The three most predominant methods for hard rock excavation and fragmentation are the use of explosives, mechanical impact/cutting and hydraulic fracturing. However, those methods have inherent drawbacks, such as non-applicability or poor performance in extremely hard and abrasive rocks. Novel rock fracturing and fragmentation methods are in need to either work individually or in combined forms to break rocks. Research shows that some rock forming minerals and water can be heated up rapidly by microwave, to induce microcracks and fractures in rocks. Microwave therefore can be regarded as a promising technology of hard rock fracturing and fragmentation, with the potential of energy and cost efficiency. This keynote first provides a brief review of the research on microwave effects on rock fracturing, followed by descriptions of experimental studies of crack formation in different rocks treated by a low power industrial microwave. Possible fracturing mechanisms by microwave treatment are discussed, and the applications of microwave treatment assisting rock excavation coupled with mechanical means are outlined at the end of this keynote.
Fracturing and fragmentation of hard rocks is one of the most important tasks in rock engineering in the fields of mining, petroleum, and tunneling industries. While drilling and blasting remains to be the most powerful and effective method to break hard rocks, the uses of explosives are often limited by various constraints. Hard rocks are excavated and fractured by other means, including mechanical cutting and drilling, hydraulic fracturing and heating. Mechanical cutting becomes a dominant method for large scale rock excavation, particularly in tunneling, with the extensive uses of tunnel boring machines (TBMs) and roadheaders. However, hard rocks can pose great challenges to mechanical cutting and fracturing due to the extreme hardness and abrasiveness, which often lead to reduced advance rates and increased cutting tool wear. Other methods of rock fracturing and cutting have been investigated, including waterjet, laser, millimeter wave and microwave, as a sole mean for rock cutting or in combination with mechanical excavation.
New technologies explored for possible use of rock fracturing and cutting are the electrical methods and the electromagnetic (EM) methods. The electrical methods include plasma blasting, electron beam and electric current. The EM methods use electromagnetic waves, including laser cutting, infrared irradiation, torch heating, and microwave heating. Mostly those technologies are investigated as a sole mean for rock fracturing or vaporization. The study presented in this keynote uses microwave technology to induce microcracks in hard rocks hence to ease the mechanical excavation. The objective is on adapting and developing low powered microwave tools to assist mechanical excavation of hard rocks, as an economical alternative.
Rathnaweera, Tharaka D. (Monash University /Nanyang Technological University) | Gamage, Ranjith P. (Monash University) | Wei, Wu (Nanyang Technological University) | Perera, Samintha A. (The University of Melbourne) | Haque, Asadul (Monash University) | Wanniarachchi, Ayal M. (Monash University) | Bandara, Adheesha K. (Monash University)
Over the last several decades, many studies have generated a large amount of proppant performance data, but these studies have only focused on proppant conductivity, with no attention to how proppant mechanical properties vary under loading conditions. The impact of mechanical behaviour on proppant performance can only be fully understood by the combined investigation of micro-structural and mechanical changes with increasing loading. Therefore, this study aims to identify such micro-structural behaviour, and in particular the impact on proppant mechanical properties. Proppant samples were tested under one-dimensional compression loading using high-resolution X-ray CT scanning technology. The reconstructed images taken at different load stages were analysed to capture the micro-structural behaviour and finally correlated with the mechanical behaviour of the proppant.
According to the results, there are significant micro-pore voids inside the proppant mass. When the proppant has a higher degree of porosity, there is a considerable reduction of the compressive strength which is not favourable for hydro-fracturing treatment designs. Moreover, it is clear that the brittleness of the proppant decreases with increasing porosity, as its Young’s modulus reduces with increasing pore voids. Therefore, it is important to have high manufacturing standards to achieve effective proppant performance at great depths. The micro-structural behaviour under increasing loading was investigated by performing comprehensive CT image analysis using Drishti software. According to the results, under compressive loading, proppants cleave and generate large fragments like a flower, and this happens suddenly and quite violently through the material. Interestingly, post-failure analysis revealed that the failure mechanism of a single proppant consists of three major stress levels, where initially proppant fails at a high stress level and gains some crushing-associated strength at later stages.
