Chung, Traiwit (University of New South Wales) | Wang, Ying Da (University of New South Wales) | Armstrong, Ryan T. (University of New South Wales) | Mostaghimi, Peyman (University of New South Wales)
Direct simulation of flow on microcomputed-tomography (micro-CT) images of rocks is widely used for the calculation of permeability. However, direct numerical methods are computationally demanding. A rapid and robust method is proposed to solve the elliptic flow equation. Segmented micro-CT images are used for the calculation of local conductivity in each voxel. The elliptic flow equation is then solved on the images using the finite-volume method. The numerical method is optimized in terms of memory usage using sparse matrix modules to eliminate memory overhead associated with both the inherent sparsity of the finite-volume two-point flux-approximation (TPFA) method, and the presence of zero conductivity for impermeable grain cells. The estimated permeabilities for a number of sandstone and carbonate micro-CT images are compared against estimation of other solvers, and results show a difference of approximately 11%. However, the computational time is 80% lower. Local conductivity can furthermore be assigned directly into matrix voxels without a loss in generality, hence allowing the pore-scale finite-volume solver (PFVS) to be able to solve for flow in a permeable matrix as well as open pore space. This has been developed to include the effect of microporosity in flow simulation.
Suo, Yu (University of New South Wales) | Chen, Zhixi (University of New South Wales) | Rahman, Sheik (University of New South Wales) | Xu, Wenjun (University of New South Wales and Southwest Petroleum University)
Hydraulic fracturing is a significant way to improve the productivity of the unconventional reservoir with low permeability and porosity. Current hydraulic fracturing simulation models are mostly based on poro-elastic theory. However, for rocks such as shale, the viscoelastic feature has been observed in both field investigations and laboratory experiments. This paper presents a 3D numerical model for fracture propagation in viscoelastic shale gas formations using ABAQUS platform. The cohesive elements based on damage mechanics were adopted to simulate the initiation and propagation of hydraulic fractures. The model was used to investigate formation properties and treatment parameters on fracture geometry, especially the fracture behaviour when entering into the barrier formations. It is found that higher treatment pressure is required to initiate and propagate the hydraulic fracture and the fracture is wider but shorter in poroviscoelastic formation comparing to poro-elastic formation. The higher differential in-situ stress, tensile strength and Young modulus in barrier formations and lower fracturing fluid injection rate and lower fracturing fluid viscosity have positive effect on the controlling of fracture vertical growth and restricting hydraulic fracture within the pay zone. Results of this study will provide the industry a better understanding of hydraulic fracture behaviour in shale gas formations.
Xu, Wenjun (Southwest Petroleum University) | Zhao, Jinzhou (Southwest Petroleum University) | Li, Yongming (Southwest Petroleum University) | Rahman, Sheik S (University of New South Wales) | Fu, Dongyu (Southwest Petroleum University) | Chen, Xiyu (Southwest Petroleum University)
Complex fracture network makes it possible for commercial exploition of shale gas by means of hydraulic fracturing. It was believed that the interaction between hydraulic fracture (HF) and natural fracture (NF) had a significant impact on HF complexity. In this paper, a new numerical model has been developed to investigate HF/NF intersection under different geological and engineering parameters. Displacement discontinuity method (DDM) and finite volume method (FVM) are used to numerically model and solve the problem of coupled rock deformation, fluid flow, interface slipping, and opening associated with HF propagation and its interaction with NF. In addition, the model also considers the effects of fracture fluid leak-off. Based on the model, sensitivity analyses of key influence parameters are implemented. The numerical model results provide detailed quantitative information on fracture-geometry evolution, interfacial stress distribution and injection-pressure history. The simulation results show that the HF tends to cross the NF under the conditions of high principal stress difference, high intersection angle, high interfacial friction, high injection rate, high fracturing fluid viscosity and low initial conductivity of the NF. Moreover, the morphology of HF is significantly affected by two engineering parameters, the injection rate and the viscosity of the fracturing fluid. The effect of these two engineering parameters on the morphology of HF can be expressed as the product of them. The same value of the product results in the same HF morphology at the times of same injected-fluid volumes. In addition, the injection pressure curves can also help determine whether a crossing HF is developed when a HF interacts with a NF. The numerical model provides an effective approach for quantitatively analyzing the development of various types of HF/NF interaction behavior. It allows us to gain a better insight to the performance of hydraulic fracturing treatments in naturally fractured reservoirs.
