Erik Eberhardt

Fluid-injection-induced seismicity, e.g., that associated with hydraulic fracturing operations, is of high importance and interest to various stakeholders, including indigenous and local communities, regulators, operators and insurance companies. Failure to perform a thorough hazard assessment may result in undesired large magnitude events (>Mw2) that could impact safety, cause damage to infrastructure, and delay or stop projects. This thesis focuses on three key objectives: 1) to investigate whether hazard assessment relationships established for natural earthquakes can be used for fluid-injection-induced seismicity, 2) to understand the mechanism(s) of fluid-injection-induced seismicity events with focus on hydraulic fracturing operations in the Montney play of northeastern British Columbia (NEBC), and 3) to propose mitigation measures to potentially reduce fluid-injection-induced seismicity hazards. The methodology employed integrates empirical analyses with advanced 3-D numerical modelling. The empirical analyses are first applied broadly to a database compiled from global records of induced seismicity over a wide range of industrial operations, and then specifically using datasets from two well-pads in the Kiskatinaw area of NEBC. Numerical modelling supported these analyses and provided mechanistic understanding of them.Main findings include: 1) The commonly held belief that larger earthquakes occur in compressional stress regimes (as observed for natural earthquakes) is not necessarily true for fluid-injection-induced seismicity, and thus the use of focal mechanisms and b-values to assess hazard susceptibility and maximum magnitude potential should be approached with caution. 2) The dominant mechanism for large magnitude induced events in the Kiskatinaw area was found to be the direct effect of fluid-injection pressure perturbation associated with geological structures in the targeted reservoir volume that were limited in their hydraulic connectivity (i.e., where fracture intensity is low). 3) Higher injection pressurization rates were found to result in higher magnitude events, while lowering the rate was observed to cause the seismic moment release through many smaller events. The results point to mitigation measures such as monitoring and controlling the pressurization rate to control the rate of seismicity and event magnitudes, and increasing hydraulic fracturing fluid viscosity to reduce the fluid pressure front propagation to unfavorable (i.e., susceptible) areas outside the designed hydraulic fracturing zone.

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Hydraulic fracturing is the primary means of developing and completing production wells targeting unconventional gas reservoirs. However, these operations have been subject to public concern and regulatory oversight over their rapid growth in use, environmental footprint and potential hazards. This thesis is motivated by one of these potential hazards, induced seismicity, and the need to better understand the level of hazard present with respect to the event magnitudes possible, recognizing that these are not equal across different shale gas plays due to regional differences in the geological conditions or even within the same play due to local differences. The research presented focuses on three objectives: 1) to investigate the effect of different tectonic stress regimes on the magnitude distribution of induced seismicity events; 2) to analyze the influence of fluid injection rate and volume on induced seismicity; and 3) to compare the influence of different geological and operational parameters on induced seismicity in the Montney play in northeastern British Columbia. The methodology integrates statistical and machine learning analysis techniques with advanced 3-D numerical modelling. The empirical analyses are applied to compiled databases of recorded seismicity, hydraulic fracturing well data, and available geology and in situ stress data. Focus is placed on the Montney, but comparisons are also made to other shale gas basins. These analyses are complemented by 3-D numerical modelling used to provide mechanistic understanding to the trends observed in the empirical analyses. The main findings of this thesis are as follows. 1) Thrust faulting stress regimes have lower b-values than strike-slip stress regimes and therefore are more susceptible to larger induced seismicity events. 2) Specific to Montney, injection volume influences susceptibility to induced seismicity for wells that target naturally fractured formations, such as Middle Montney formation, whereas injection rate was seen to be an influencing factor for wells targeting the Upper Montney formation. 3) Machine learning provides a valuable means to assess the importance of different geological and operational parameters on induced seismicity for individual shale gas plays, allowing for susceptibility maps and mitigation options to be determined in advance.

