Saydam, Serkan (University of New South Wales) | Wu, Saisai (University of New South Wales) | Ramandi, Hamed Lamei (University of New South Wales) | Crosky, Alan (University of New South Wales) | Timms, Wendy (Deakin University) | Hagan, Paul (University of New South Wales) | Hebblewhite, Bruce (University of New South Wales) | Vandermaat, Damon (University of New South Wales) | Craig, Peter (University of New South Wales) | Chen, Honghao (University of New South Wales) | Elias, Elias (University of New South Wales)
Catastrophic failure of rockbolts and cable bolts, due to stress corrosion cracking (SCC), is a major problem in many underground excavations that can compromise both safety of the workers and the economic viability of the operations. This paper reports on development of laboratory instruments and methodologies at UNSW Sydney for simulating SCC in laboratory environments. Both representative coupon testing, and full-scale rockbolt and cable bolt testing methodologies are presented. Coupled with a detailed environment characterisation and field tests, the laboratory methodologies will aid in further understanding of SCC and identifying the potential countermeasures to prevent SCC occurrence in underground excavations.
In underground structures, excavation of rocks reduces the confining pressure on the surrounding rocks, allowing the strata to separate, fold and buckle into the void created (Aydan, 2018). Because rock is weak in tension, this buckling action can lead to fracturing of the strata and a roof failure. To prevent the relative movement and fracturing of the strata, rockbolts and cable bolts are often used to stabilise an excavation (Chen et al., 2016; Hadjigeorgiou and Potvin, 2011; Kilic et al., 2002; Oliveira and Diederichs, 2017; Windsor and Thompson, 1994). Rockbolts used in underground coal mines are usually manufactured from steel rods, typically 22 mm in diameter and 1200-2200 mm long, which are installed by drilling a hole into the rib or roof strata. Cable bolts are an evolution of rockbolting technology which are usually comprised of a number of wires wound together around a central king wire. Cable bolts usually offer a higher flexibility and load capacity than regular rockbolts (Chen et al., 2015; Galvin, 2016; Windsor, 2004). These, together with cable bolts greater length, allow for anchoring to a greater depth where the potential of presence of stable rockmass is high.
With the decline in the global coal reserves accessible for open-cut mining, underground mining at greater mine depths has increased the reliance of coal industry on rock reinforcing techniques. As the mining operations continue in greater depth, rockbolts and cable bolts encounter more challenging geological conditions. In the past few decades, a particular attention has been paid to failure of rock bolts and cable bolts in underground mines. One of the main causes of such failures has been identified to be stress corrosion cracking (SCC), which had been simply overlooked in the past. SCC requires synergistic occurrence of three key elements: stress, an appropriately corrosive medium and a material susceptible to SCC (Gamboa and Atrens, 2003; Jones, 1998). This synergy is described in the schematic shown in Fig. 1. The conditions required to induce SCC vary depending on each of the key element. The stress required to induce SCC is usually below the yield stress of the material. Stress corrosion cracks generally grow at a slow rate until the stress in the remaining section exceeds the fracture strength of the material, at which point the material will fail (Enos and Scully, 2002; Scully, 1975; Wu et al., 2018b). SCC results in a dramatic reduction in mechanical strength with only a very minor removal of material. In most cases, SCC is not noticeable by a casual inspection. Structures affected by SCC generally fail in a fast, sudden, brittle and catastrophic manner (Schweitzer, 2010).
Despite decades of numerical, analytical and experimental researches, sand production remains a significant operational challenge in petroleum industry. Amongst all techniques, analytical solutions have gained more popularity in industry applications because the numerical analysis is time consuming; computationally demanding and solutions are unstable in many instances. Analytical solutions on the other hand are yet to evolve to represent the rock behaviour more accurately.
We therefore developed a new set of closed-form solutions for poro-elastoplasticity with strain softening behaviour to predict stress-strain distributions around the borehole. A set of hollow cylinder experiments was then conducted under different compression scenarios and 3D X-Ray Computed Tomography was performed to analyse the internal structural damage. The results of the proposed analytical solutions were compared with the experimental results and good agreement between the model prediction and experimental data was observed. The model performance was then tested by analysing the onset of sand production in a well drilled in Bohai Bay in Northeast of China. Acoustic and density log along with core data were used to provide the input parameters for the proposed analytical model in order to predict the potential sanding in this well. The proposed solution predicted the development of a significant plastic zone thus confirming sand production observed by today sanding issue in this well.
Ramandi, Hamed Lamei (School of Petroleum Engineering, The University of New South Wales) | Armstrong, Ryan T. (School of Petroleum Engineering, The University of New South Wales) | Mostaghimi, Peyman (School of Petroleum Engineering, The University of New South Wales) | Saadatfar, Mohammad (Department of Applied Mathematics, Australian National University) | Pinczewsk, W. Val (School of Petroleum Engineering, The University of New South Wales)
An Australian bituminous coal is imaged at high resolution of 16.1 µm with (wet) and without (dry) X-ray attenuating fluids present in the pore space using a large-field three-dimensional microfocus helical X-ray computed tomography (micro-CT) instrument. Scanning Electron Microscope (SEM) is conducted on slices of the specimen to visualize coal micro-features up to resolution of about 15 nm. Two- and three-dimensional image registration techniques are used to precisely overlay micro-CT tomograms of the core plug in dry and wet conditions and SEM images to yield detailed three-dimensional visualizations of the geometry and topology of the fracture systems in coal. SEM images are also used to produce a calibration curve based on the relationship between the micro-CT intensity values and the true apertures of fractures within coal. This eliminates the need for two sets of imaging. Advanced filtering algorithms are applied to segment the micro-CT image into four distinct phases: resolved fractures, sub-resolution pores and fractures, macerals, and minerals. The application of micro-CT in determination of relative age relationships between adjacent geological features is presented. The distribution of resolved aperture size within the coal sample is investigated and the variation of permeability and porosity in several sub-samples of the coal is plotted. The analysis suggests that coal permeability is independent of porosity and is likely affected by other petrophysical properties such as lithotype. To include the effects of mineral phase on coal properties, we remove the segmented mineral phase and merge it to the resolved fracture phase. This analysis affirms that minerals are deposited in highly connected regions.