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ABSTRACT: A seismic monitoring case study is documented for stimulation of multiple wells completed in the Lower Montney Shale. A network of 4 broadband stations and 1 accelerometer were deployed and recorded numerous minor seismic events with moment magnitude between 0.5 and 2.8. Sequential hydraulic fracture stages progressively activated several parallel, critically- stressed faults, resulting in induced seismicity during the injection. The case study provides key insights about the spatial and temporal characteristics of the seismogenic faults, and the relationship to fracturing operations. Source mechanisms show a predominantly strike-slip mechanism consistent with lineaments apparent from the seismic locations. The case study highlights observations during multiple fault activation with progressive fault activation and the corresponding ground motion, and provides seismic observations conducive to effective mitigation following a traffic light protocol.
Injection induced seismicity is an increasing concern throughout North America, particularly associated with salt water disposal in the U.S. Mid-continent and hydraulic fracturing in Western Canada. Hydraulic fracturing operations within localized regions of three reservoirs in Canada has documented induced seismicity with local magnitudes up to 4.6 Ml in the case of the largest event in the Montney trend. Regulators have imposed mandatory monitoring and a traffic light system where operations are modified to mitigate events where seismic magnitude exceeds a specific level.
The Montney shale is an active unconventional reservoir in Western Canada, where large scale hydraulic fracturing has been utilized to economically recover hydrocarbons. In NE British Columbia, the Montney operations occur near the Fort St. John graben structure. High volume hydraulic fracturing has resulted in felt seismicity, including some of the largest recorded induced seismicity associated with hydraulic fracturing (British Columbia Oil Gas Commission (BCOGC, 2014). In addition to the imposition of a traffic light system where operations are required to cease if magnitudes greater than 4 occur, ground motion requirements have also been introduced to understand local effects of felt activity.
In this paper, a case study is described using a local seismic array to monitor stimulation of four Lower Montney treatment wells. The resulting seismicity indicates several parallel, pre-existing faults were activated during treatment of the pad.
ABSTRACT: The stability of a slope can be evaluated according to its factor of safety. There are two approaches used to evaluate the factor of safety: the limit equilibrium method (LEM) and the shear strength reduction technique (SSRT). The LEM defines the factor of safety (FOS) as the ratio of nominal capacity to the demand of the system, and it is a force and/or moment equilibrium calculation. The SSRT calculates the strength reduction factor (SRF) using a stress/strain analysis in a numerical model.
Comparisons between the FOS and SRF are presented in this paper. This study aims to evaluate the impact of the slope angle, the dilation angle, and the elastic parameters on the factor of safety using LEM and the SSRT. Results have shown that the limit equilibrium methods generate factors of safety varying in the order of a tenth under the same slope conditions, and that the SRF can vary from the FOS by hundredths. This investigation has concluded that the SRF values are not necessarily always smaller than the FOS.
A slope is designated as a surface of which one end or side is at a higher elevation than another. The construction of roads, earth-dams, embankments, and the ore extraction process generally require the engineering of new slopes.
The evaluation of the stability of the slope is paramount to its design. The concept of slope stability varies for civil and mining purposes. In civil engineering designs that are focused in transportation and urban development, slopes are planned to be static, to avoid potential risks to public safety. Contrarily, surface mines are a dynamic environment, they are constantly changing, and therefore small-scale movements are acceptable in certain conditions.
Independent of the field of application, and of how conservatively the slope is designed, the definition of a standard criterion to evaluate how prone a slope is to failure is vital. This criterion is known as the factor of safety.
A slight change on the factor of safety causes major impacts on the Net Present Value (NPV) of a mining project. It is crucial to achieve the optimal slope conditions to maximize ore extraction, but still provide a safe environment for personnel and equipment.
Li, W. (Massachusetts Institute of Technology) | Opolot, M. (Masdar Institute of Science and Technology) | Sousa, R. L. (Masdar Institute of Science and Technology) | Einstein, H. H. (Massachusetts Institute of Technology)
ABSTRACT: The dissolution of rocks in underground flow paths is transport-controlled if their dissolution rates are relatively high, while the dissolution is reaction-controlled if their dissolution rates are relatively low. Transport-controlled dissolution is a common process in the formation of gypsum karst and oil reservoir acid stimulations. As an initial step to understand groundwater flow and dissolution in fractures and in wormholes, pipe flow is used as a simplified representation of groundwater flow. The extended Graetz solution was developed by Li and Einstein, 2017 to simulate the transport-controlled dissolution process when water flows through a tube in gypsum. This model can predict both the effluent concentration and the evolving geometry of the tube during the dissolution process. This paper focuses on the experimental validation of this model. An effluent chemistry monitoring system (ECMS) was developed and integrated in the triaxial system at MIT. Flow tests were conducted with cast gypsum tubes in this triaxial system. The experimental results of the effluent chemistry and tube geometries confirmed the validity of the extended Graetz solution.