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The continued demand for new resources and infrastructure in the civil, mining, and energy sectors has pushed these industries to greater depths. This has exposed projects to higher stresses and more complex failure modes involving massive brittle rock, which in turn has pushed engineers and designers to the limits of conventional empirical and numerical design capabilities, thus representing a significant challenge to the rock engineering profession. Massive rock becomes susceptible to brittle fracturing in high stress environments, surprising operators with behaviours ranging from progressive non-violent failure and dilative behaviour in the form of spalling to sudden and violent failure in the form of strainbursting. Needed are new solutions and engineering design tools tailored specifically to brittle fracturing mechanisms.However, the dominance of shear failure in weak/jointed rock masses under lower stress environments encountered during the development of rock mechanics as a discipline has led to a shear failure paradigm, for which σ₂-independent Mohr-Coulomb based strength /dilation models form the basis for present-day design tools. Not accounted for is the extensional fracturing observed under high stresses and low confinement conditions and its 3-D characteristics. This has led to serious consequences for deeper tunnelling and mining projects. Critical is the recognition of the dual nature of brittle failure, incorporating extensional and shear fracturing, and their 3-D directionality and 3-D confinement-dependency that affect the corresponding mobilization of strength and dilation.This thesis addresses these knowledge gaps with the objective to investigate and develop a series of new formulations specific to the brittle failure mechanism that more correctly models the progressive failure, strength mobilization, and bulking deformations experienced under complex stress paths for deep excavations in massive to moderately jointed brittle rock. First, a theoretical framework is developed that can describe the dual extensional/shear fracturing mechanisms, and their 3-D directionality and 3-D confinement-dependency. This understanding was then used to develop and formulate: i) an integrated 3-D confinement-dependent strength criterion that captures both extensional and shear fracturing, ii) a cohesion-weakening friction-strengthening strength mobilization model, and iii) a 3-D confinement-dependent dilation mobilization model capable of capturing the 3-D directional nature of spalling rock.

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The demand for metals combined with diminishing near surface resources has prompted the increasing development of complex and unprecedented open pit designs to recover deeper resources. These designs include pushback extensions, intentional over-steepening of toes, or the transition to underground retreat or mass mining methods. While past designs rarely involved pit depths exceeding 500 m, steeper and deeper designs approaching or exceeding 1000 m are now considered. Experiences with large open pits demonstrate that complex failure mechanisms occur with higher propensity within these slopes. New technologies used to monitor slope displacement, such as radar interferometry, along with increased real-time data processing have given engineers more data and faster tools to investigate the fundamental rock mechanics that occur within large slopes. Radar allows for the collection of large amounts of real-time data with millimeter precision. Emphasis is given in this thesis to the use of radar monitoring in resolving displacements in proximity to fault damage zones. Research was conducted to develop and execute a first of its kind 3-D radar experiment involving the simultaneous deployment of two radar systems. This experiment demonstrates that valuable knowledge, in the form of a 3-D displacement map, was used to resolve the influence of large fault zones in promoting complex slope deformation kinematics and failure mechanisms.In parallel, numerical modelling continues to develop as a key tool in understanding deep-seated rock slope deformation mechanisms. Research was conducted to investigate the characterization and representation of key fault properties within sensitivity analyses used to provide guidance on the impact of simplification of these complex structures. Representative geometries and input parameters based on case studies were used to show the influence of fault location, orientation and complexity, on stress heterogeneity created by the interaction between faults and deepening large open pits, as well as the transition to underground massmining. These interactions can create zones of plastic shear strain or extensional strain damage not typically accounted for in most stability analyses. The inclusion of stress heterogeneity and subsequent rock mass damage is shown to modify the observed mechanisms of slope movement and allow previously unviable kinematics to develop.