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).
The overlying strata is often destroyed in large-scale during shallow coal seam mining, and the sliding instability of the caved roof seriously threatens the safety of the mining field. Based on the monitoring data of the roof weighting of a typical shallow coal mining, the load distribution characteristics of the roof along the strike and trend of the mining field were analyzed, and the mechanical model of the pressure-arch in the surrounding rock was established. Then the evolution characteristics of the pressure-arch and the elastic energy of the surrounding rock were revealed during shallow coal mining by theoretical analysis and numerical simulation. The results show that the continuous pressure-arch was formed when the horizontal stress exceeded the primary vertical stress of the mining field, and the elastic energy of the roof was released by the mining unloading effect. The caved zone of the overlying strata was formed below the inner boundary of the pressure-arch. The elastic energy was accumulated in the pressure-arch and the energy arrived the highest at the front arch foot. The accumulated energy at the arch foot was released by coal mining and the shear zone could be formed. So the sliding of the caved zone along the shear zone would induce the strong roof weighting. The concentrated stress and the released energy during each mining increased with the panel advancing, and the height of the shear zone also increased. The conclusions obtained in the study are of important theoretical value to direct the similar engineering practice.
The instability of overlying strata during shallow coal mining, such as the large-scale roof falling and step-like ground subsidence, is a key problem that can restrict the safety mining in the mines (Ju &; Xu 2015). The self-bearing structure of the pressure-arch can form in the overlying strata after the coal mining, and this structure can support the load of the upper strata and soil layer, so the weighting intensity of the panel is determined by the caved rock in the unloading zone under the inner boundary of the pressure-arch. A large amount of elastic energy is accumulated in the pressure-arch under the concentrated stress, and the released energy for mining is the internal cause of rock failure (Wang et al. 2017). It is an important problem to reveal the distribution characteristics of the stress and energy fields in the mining field, and to analyze the stability of the overlying strata during shallow coal mining based on the evolution characteristics of the pressure-arch.
There is a worldwide increase in tunnel excavation especially for infrastructure development which aims to improve the amenity of urban life. When construction of these tunnels is near urban dwellings and/or rockmass conditions dictate, mechanised tunnelling methods are often favoured. In Sydney, Australia, the geotechnical conditions suit the use of roadheaders for construction in massive sandstone of multi-lane vehicular tunnels with project costs measured in the hundreds if not billions of dollars. Despite their enormous costs, these projects are highly competitive with tight budgets and of relative short duration, consequently there is little incentive to invest in the development of computerised data collection and monitoring systems.
A recent project was undertaken involving a major road tunnelling project having multiple faces and construction sites where a system was developed comprising data collection, aggregation and analysis of a fleet of roadheaders with the aim of providing information to improve construction management. Over a 36-week period, data was collected from 22 roadheaders entailing 122,000 shift activities across 9200 shifts together with changes in geotechnical conditions. The data was combined into a single database that provided useful productivity metrics to the various project management teams and other stakeholders.
The project demonstrated the benefits of a largely automated, centralised data model that provided timely and reliable information and, eliminated a significant amount of data-entry work resulting in more productive time for site engineers. The value of such a system was realized when it was implemented in a subsequent construction project and used to provide reliable data in the planning and tendering of future projects. The system enabled roadheader productivity to be optimised in the project, by for example confirming the critical path in the roof support installation stage of the excavation cycle could be reduced in adopting split-face headings rather than full-face headings.
The use of large-scale data analytics to drive business improvement is increasingly being used across many industries as the technology used to collect and interrogate data becomes more accessible, and the commercial benefits of such information become apparent. For example, as the mining industry moves towards integrating robotics and automation into its processes, an unprecedented amount of data relating to every stage of a mining process is available to organisations to drive improvements in productivity, design and planning.