The dissolution of rock minerals is a very common process that is of interest in a number of contexts: (1) underground gypsum and limestone karst formations (Benito et al. 1995; Johnson, 2008); (2) petroleum reservoir exploitation; (3) carbon dioxide sequestration (4) geothermal reservoir exploitation and heat storage; (5) mining and mineralization processes (in situ leaching); and (6) geotechnical applications (including effects on underground storage reservoirs, tunnels and other structures) (Cooper, 1986). The dissolution changes the geometry of the flow paths, which in turn affect the rate of dissolution in the rock-fluid system. For example, the dissolution of rock minerals in a rock matrix enlarges the pore space and concentrates flow in wormholes, which are the long, finger-like channels that form due to the nonuniform dissolution of the matrix. The dissolution of rock minerals in rock fractures enlarges the fracture and leads to channeled flow in rock fractures. The evolution of these cavities (wormholes and fractures) may lead to undesired consequences such as sinkholes, subsidence and cap rock leakage. Therefore, it is important to have a better understanding of the dissolution induced evolution of the underground cavities.
Donati, D. (Simon Fraser University) | Stead, D. (Simon Fraser University) | Elmo, D. (University of British Columbia) | Karimi Sharif, L. (University of British Columbia) | Gao, F. (China Coal Mine Research Laboratories) | Borgatti, L. (University of Bologna) | Spreafico, M. (University of Bologna)
ABSTRACT: The importance of considering brittle fracture in rock engineering is being increasingly recognized, and considerable advances in the modelling of brittle fracture have occurred in recent years. In this paper we present selected case studies illustrating the application of a wide variety of brittle fracture modelling methods, including simple boundary element, distinct element approaches with Voronoi/Trigon tessellation, particle flow codes, hybrid finite-discrete element, and lattice spring approaches, in some case in both 2D and 3D. Based on our experience in using all the major methods of brittle fracture modelling and using selected case studies we compare and contrast the different approaches highlighting their practical advantages and limitations. Finally, we briefly summarize recent advances in brittle fracture modelling including the importance of threedimensional kinematics, damage, water, and time dependency. Although challenges are often encountered in the use of brittle fracture models in rock engineering, the authors contend that observations of rock failure processes at all scales suggests that realistic rock engineering models must adequately consider brittle failure mechanisms.
Brittle fracturing of intact rock constitutes a critical factor in the stability and damage evolution in rock slopes and underground excavations. Earlier studies focusing on the mechanisms underlying intact material fracturing date back to the first half of the 20th century (e.g., Griffith, 1921). Ever since, the effect of stresses on crack growth and intact rock fracturing at the laboratory sample scale has been investigated by numerous authors (Hoek and Bieniawski, 1965; Bieniawski, 1967; Lajtai, 1974; Eberhardt et al., 1998; Diederichs, 2003; Cai et al., 2004). However, due to the intrinsic complexity of rock masses, fracture mechanics principles were at first only indirectly applied to stability analysis at the medium scale (e.g., pillar/underground excavations) and large scale (e.g., high rock slopes/block caving mines). Jennings (1970) investigated the effects of rock bridges on rock slope stability by assigning apparent shear strength parameters to equivalent, fully-persistent rupture surfaces. Einstein et al. (1983) explored the influence of out-of-plane rock bridges on the slope stability using a probabilistic approach. In the past two decades, the exponential improvements in computer processing power of modern workstations has considerably increased the complexity of processes that can now be modelled using a numerical approach. Numerous methods for the direct simulation of brittle fracturing of intact material have been developed taking advantage of a variety of numerical techniques, including Continuum, Discontinuum, Hybrid, and Lattice-Spring methods. We provide an overview of the most common numerical techniques that have been developed for the investigation of brittle fracture in rock. For each method, we also provide representative example applications, and discuss the advantages and limitations of the different approaches.