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Block cave mining is experiencing a global growth in importance as new large, lower grade and deeper ore bodies favouring underground mass mining methods are developed. With block caving, the rock mass fragmentation process is decisive in the design and success of the operation. The last stage of this fragmentation process known as secondary fragmentation, plays a major role in the design and success of a caving operation. Despite this, it is the least understood fragmentation stage due in part to the complex mechanisms and the numerous variables involved in this phenomenon. The broken ore density (BOD) and the inter-block friction angle (ϕ') are comprehensively investigated here. A conceptual framework describing the BOD distribution and a procedure to evaluate this parameter under both an isolated movement zone and interactive flow are proposed, and an approach to evaluate ϕ' under different broken ore properties and draw column conditions is developed to be applied to early stage feasibility studies and design. A comprehensive laboratory testing program was carried out using concrete cuboids, controlling their size, shape and compressive strength. These are used as a proxy for broken ore fragments. These results were used to develop empirical design charts for assessing secondary fragmentation and hang-ups potential. Several factors influencing the secondary fragmentation for feasibility and advanced engineering assessments have been investigated including: air gap thickness, BOD, segregation of large blocks due to draw column surface topology, broken ore strength heterogeneity, block strength damage and crushing under high confining stresses, water within draw columns, and cushioning by fines. This new knowledge will contribute to more accurate secondary fragmentation predictions at the drawpoints. Finally, a new empirical approach to predict secondary fragmentation and drawpoint block size distribution (BSD) directed at early-stage conceptual and feasibility engineering design studies is developed. This methodology, built with relevant data from related fields and supplemented by generated data, was tested against field data from the El Teniente mine, Chile, confirming satisfactory predictions for stronger rocks and mixtures of strong and weak broken ore materials. The results were not as reliable for predicting drawpoint BSD for weak rocks.

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My research on rock falls over the last five years is an extension of my 45 year professionalcareer that has included a wide variety of rock fall projects. This experience has provided mewith an excellent understanding of rock fall behavior; the objective of my research has been toapply this to developing improvements in rock fall modeling methods and design of rock fallcontainment structures. My research to meet these objectives has involved the following: Case studies – details of rock fall behavior at six locations with varied topography and geologyare presented, and the results have been used to verify the application of impact mechanicstheory to rock falls, and to calibrate modeling programs. Rock fall trajectories and velocities – the application of Newtonian mechanics to rock falltrajectories and velocities is described, and results compared with actual translational andangular velocities, and trajectory heights. Impact mechanics – the application of theoretical impact mechanics to rock fall impacts isdiscussed in terms of [normal impulse – relative velocity] diagrams for rough, rotating bodies,and equations relating impact and restitution velocities and angles. Coefficient of restitution – it is shown that the normal coefficient of restitution defined by the normal final and impact velocities is related primarily to impact angle rather than slope materialproperties. Furthermore, for shallow impact angles less than about 20 degrees, the normalcoefficient of restitution can be greater than 1.0. Energy changes – energy is lost during impact and gained during trajectories. Equations forenergy changes are developed, as well as diagrams showing values of changing potential, kineticand angular energies during rock falls. Rock fall modeling – results of rock fall modeling using the RocScience program RocFall 4.0 forfive case studies are presented; the applicable input parameters are listed. Design of protection structures –impact mechanics and scale model tests of protection netsshow that these structures can be designed to redirect rather than stop rock falls, and to absorbenergy uniformly during impact. These properties mean that only a portion of the impactenergy is absorbed by the net and that forces induced in the net are minimized.

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Hydraulic fracturing provides a means to optimize shale gas completions by enhancing the permeability of what is otherwise very tight rock. However, the coupled nature of the processes involved (e.g., thermo-hydro-mechanical-chemical), interlinked with geological variability and uncertainty, makes it extremely difficult to fully predict the spatial and temporal evolution of the hydrofrac and surrounding invaded zone. Numerical design tools have been developed to contend with this complexity, but these have largely focused on the mechanics of brittle fracture propagation at the expense of making simplifying assumptions of the host geology within which the hydraulic fracture is propagating, namely treating it as a linear elastic continuum. In contrast, the reservoir rock conditions are much more complex. Present are natural discontinuities, including bedding planes, joints, shears and faults superimposed by the in-situ stress field. The natural discontinuities under the applied in-situ stress have the potential to either enhance or diminish the effectiveness of the hydraulic fracturing treatment and subsequent hydrocarbon production. Improved understanding of the interactions between the hydraulic fracture and natural fractures under the stress field would allow designers and operators to achieve more effective hydraulic fracturing stimulation treatments in unconventional reservoirs. To better account for the presence of natural discontinuities in shale gas reservoirs, this thesis investigates the use of the 2-D commercial distinct-element code UDECTM (Itasca Consulting Group, 1999) to simulate the response of a jointed rock mass subjected to static loading and hydraulic injection. The numerical models are developed to illustrate some important concepts of hydraulic fracturing such as the effect of natural fractures in fracture connectivity, effects of stress shadowing in multiple horizontal well completion, and the effect of fluid injection in induced seismicity, so they can be used to qualitatively evaluate the effects of the in-situ environment on the design and the consequences of the design on the in-situ environment.