Unconventional oil/gas production has recently attracted the research community due to the uncontrollable increasing demand for primary energy sources (Perera et al., 2016; Wu et al., 2017). Since this method provides a good solution to energy scarcity, over the last several decades, the industry has tried to enhance the production rate, mainly focusing on production enhancement techniques which can be effectively used in the energy extraction from sub-surface geological formations. Of the various options, hydraulic fracturing is one of the best ways to enhance oil/gas extraction, as it increases the formation’s permeability, allowing easy movement of the extracted oil/gas towards the production well (Rutledge and Scott, 2003; Orangi et al., 2011; Vengosh et al., 2014; Wanniarachchi et al., 2015). However, this process may be jeopardised due to the high stress levels acting on the formation at great depths (both vertical overburden and confining pressures). One possible consequence is re-closure of the fracture network under downhole stress conditions, which severely affects the post-fracturing production. Such issues can negate the use of proppant as a hydraulic fracture treatment method where proppants injected with the fracturing fluid prop the fractures, withstanding the fracture-closure stress (Wanniarachchi et al., 2015). Although the proppant gives a reliable solution to overcome this issue (propping the fracture network), sufficient closure stress can cause mechanical failure of the proppant, changing the fracture conductivity, causing re-closure of the fracture network, and altering the bulk properties of the proppant pack, which can negatively influence oil/gas extraction. Therefore, it is important to understand the mechanical behaviour of proppants under downhole stress conditions before injecting proppant with the hydro-fracturing fluid.
Xing, Haozhe (Monash University / Commonwealth Scientific and Industrial Research Organisation) | Wu, Gonglinan (Monash University / Commonwealth Scientific and Industrial Research Organisation) | Zhang, Qianbing (Monash University) | Dehkhoda, Sevda (Commonwealth Scientific and Industrial Research Organisation) | Zhao, Jian (Monash University)
Dynamic compression and Brazilian Disc (BD) tests were performed on heated sandstone with split Hopkinson pressure bar (SHPB) at different strain rates. The sandstones were treated under the temperatures of 20 °C, 200 °C, 400 °C, 800 °C and 1200 °C. The full-field and real-time fracturing processes were captured by the high-speed 3D-DIC technique with resolution of 256 × 256 pixels and 200,000 frames per second (fps). The effect of heat on crack initiation stress thresholds, and the stability of crack development were investigated together with the dynamic Poisson’s ratio and elastic modulus. The density and wave velocity were found to decrease with temperature increase of the heat treatment. The results also showed that strain rate effect still exists in the high temperature treated sandstones; however, within a critical range (temperatures between 400 °C and 800 °C), the reduction in compressive and tensile strength is followed by a rise. The thermal effect on the distribution and evolution of the strain localization under compressive loading were discussed.
With an increasing demand in resource and space, the utilization of underground spaces, where high temperature may occur, is more and more important. The thermal effect on the rock mechanics will influence the efficiency of the rock excavation and the safety of the rock engineering. The underground spaces, also, experience dynamic loadings from various sources such as impact, explosion and earthquake. However, temperature and strain rate have opposite effects on the stress and strain. Increasing the strain rate or decreasing the temperature will lead to higher stress levels, but lower values of strain (Zhang and Zhao, 2014). Therefore, the understanding of the coupled effect of high temperature and strain rate on the dynamic behaviour of rocks is essential.
