B.H. Brady

Evaluation and application of controlling parameters for seismic events in hard-rock mines

A method is proposed for quantitative assessment and interpretation of the seismic hazard of planned excavation in a hard-rock mine. The quantitative assessment is derived from three-dimensional, elastic, boundary element modelling of the host rock mass. Parameters obtained from the modelling are associated with historical observations of seismicity in the mine to generate probabilistic relations between seismic event occurrence and event strength. The parameters used in the event spatial occurrence relations and the event strength estimates are the Factor of Safety against seismic failure for different types of seismic events, which are inferred from back analysis of field observations, and the Modelled Ground Work. The estimates of these parameters are related to the controlling quantities for a seismic event, which are a local state of stress sufficient to cause rock mass failure, and an unstable local concentration of strain energy, i.e. energy in excess of that which can be dissipated non-violently in rock mass failure. Application of the method is illustrated by reference to several case studies of mine seismicity.

Simulations of a coupled hydro-chemo-mechanical system in rocks

A coupled hydro-chemo-mechanical numerical model is developed for these coupled phenomena in many engineering fields. The model has been applied to predicting the response of a stressed rockmass column to an injected reactive fluid (reagent) flow. The response includes evolutions of porosity, permeability, reagent and mineral concentrations during dissolution. In the model, the progress of dissolution is defined by the change in porosity ratio and the porosity increases with dissolution assuming there is no precipitation. The numerical evolutions of porosity, permeability, reagent and mineral concentrations during dissolution are validated against steady state solutions. The model results show that these evolutions are regulated to a certain extent by the applied external loadings: an applied extensional stress enhances the progress of the dissolution process while an applied compression stress slows the progress of the dissolution process.

A coupled hydromechanical system defined through rock mass classification schemes

Changes in the hydraulic conductivity field, resulting from the redistribution of stresses in fractured rock masses, are difficult to characterize due to complex nature of the coupled hydromechanical processes. A methodology is developed to predict the distributed hydraulic conductivity field based on the original undisturbed parameters of hydraulic conductivity, Rock Mass Rating (RMR), Rock Quality Designation (RQD), and additionally the induced strains. The most obvious advantage of the methodology is that these required parameters are minimal and are readily available in practice. The incorporation of RMR and RQD, both of which have been applied to design in rock engineering for decades, enables the stress-dependent hydraulic conductivity field to be represented for a whole spectrum of rock masses. Knowledge of the RQD, together with the original hydraulic conductivity, is applied to determine the effective porosity for the fractured media. When RQD approaches zero, the rock mass is highly fractured, and fracture permeability will be relatively high. When RQD approaches 100, the degree of fracturing is minimal, and secondary porosity and secondary permeability will be low. These values bound the possible ranges in hydraulic behaviour of the secondary porosity within the system. RMR may also be applied to determine the scale effect of elastic modulus. As RMR approaches 100, the ‘softening’ effect of fractures is a minimum and results in the smallest strain-induced change in the hydraulic conductivity because the induced strain is uniformly distributed between fractures and matrix. When RMR approaches zero, the laboratory modulus must be reduced significantly in order to represent the rock mass. This results in the largest possible change in the hydraulic conductivity because the induced strain is applied entirely to the fracture system. These values of RMR bound the possible ranges in mechanical behaviour of the system. The mechanical system is coupled with the hydraulic system by two empirical parameters, RQD and RMR. The methodology has been applied to a circular underground excavation and to qualitatively explain the in situ experimental results of the macropermeability test in the drift at Stripa. Copyright