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While block caving presents an economic means to develop lower grade ore deposits, it often leads to significant ground deformations threatening the safety of overlying mine infrastructure. For guidance on relationships between caving depth and surface subsidence, a comprehensive cave mining database was developed and the database clearly shows caving-induced surface deformations tend to be discontinuous and asymmetric due to large movements around the cave being controlled by geologic structures, rock mass heterogeneity and topographic effects. Also shown is that as undercut depth increases, the magnitude and extent of the caved zone on surface decreases. Numerical modelling conducted in a benchmark study testing several different numerical methods (finite-element, distinct-element, FEM-DEM with brittle fracture and 3-D finite-difference) indicates this is only the case for macro deformations and the lateral extent of smaller strain deformations increases as a function of undercut depth, which indicates caution should be taken against relying on existing empirical design charts for estimates of caving-induced subsidence where small strain subsidence is of concern, as the empirical data does not properly extrapolate beyond the macro deformations. In addition,sophisticated 3-D numerical modelling was investigated as a means of predicting the extent and magnitudes of caving-induced surface subsidence. Results from a back analysis of the cave-pit interactions at the Palabora mine were used to constrain the rock mass properties and far-field in-situ stresses derived from field characterization data. The “best fit” set of input properties obtained was then used for forward modelling. Further calibration was performed using high-resolution InSAR monitoring data. The close fit achieved between the predictive 3-D numerical model and InSAR monitoring data demonstrates the significant value of InSAR calibrated 3-D numerical models.Collectively, the results of this research help to further the characterization, assessment and understanding of block-caving subsidence, by addressing existing limitations in the use of empirical and numerical subsidence analysis methods. The limitations and uncertainty arising from mine site data are described, specifically the representation of mine geology, rock mass properties, in-situ stresses and cave propagation, together with means to constrain these inputs and calibrate sophisticated 3-D numerical models through back analysis and integration with InSAR data.

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A dip slope is a natural or man-made rock slope with a persistent discontinuity behind the slope face that is coincident with, or similar to the slope inclination. Most dip slope failures occur in weak, orthogonally jointed sedimentary rock with toe breakout involving sliding along joints, plastic failure of intact blocks, and/or intense deformation of the slope that facilitates kinematic release. Dip slope failures have been reported to extend more than 20 percent of the slope’s height behind the crest, making this rock slope failure mechanism relevant in the context of engineering projects. Nonetheless, dip slope failure mechanisms and evaluation methods are not well understood because of the complexity of the toe breakout. This thesis provides an overview of dip slope failure mechanisms where bi-planar failures may occur. It provides specific guidance for evaluating a dip slope’s stability state predicated on an extensive literature review, numerical modeling, and parametric evaluations. The thesis provides methods for effectively planning and executing geotechnical investigations with the goal of establishing a dip slope’s stability state. Finally the thesis uses a comprehensive case study where very detailed geotechnical information is available as data for dip slope design. In summary, the results of this study and research suggest that: 1) typical failure mechanisms for dip slopes can be characterized and anticipated based on documented case histories and therefore site investigations should be customized towards evaluating the potential for those failure mechanisms, 2) typical geotechnical investigation and analysis methods (supplemented with numerical modeling where appropriate) may be used to evaluate a dip slope's stability state, 3) the influence of the rock mass shear strength at the toe and the failure mechanisms assumed for toe breakout is paramount while establishing a dip slope’s stability state where slopes are steeper than about 45 degrees, 4) the stability state of shallow dip slopes is dominated by the shear strength of the slope-coincident sliding surface, 5) statistical evaluations of other geotechnical parameters that dictate a dip slope’s stability suggest that at a scoping level, geotechnical investigation methods can be cost effectively planned to provide value to the geotechnical project, and 6) risk sharing dictates the current methods of dip slope evaluation and these methods can be improved based on the research contained herein.

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