The thermal effect on rock dynamics has attracted extensive attentions from researchers. The dynamic fracture toughness of Fangshan gabbro and Fangshan marble subjected to high temperature was measured by (Zhang et al., 2001) with the short rod (SR) method on SHPB. It was found that temperature variation affects the dynamic fracture toughness of the two rocks to a limited extent within the tested temperature ranges. This result was different from the results obtained under the static loading condition. Yin et.al. investigated effect of thermal treatment on the dynamic fracture toughness of Laurentian granite (LG) conducted based on notched semi-circular bend (NSCB) test (Yin et al., 2012). The thermally induced micro-cracks within the rock samples were then examined by scanning electron microscope (SEM). They found that at temperatures below 250 °C, the thermal expansion of grains led to an increase in the toughness of the rock. At treatment temperatures above 450 °C, the sources of weaknesses such as grain boundaries and phase transition of silicon were depleted resulting in decrease of fracture toughness. Similar pattern was also found in tensile strength in Brazilian disc tests done by (Yin et al., 2015) on Laurentian granite after being treated with high temperature. These results showed that dynamic tensile strength first increases and then decreases with a linear increase of loading rate. Liu and Xu employed the SHPB method to conduct uniaxial compression and split tensile tests on Qinling biotite/granite samples, which were treated under high temperatures and then cooled naturally to room temperature (Liu and Xu, 2014). These researchers also concluded the effect of high temperature on the dynamic tensile and compressive strength. Huang and Xia used computed tomography (CT) to quantify the damage induced by the heat-treatment and correlated it with the dynamic compressive strength of Longyou sandstone which was obtained by SHPB (Huang and Xia, 2015). Further investigations by (Liu and Xu, 2015) were carried out on the influences of coupled temperature/strain rate effect on dynamic compressive mechanical behaviours of sandstone. No obvious strain rate effect was observed in tests conducted under high temperature compared to ones at room temperature when ratios of dynamic compressive strength, peak strain, and energy absorption ratio of rock were studied. Similar research was implemented on granite by (Fan et al., 2017), and results showed that the dynamic energy absorption capacity increases below 400 °C but then decreases as the temperature increases to 800 °C. The effect of thermal treatment on energy absorption capacity was more obvious under a smaller impact pressure in granite samples. The dynamic mechanical behaviours of coal samples exposed to elevated temperatures were also examined with SHPB unit (Yu et al., 2017). In this study, the anthracite specimens were preheated up to 500°C in an oxygen-free environment. The results showed that coal gradually loses its dynamic bearing and anti-deformation capacities with increase in temperature, especially after 300°C.
Offshore oil and gas field developments are capital-intensive projects that require extensive facilities to drill, produce and transport the hydrocarbon from the reservoir to the processing plant. Determining the site, number and size of these facilities are amongst the most important decisions impacting a project's success. Here, we present a novel strategy to assist in these decisions by combining a stochastic optimization routine with a Virtual Reality (VR) Aided Design. The model uses a discrete-network optimization algorithm that employs a Monte Carlo Markov chain to explore feasible configurations that minimize the development's investment. It integrates the optimization with a state-of-the-art VR environment to allow the engineer to both monitor the progress of the optimization and help guide the field development in real time.
We present results illustrating how the approach can be employed in field developments to connect well targets to processing facilities. The model determines the optimum location, size and number of offshore well-head platforms, tie-in facilities, well paths and pipeline routes. It incorporates critical technical considerations for the design of drilling paths (e.g. dog leg severity) and surface facilities (e.g. water depth). The model has been applied to real data from offshore field developments in the North Sea and the Gulf of Mexico. Results including the investment value and optimum configuration are shown and supplemented with graphics from the VR environment. The VR technology enables a novel approach to optimize the development. The immersive platform lets the user not only visualize the field, it is also capable of providing real-time interaction with the computer-generated design. This allows the integration of engineering intuition and experience to enhance the development and eliminate infeasible or unfavorable configurations.
