by the hydraulic fracture once the fracture tip was well past. The instruments were logged at 15 second intervals throughout the hydraulic fracture treatments to provide a time history of the complete three dimensional stress changes that occurred as each hydraulic fracture grew toward and then passed close to the instruments.

    The monitoring was undertaken as part of a larger project aimed at preconditioning a rock mass with multiple hydraulic fractures. The result of only two of the fractures are considered in this paper. Analysis of the other fractures is ongoing. Analysis of the stress change data provides information about the fracture rate and mode of growth orientation, and about the excess pressure acting inside the fracture to open it. Growth-Analysis-and-Fracture-Mechanics-Based-on-Measured-Stress-Change-near-Full-Size-Hydraulic-Fracture-R.Jeffrey-K.Mills.pdf370 KB
  • Hydraulic Fracturing Applied to Stimulation of Gas Drainage From Coal - Rob Jeffrey

    Hydraulic fracturing is routinely applied to stimulation of oil, gas, and coalbed methane wells around the world. The stimulation effect is achieved in coal seams as in other reservoirs, by producing a conductive fracture, connecting the well to the coal reservoir. The conductivity of the fracture is usually maintained by placing a round and sieved sand proppant in the fracture channel. The proppant prevents the fracture faces from closing back completely on one another after the treatment. The design of the fracture treatment, therefore, centers on selecting fluids, injection rates, and slurry concentrations that will produce the desired propped fracture channel. Hydraulic-Fracturing-Applied-to-Stimulation-of-Gas-Drainage-From-Coal-R.Jeffrey.pdf501 KB
  • Propogation of a Penny Shaped Hydraulic Fracture Parallel to a Free Surface, with Application to Inducing Rock Mass Caving for Mining - Rob Jeffrey

    In this paper, the problem of a penny-shaped hydraulic fracture propagating parallel to the free surface of an elastic half-space is studied. The fracture is driven by an incompressible Newtonian fluid injected at a constant rate. The flow of viscous fluid in the fracture is governed by the lubrication equation, while the crack opening and the fluid pressure are related by singular integral equations. We construct two asymptotic solutions based on the assumption that the energy expended in the creation of new fracture surfaces is either small or large compared to the energy dissipated in viscous flow. One important outcome of the analysis is to show that the asymptotic solutions, when properly scaled, depend only on the dimensionless parameter R, the ratio of the fracture radius over the distance from the fracture to the free-surface. The scaled solutions can thus be tabulated and the dependence of the solution on time can be retrieved for specific parameters, through simple scaling and by solving an implicit equation. Propogation-of-a-Penny-Shaped-Hydraulic-Fracture-Parallel-to-a-Free-Surface-with-Application-to-Inducing-Rock-mass-Caving-for-Mining-R.Jeffrey.pdf376 KB
  • Hydraulic Fracturing Applied to Inducing Longwall Coal Mine Goaf Falls - Rob Jeffrey - Ken Mills

    This paper describes the first successful use of hydraulic fracturing to induce a goaf event and control the timing of caving events in Australia. Hydraulic fractures are initiated at 7 to 10m above the bottom of a thick conglomerate roof and, because of the low vertical stress magnitude relative to the other two principal stressed, grow as horizontal fractures. The fractures extend radially outward from the injection borehole into the rock until a goaf fall occurs. Hydraulic fracturing has provided a means to control the timing of windblast events and thereby significantly improved safety. The successful implementation of hydraulic fracturing at Moonee Colliery to control the timing of goaf events has enabled the mine to continue operating. Hydraulic-Fracturing-Applied-to-Induce-Longwall-Coal-Mine-Goaf-Falls-R.Jeffrey-K.Mills.pdf240 KB
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