Raffaldi, M. J. (National Institute for Occupational Safety and Health) | Seymour, J. B. (National Institute for Occupational Safety and Health) | Abraham, H. (National Institute for Occupational Safety and Health) | Zahl, E. (Contractor) | Board, M. (HeclaMining Company)
ABSTRACT: Underhand cut-and-fill mining has allowed for safe extraction of ore in many mines operating in weak rock or highly stressed, rockburst prone ground conditions. However, design of safe backfill undercuts poses unique geotechnical challenges that must be addressed by these operations. Hecla Mining Company and the Spokane Mining Research Division of the National Institute for Occupational Safety and Health have worked collaboratively for several years to better understand the geomechanics of cemented paste backfill (CPB) and thereby improve safety in underhand stopes. This work has included a series of laboratory strength studies and an extensive in situ backfill instrumentation program to monitor long term stope closure and resulting stress in the backfill. The fill must be strong enough to resist flexural failures, but the large stope closures (5 to 10 cm) that occur during undercutting also require the fill to have significant residual strength in order to remain stable after the elastic strain limit has been exceeded. This paper provides an overview of underhand-cut-and-fill mining with CPB as practiced at the Lucky Friday Mine, the collaborative research that has been undertaken with emphasis on the instrumentation and monitoring program, and technical insight that has been gained through this work.
Backfilling has allowed for safe extraction of ore in many mines operating in weak rock or rockburst-prone ground conditions. In the Coeur d’Alene mining district of northern Idaho, cut-and-fill mining methods have historically been used to mine narrow, steeply dipping veins of silver-lead-zinc ore (Blake and Hedley, 2003; Williams et al., 2007). At the Lucky Friday Mine, the use of cemented paste backfill (CPB) in conjunction with mechanized underhand cut-and-fill mining methods has reduced the number of injuries and fatalities caused by mining in deep, high-stress, rockburst-prone ground conditions, greatly improving the safety of underground miners (Peppin et al., 2001; Pakalnis et al., 2005).
Although the use of backfill has a sound safety record, implementation of a backfilling program is not without risk and requires technical oversight, particularly in underhand cut-and-fill mining operations where employees work directly beneath cemented backfill. Design of safe backfill undercuts poses unique geotechnical challenges that must be addressed.
ABSTRACT: The fracture response of rock, as a quasi-brittle material, is highly sensitive to its microstructural design. We present a statistical damage formulation to model dynamic rock fracture. The damage model is rate-dependent and the corresponding damage evolution is a dynamic equation which introduces a timescale to the problem. The introduced timescale preserves mesh objectivity of the method with much less computational efforts in comparison with other conventional non-local formulations. We define a statistical field for rock cohesion to involve microstructure effects in the proposed formulation. The statistical field is constructed through the Karhunen-Loève (KL) method. The damage model is coupled with the elastodynamic equation. The final system of coupled equations is discretized by an asynchronous Spacetime Discontinuous Galerkin (aSDG) method. Robustness of the proposed formulation is shown though dynamic fracture simulation of rock under uniaxial compressive load. The numerical investigation indicates the importance of load amplitude and microstructure randomness on failure response of rock.
Brittle materials have a significant role in various applications: glasses, ceramics, concrete, bone, etc. These types of materials are susceptible to sudden rupture by cracking as they have many microdefects and microcracks. Before reaching the ultimate load capacity of a material sample, existing microcracks and microdefects propagate at microscale. At ultimate capacity, microscale degradation processes coalescence and cause fracture initiation at macroscale. Macroscale degradation continues under a softening process until the material completely fails.
An important aspect in fracture of rock, and in general quasi-brittle materials, is the effect of microstructure on their fracture response. As shown in  due to the high sensitivity of these materials to their defects, even for the same loading and geometry set-up, different fracture patterns can be observed. Same observations are made in  where high variations on material response, especially beyond elastic range—e.g., ultimate load and fracture energy— were observed due to sample to sample variations. Size effect is another consequence of the high sensitivity of response to microscale defects, as for example demonstrated in [3,4]. In fact, the Weibull's weakest link model [5,6] has proven very effective in capturing the size effect and statistical variation of fracture strength. We have used the Weibull model in the context of an interfacial damage model to capture statistical fracture response of rock, in hydraulic fracturing , fracture under dynamic compressive loading , and in fragmentation studies [9,10]. However, as will be discussed below, these models first can become quite expensive due to the use of a sharp interface model for fracture and second are not derived from a homogenization approach.