We acquired microtremor array data at 11 sites in the Seattle basin, Washington State, and applied the wavenumber-normalized SPAC method (krSPAC) to obtain
Presentation Date: Monday, October 15, 2018
Start Time: 1:50:00 PM
Location: 213B (Anaheim Convention Center)
Presentation Type: Oral
ABSTRACT: According to the studies on tight gas reservoirs, liquid CO2 (L- CO2) is much superior in hydraulic fracturing compared to conventional fracturing fluids. However, the applicability of this technique for hydraulic fracturing of coal seams has been hindered due to the lack of understanding. This paper investigates the superiority of L-CO2 as a fracturing fluid for coal seam gas extraction, in terms of break-down pressure and acoustic emission (AE) energy release during fracture propagation. The results reveal that L-CO2 induced break-down pressure is around 19.6% lesser than the water induced break-down pressure, whereas the time taken to break-down is 59.3% higher for L-CO2 compared to water. Importantly, due to the low compressibility, water injection pressure showed an exponential pressure development closer to the break-down, resulting a higher break-down pressure within a short time period. Conversely, the highly compressible L-CO2 exerted only a gradual increment in pressure in the sample over a considerable time, which caused a more controllable fracturing process. The observed AE suggests that the ability of low viscous L-CO2 to penetrate through the tiny pores in the coal mass has the potential to create a stable and dense fracture network, instead of the uncontrolled unstable sudden failure occurred under the water based fracturing. This well-developed fracture network can significantly enhance the rock mass permeability and the reservoir gas extraction. Overall, it can be concluded that the combined characteristics of L-CO2 (high compressibility, low viscosity) lead more controllable and effective coal seam hydraulic fracturing process compared with the water based fracturing.
Hydraulic fracturing is one of the well-established well-stimulation methods, which has been used for enhanced coal bed methane extraction (ECBM), over last few decades. The process involves mechanically fracturing of the reservoir by injecting a pressurized fluid into the rock formation through vertical and horizontal wellbores drilled in the reservoir. A mechanically induced fracture network, as provided by this process, is necessary to deliver an economic gas production, especially from low permeable reservoirs like coal. The efficiency of the fracturing process depends on the characteristics of the induced fracture network, which ultimately governs the permeability enhancement of the formation. Thus, the induced fracture network should have a large surface contact area between fractures and the reservoir and a high fracture interconnectivity, in order to provide an easy pathway for the gas to move towards the wellbore (King, 2010).
ABSTRACT: Cable bolts are extensively used as the most reliable support systems in underground and surface structures leading to some investigations on the characterization of the performance of the cable bolts under different conditions including field and laboratory environments. This paper aims to develop an analytical model for prediction of the full load-displacement performance of the Sumo cable bolt under constant normal stiffness which due to its unique design it has been classified as the modified cable bolt. The model includes nonlinear dilation and incremental shear load equations inspired by rock joint discipline. The nonlinear shear displacement between the cable bolt and the grout is captured through a loop of incremental raise in the axial load for a given unit displacement. The model is then calibrated against the experimental data obtained from Sumo cable bolt. A good agreement was found between the model simulations and the experimental results confirming the applicability of the proposed constitutive model for the performance prediction of the Sumo cable bolts tested under constant normal stiffness.
Fully grouted cable bolts are the conventional ground support systems in both civil and mining operations (Aziz et al, 2015). They are employed to transfer the load from the unstable layer of strata above the underground excavation to the stable layer (Ghadimi, S hahriar and Jalalifar, 2015). Therefore, understanding the failure mechanism in cable bolt is essential for the safe design of structures on or within the rock mass. It has been reported by different researchers that the cable bolt failure generally occurs at the bolt to grout interface if the strand rupture has not initiated (Goris, 1991; Hyett, Bawden and Coulson, 1992; Hyett, Bawden and Reichert, 1992; Yazici and Kaiser, 1992; Hutchinson and Diederichs, 1996). Thus, the research focus primarly should be on the failure mechanism at the cable bolt to grout interface.