Tahmasebinia, F. (University of New South Wales) | Zhang, C. (University of New South Wales) | Canbulat, I. (University of New South Wales) | Vardar, O. (University of New South Wales) | Saydam, S. (University of New South Wales)
Stability of underground excavations in mining and tunneling is one of the most significant concerns of geotechnical engineers. Rock reinforcement methods can be used to increase rock strength and minimise the displacement of unstable rock mass. It is important to understand how the reinforcement systems work to ensure the stability of underground excavations. Rock bolts together with cables bolts have been commonly used as an effective underground support system and an element of reinforcement to improve excavation stability. The principle of rock bolting is one of the most researched aspects of ground control as they are used under a variety of loading conditions and geological environments. Both support systems are usually considered to be subjected to static loads under relatively low stress conditions; however, under high stress conditions, especially burst-prone conditions, they may be subjected to dynamic loading. Cable bolts and other support elements in such conditions are used to absorb the kinetic energy of the removed rock to avoid sudden and violent failures. Therefore, cable bolts that have high energy-absorption capability are becoming more widely used for burst control. In this paper, numerical simulation analyses are developed for cable bolts, grout and rock mass, as well as the interactions between them under static and dynamic loading, to evaluate the performance of cable bolts and surrounding materials. To validate the suggested numerical models, the simulated results are compared with the reported experimental observations in the literature. A novel numerical model is suggested to predict the dynamic behaviour of the cable bolts under impact loading.
Rock reinforcement methods have been used to increase rock mass stability by increasing its strength and minimising the displacement of unstable rock mass in underground excavations. Both rock bolts and cable bolts are widely used in underground mines and tunnels. A rock bolt is usually a steel bar, which is fixed into rock mass either by grouting or mechanically, and it can be pre-tensioned. A cable bolt is a flexible tendon that is made of twisted steel wires, with high tensile strength to support rock mass, and it is usually fixed into rock by resin and/or grouting. There are various types of rock bolts which vary in size, capacity and geometry to suit different ground conditions.
A large amount of research, including experiments, has been conducted to investigate the tensile failure and load transfer capacity of rock bolts and cable bolts. The pull test and the shear test are the two methods used to examine rock bolt performance. The short-encapsulated pull test, which can be carried out in both the laboratory and the field, evaluates the axial reinforcement behaviour of rock bolts and cable bolts. The shear test is normally undertaken in the laboratory and it includes two methods: the single shear test and the double shear test. The single shear test can underestimate the shear strength of the bolts under certain circumstances. The double shear test is usually conducted on fully grouted and axially tensioned bolts installed in different types of three-piece adjoining concrete blocks.
Rock bolts and cable bolts are usually considered to experience static loads under relatively low stress conditions. However, in burst-prone conditions, support elements are subjected to dynamic loading. Therefore, it is important to understand the bolt behaviour under dynamic loading conditions, especially from the perspective of energy absorption. This paper presents the results of explicit modelling of cable bolt behaviour and interactions with the host rock mass under static and dynamic loading to improve the understanding of support responses under dynamic loading conditions, which can assist in controlling dynamic failures.
Recent laboratory studies have shown fines migration induced decrease in rock permeability during CO2 injection. Fines migration is a pore scale phenomenon, yet previous laboratory studies did not conduct comprehensive pore scale characterization. This study utilizes integrated pore scale characterization techniques to study the phenomenon.
We present CO2 injection experiments performed on two Berea sandstone samples. The core samples are characterized using nitrogen permeability, X-ray micro-computed tomography (micro-CT), Scanning Electronic Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDS) and Itrax X-ray Fluoresence (XRF) scanning. The core samples were flooded with freshwater, then CO2-saturated water, and finally water-saturated supercritical CO2 (scCO2). To calculate permeability, the pressure difference across the core samples was monitored during these fluid injections. The produced water samples were analysed using Inductively Coupled Plasma-Optical Emission Spectrometry (ICPOES). After the flooding experiment, nitrogen permeability, micro-CT, SEM-EDS and Itrax-XRF scanning was repeated to characterize pore scale damage. Micro-CT image based computations were run to estimate permeability decrease along the core sample length after injection.
Results show dissolution of dolomite and other high density minerals. Mineral dissolution dislodges fines particles which migrate during scCO2 injection. Berea 1 and Berea 2 showed respectively 29% and 13% increase in permeability during CO2-saturated water injection. But after water-saturated scCO2 injection, both Berea 1 and Berea 2 showed 60% decrease in permeability. The permeability damage of the sample can be explained by fines migration and subsequent blockage. SEM-EDS images also show some examples of pore blockage.
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.
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.