Sharp interface (SI) models represent fracture on crack surfaces. Some examples include the linear elastic fracture mechanics (LEFM) model, cohesive models [11,12], and interfacial damage models [13-15]. Each of these models has its own advantages/disadvantages. SI models explicitly track real pattern of fractures, but their implementation is cumbersome and their computational cost is high. Also, in applications such as multiscale methods, it is hard to track explicit discontinuities in all scales of interest. If it is even possible, the computation cost will be extremely high. These facts have lead many efforts to develop fracture models based on continuum mechanics.
Islam, N. (University of Utah) | Vijapurapu, R. (University of Utah) | Jones, M. (University of Utah) | McLennan, J. (University of Utah) | Moore, J. (University of Utah) | Rickard, W. (Geothermal Resources Group) | Balamir, O. (Geothermal Resources Group) | Vagnetti, R. (U.S. Department of Energy National Energy Technology Laboratory)
ABSTRACT: Mechanical Specific Energy (MSE) characterizes the input and expenditure of energy during drilling. The rate of penetration of a drill bit (ROP) is the result of how much effective energy can be applied to break and remove rock. With evolving technologies and downhole sensors, Mechanical Specific Energy is widely used in the oil and gas industry to improve performance and provide real time feedback. This feedback loop to the driller signals impending operational issues. Rate of Penetration improvements have been attributed to adjusting procedures according to bottom hole feedback.
This paper evaluates MSE and other drilling issues experienced while drilling a granitic, geothermal reservoir. Drilling data were acquired from a recent geothermal drilling operation near Milford, Utah; one of the Department of Energy's FORGE geothermal sites. Since this is an experimental well the data has been analyzed in a reactive fashion to determine rock properties and compare them with actual core and log data rather than in a proactive fashion for improving performance. MSE will be proactively used as a real time application for the next phase of FORGE drilling.
Mechanical Specific Energy has been used in the oil and gas industry for the past five decades in various forms and flavors. Simply stated, Mechanical Specific Energy (MSE) is a representation of conservation of energy. The energy and work that are input and expended to cut a volume of rock should be conserved in a hypothetical closed system. This closed system includes energy input from a top drive or rotary rig, as well as energy associated with circulation of drilling fluids and cuttings. The energy balance also includes losses due to friction, drillstring vibrations, heat, sound, and energy associated with the mechanical disaggregation of the rock. The rate of penetration of a drill bit (ROP) is directly impacted by how much effective energy is actively applied to break and remove rock.
Many different manifestations of specific energy have been used to implicitly correlate with formation characteristics that influence drilling. In most instances, the relationship has been passive - drillers have been interested in improving footage rather than concurrently understanding the mechanical properties of the reservoir. One goal of this paper is to advocate for concurrent use of MSE and other at-the-bit measurements for inferring mechanical properties in real time. Originally, MSE applications were restricted to conventional shallower drilling applications in nominally straight holes. With evolving technologies and downhole sensors, Mechanical Specific Energy is widely used in the oil and gas industry to improve performance and provide real time feedback. This feedback loop to the driller indicates operational issues, highlights downhole formation transitions and reflects rock properties. Rate of Penetration improvements have been attributed to adjusting drilling parameters (mainly WOB and RPM) according to this bottomhole feedback. Similarly, re-engineering of technical limits that hinder performance - such as increasing top drive limits, altering bottomhole design specifications and redesigning bits - have been undertaken based on specific energy measurements at the bit.
ABSTRACT: The estimation of the in-situ stress state is required for the design and execution of deep engineering operations related to Enhanced Geothermal System (EGS). Borehole failures, often referred as borehole breakouts, which are controlled by local stress concentration around the wellbore, are recognized being a useful indicator to assess in-situ stress conditions. However, breakouts evolve with time and this may affect our ability to use them for quantifying the stress state. We use a unique data set from the deep geothermal well of Rittershoffen GRT-1 in order to verify the hypothesis concerning wellbore breakout geometrical evolution. In GRT-1 wellbore, imaging has been acquired 4 days, 348 days and 946 days after drilling completion. Thermal, hydraulic and chemical stimulations have been performed between the first and the second image acquisition. Using this data set, we were able to describe in-situ the breakout evolution with time. We show increase in the extension of breakouts along the well. Contrary to the common assumptions, we also show that breakout widen, but within the limit of the accuracy of our analysis they do not deepen. The consequences of the breakout evolution for stress characterization are significant and add up to other important uncertainties in such analyses like the estimation of strength parameters.