Li et al (2017) has proposed two classifications for the current cable bolts used in industry based on the surface geometry including conventional and modified. The former is made of several plain steel strands such as plain strand cable bolt while the latter consists of bulbed, nutcase and birdcaged that exhibit deformed structure. To date, the performance of conventional cable bolts under axial loading has been extensively studied from both laboratory (Stillborg, 1984; Farah and Aref, 1986; Hassani and Rajaie, 1990; Reichert, 1991; Goris, Martin and Brady, 1993; Macsporran, 1993; Benmokrane, Chennouf and Mitri, 1995; Hyett et al, 1995; Blanco Martín et al, 2013) and analytical viewpoints (Hyett et al, 1995; Blanco Martin, 2012; Chen, Saydam and Hagan, 2015).
ABSTRACT: We use a direct shear apparatus with embedded ultrasonic transducers to correlate the macroscopic frictional response with the microscopic contact processes occurring between two blocks of shaly sandstone. At constant normal load, we observe stable sliding at low velocities and oscillatory stick-slip at high velocities. For slow sliding, or during the stick phase of the stick-slip cycle, variations in the transmitted compressional P-wave amplitude show the existence of healing processes occurring at the joint, e.g. associated to the increase in contact area with contact time. Moreover, the transmitted shear S-wave amplitude is sensitive to other processes with opposite velocity dependence. The interplay between these processes, displaying velocity weakening (VW) and velocity strengthening (VS) respectively, explain the observed maximum in the steady state shear response as function of shearing rate. We also observe that the wave velocity is sensitive to gouge formation at the joint. This mechanism is induced by sliding and enhanced with shearing rate. At low shearing rates, in the VS region, small amounts of debris are formed, slightly strengthening the joint. At high shearing rates, in the VW region, the wear material significantly increases the mean separation between the two surfaces, resulting in a weaker joint.
Understanding contact scale physical processes is necessary to interpret in detail the frictional behavior of natural and simulated faults. The extent of these processes, such us plastic creep, gouge comminution or capillary condensation, directly determine the state of contact and overall strength of the fault, either by affecting the real contact area or the local strength of contact zones. Geophysical imaging of ultrasonic waves interacting with rock joints have been used as a non-destructive technique that measures the state of contact and real contact area (Kendall and Tabor, 1971; Nagata et al., 2008, 2012; Hedayat, 2013; Hedayat et al., 2014a-d; Gheibi and Hedayat, 2018a). Ultrasonic waves transmit through the points in contact and reflect at the air void spaces and in this way they can provide an indirect measure of the state of contact within the zone of measurement. Thus, the intensity of the wave transmitted through the asperities in contact can be a measure of the stiffness of the asperities and the real contact area.
Shear experiments conducted on gypsum rock joints (Hedayat et al., 2012; 2013) and Indiana limestone rock joints (Hedayat et al., 2014a, 2018; Hedayat and Walton, 2017) show that precursors in the form of maximum amplitude of ultrasonic waves appear before reaching the static shear strength of contacting surfaces. Based on the variations in the amplitude of transmitted and reflected ultrasonic waves, Hedayat et al. (2018) identified two major processes occurring during both frictional sliding and stick-slip oscillations, as follows: (a) interseismic phase and (b) preseismic phase. The interseismic phase was associated with small local slip rate and increasing ultrasonic transmission along the contact surfaces while the rock joint was locked. Such increase in the ultrasonic transmission represented an increase in the real (true) area of contact. The onset of preseismic phase was associated with the appearance of precursors for different regions of the rock joint.
ABSTRACT: A modified version of conventional Hoek cell (QRT Cell) has been designed and developed for advanced triaxial te sting. QRT cell is capable of capturing radial deformation of the core samples through a new set of Linear Variable Differential Transducer (LVDT) designed to attach to core sample diametrically. The cell has significant advantage of minimal setup time, measuring permeability and controlling the temperature during the experiment with high accuracy. A number of triaxial experiments were performed on Gosford sandstone using the new cell to demonstrate its intrinsic advantages.