A Coupled Flow-Transport-Deformation Model for Underground Coal Gasification

Abstract A coupled flow-transport-deformation model is developed to simulate underground coal gasification (UCG) though which coal is converted to useful gases in-situ without the need for mining. During UCG the carbon in a coal seam is burnt while high heat energy is released. As a result of burning, new cavities are created in the coal seam. These cavities will modify the permeability field dramatically. The modified permeability field will in turn affect the gasification. It is apparent that UCG is a coupled phenomenon involving hydrological, chemical, thermal and mechanical processes. In this study, UCG is simulated as a selective mining method through introducing a cavity evolution equation. The equation defines the selectivity as a function of porosity, and can be understood as the influence of the residual carbon on the gasification rate. When the porosity is equal to the original value (the initiation of gasifying), the gasification rate reaches its maximum; when the porosity is equal to the final value (the completion of gasifying), the gasification rate is reduced to zero. The introduction of this equation into classical flow and transport equations constrains the whole gasification process from initiation to completion. Numerical results of a typical five-spot gasification layout have confirmed the validity of this approach.

EVALUATION OF IN SITU LEACHING PROCESSES: DUAL-POROSITY MODEL

A dual-porosity model is developed to study processes of in situ leaching. The model involves two overlaying continua at the macroscopic level: a permeable fracture system that determines the flow field of leach solution and a relatively impermeable matrix system that determines the leaching kinetics. The most obvious advantage of the model is that parameters it requires are minimal and easily available in practice. These parameters include the in situ hydraulic conductivity, longitudinal and transverse dispersivities, the lumped rate constant, and an empirical rock mass classification index, RQD (Rock Quality Designation). The simulation of in situ leaching processes is linked to RQD through the effective porosity of fractured media. The incorporation of RQD enables the simulation of in situ leaching processes to be carried out for a whole spectrum of ore deposits. When RQD approaches 0, it represents that the ore deposit may be a porous medium with a high effective porosity. This may reduce the double-porosity model to a single porosity model. When RQD approaches 100, it represents that the ore deposit may be considered as impermeable and unleachable. These values bound the possible ranges in behavior of the system. Based on the double-porosity model, the relation between particle size and leachability is developed, and the effects of double porosities on the concentration of a dissolved mineral are investigated. It is demonstrated through model results that the recovery rate of a valuable mineral is mainly determined by the effective porosity of the fracture pore system, the porosity of the rock matrix system, and their ratios in addition to the concentration of reagent and the ore grade.

Simulation of Progressive Borehole Failure at Multi-Lateral Junctions

Abstract Multi-lateral junction failure, one of the challenges introduced by employing multi-lateral technology to improve production of oil and gas wells, has imposed significant impacts on the employment of this new technology. Therefore, a powerful computer simulator is required for analyzing the issues of both fracturing and collapse at the multi-lateral junction. In this study, we present such a simulator, Rock Failure Process Analysis (RFPA2D), in which rock is assumed to be heterogeneous at a mesoscopic level and an elastic-damage based constitutive law is used to describe the constitutive law of elements. The applicability of RFPA2D to simulate progressive borehole failure processes at junction locations is demonstrated through (1) successful comparisons of the simulated distribution of stress concentration factors with the known analytical solutions for the elastic case; (2) successful comparisons of the simulated borehole failure patterns with experimental observations reported in previous studies; and (3) simulations of progressive borehole failure processes under a variety of geometrical and loading conditions.

Evaluation of one‐dimensional in situ leaching processes

A methodology to characterizing processes of in situ leaching is developed to investigate various parameters that may affect the recovery of a valuable mineral and assure a successful application of the in situ leaching technology. The governing equations of in situ leaching processes for the one-dimensional case are solved both analytically and numerically for both the consumption rate of a lixiviant and the production rate of a target mineral. A numerical simulator, which is developed to evaluate coupled effects among leaching kinetics, solution flow and transport of the dissolved mineral species in saturated ore deposits, is validated against the steady-state solutions and applied to investigate the transient effects of various parameters on the mineral recovery. Results from the evaluation indicate that there exists an optimal flow velocity range of leach solution for the effective leaching of a particular ore deposit. The determination of this optimal leaching velocity may become a key to the design of a real in situ leach mine. Results of the parameter sensitivity study illustrate the relative importance of other parameters such as porosity, ore grade, and reaction rate.