Conducting triaxial experiment is vital for detailed characterization of the mechanical properties of rock under confinement in many engineering projects. In addition, with emerge of Multiphysics studies and its importance in investigating reactive rocks behavior (Roshan and Oeser 2012, Roshan and Aghighi 2012, Roshan and Fahad 2012) the use of triaxial cells capable of measuring thermo-chemo-hydro-mechanical interactions are highlighted in recent years. Currently two devices are available for such test: Hoek cell (Hoek and Franklin 1968) and Triaxial cell (Kovari et al. 1983). The former is fast and easy to use but is incapable of providing the radial deformation of the tested sample. The latter can measure on sample deformation but has a long setup time especially due to its size and weight and associated portability issues (Masoumi et al. 2015).
In this paper a newly designed triaxial cell is presented. The design of new cell is inspired by the initial Hoek cell. The cell has all the benefits of the conventional Hoek cell plus the capability of measuring the radial deformation of the core samples from the commencement of loading up to the residual stress using the miniature Linear Variable Differential Transduces (LVDT) i.e. axial deformation can be measured using displacement of loading frame or on platen LVDT. A number of triaxial tests were performed on Gosford sandstone samples at three different confining pressures including 10, 30 and 50 to demonstrate the characteristics of this triaxial cell.
2. CONVENTIONAL TECHNIQUES
2.1. Triaxial cell
International Society for Rock Mechanics (Kovari et al. 1983, ISRM 2007) and American Systems for Testing and Materials (ASTM 2000) have suggested the application of the triaxial cell for rock testing under triaxial loading. The axial and radial deformations of the samples from the commencement of loading up to large shear strains beyond the post-peak can be measured and recorded with this cell. LVDTs are used for recording both axial and radial deformations. However, there are two drawbacks associated with this methodology including the setup time and the accuracy of the radial deformation measurement. Due to the unique design of the circumferential extensometers which are used in triaxial cells, the measured radial deformations need some additional modifications as suggested by Masoumi et al. (2015). In addition, considering the high cost of triaxial systems, they are not economically justified in many instances.
ABSTRACT: A new closed-form equation is presented to measure the tensile strength of rock based on point load strength index. The validity of the proposed equation is further examined experimentally through a set of diametral point load testing of monzonites and two different types of granite having various slenderness (length to diameter) ratio. Based on the results, it is concluded that the induced tensile stress at the centre of a diametral point load specimen as well as the point load strength index are a function of both Poisson's ratio and sample length. In general, when either Poisson's ratio or the length of the sample increases, the induced tensile stress decreases. However, results indicate that the gradient of the induced tensile stress is more sensitive to the sample length than the materials Poisson's ratio. The proposed formula successfully explains such behaviors and provides estimations that are in good agreement with the experimental data.
Tensile strength is the most critical design parameter of brittle materials in a wide range of engineering applications. Accurate and reliable measurement of the tensile strength is therefore necessary, although this has always inherited a major uncertainty. The rock mechanics literature is replete with both direct and indirect testing methods to assess this critical rock property. However, the direct test methods, e.g. the direct uniaxial pulling test (DPT) on rock cores or testing “dog-bone” samples, are notoriously too difficult and expensive for routine application to large numbers of specimens. Hence, indirect tests have been reported to offer the most desirable alternative. Among the indirect methods, the Brazilian test (BT) developed by Carneiro (Carneiro, 1943), the Ring test (Ripperger and Davids, 1947; Serati and Williams, 2015), and the point load strength test (PLST) are perhaps the most widely accepted methods, with their initial applications trace back to concrete testing. The International standard testing communities such as American Systems for Testing and Materials (ASTM) and the International Society for Rock Mechanics (ISRM) have also recognized these tests for use in practice and design (ASTM, 1995; Ulusay, 2014). In the PLST, a rock specimen separates usually into two pieces mainly due to the propagation of a single tensile crack, similar to the failure pattern in the BT conditions. The PLST results can be therefore correlated to the material’s tensile strength (BTS) and Unconfined Compressive Strength (UCS) according to the following imperial relationships (ISRM, 1985; Chau and Wong, 1996; Li and Wong, 2013):