A large amount of energy is available at depth. This energy can be extracted by circulating fluids between boreholes through the hot rock mass, but this requires that sufficient permeability is present at depth. As permeability tends to decrease with depth (Manning and Ingebritsen 1999), it is necessary to target deep structures with locally higher permeability (e.g. fault zones) and/or to perform permeability enhancement operations. The later approach is referred as Enhanced Geothermal Systems (EGS). The principle underlying this technology consists of increasing the hydraulic performance of the reservoir through different types of stimulations so that commercially interesting flow rate can be achieved. The stimulations consist of high-pressure injection (hydraulic stimulation), cold water injection (thermal stimulation) or chemical injection (chemical stimulation). In the two first cases, the permeability increase is obtained by inducing a thermohydromechanical perturbation to the rock mass which reactivates existing structures or create new ones. The in-situ stress state is central to understand the response of the rock mass to injections and to design such operations.
ABSTRACT: Clogging of porous media is known to have profound effects on hydrological and mechanical properties of geological reservoirs. Blockage of in situ pores and formation of impermeable barriers against fluid flow poses several orders of magnitude reduction in reservoir permeability, leading to major efficiency losses in plethora of fluid injection associated activities such as enhancement of oil recovery and deep waste disposal operations. Various mitigation, rehabilitation, and cleaning techniques have been developed over the years across various disciplines aiming at enhancing fluid flow by unclogging the blocked porous matrix. There are major inconsistencies in the available reports on the efficiency of current unclogging techniques, which highlight the challenge of selecting the optimal mitigation method in practice. The current article presents a review on the clogging phenomenon in geological formations. The state-of-the-art mitigation techniques adopted in various practices are also presented to offer a holistic insight into the most practical ways of flow enhancement aimed to ensure both sustainability and minimal environmental contamination. The final part of the paper presents results from a newly developed fully-coupled numerical simulation of column tests to assess geomechanical clogging in soils. Findings provide a foundation for rigorous numerical simulations, as well as in depth physical assessment of clogging phenomenon in soils.
Pore blockage, referred to as clogging, is a ubiquitous phenomenon in the natural subsurface profoundly effecting hydrological, mechanical, and dynamic properties of geological formations. Deposition of suspended solids, microbial growth, and proliferation of bio-products in permeable zones progressively increase the resistance against injection flow in plethora of fluid- injection-associated activities such as enhancement of oil recovery, maintenance of in-situ pressures in long-term producing geothermal reservoirs, aquifer storage and recovery (ASR), and permanent disposal of wastes deep into geo-reservoirs. To alleviate the elevated resistance against flows in a clogged formation injection pressures have to be continuously increased. Such overpressure results in wear and tear of the injection facilities, breakthrough of sealing cap-rocks, and pressure-induced leakage of contamination into the surrounding environment and water resources. Health and environmental hazards associated with injection into clogged formations remain a topic of intense dispute, especially in hydrocarbon waste or waste water disposal operations.
ABSTRACT: Accurate prediction of softening and failure behavior of rocks are essential to hydraulic fracturing simulation using strain-softening type models. Failure to preserve the fracture energy causes these continuums based numerical models to suffer from mesh-size dependency. The virtual Multi-dimensional Internal Bond Model (VMIB) is derived from a particle-based constitutive law at the micro scale. It has been implemented in a 3D Finite Element Method in which material softening and energy dissipation occur over the “representative elementary volume”. However, in realistic materials, energy dissipation is due to fracture surfaces creation instead of material softening in the element. In this work we present an improved VMIB model to bridge the energy dissipation over the representative elementary volume and the fracture surfaces using a virtual bond potential that incorporates the material fracture energy to eliminate the mesh-size sensitivity. The virtual bond potential considers both the critical fracture energy and element size. The 3D model is calibrated and verified by carrying out simulations of a group of three-point-bend tests using different mesh sizes. Then, by incorporating a three-dimensional element partition method, the model is applied to a series of laboratory scale hydraulic fracturing experiments. Furthermore, multiple hydraulic fracturing from closely-staged clusters is simulated. It is found that the model can accurately capture the fractures growth pattern that is influenced by the stress boundary conditions and the stress shadow interaction among the fractures. The results also show the predicted breakdown pressure reasonably agree with the experiment data.
In hydraulic fracturing simulation, capturing rock mechanical behaviors in response of fluid pressurization is crucial for fracture pattern and fluid pressure prediction. Many analytical solutions [Sneddon, 1946; Sneddon and Elliot,1946; Khristianovic and Zheltov, 1955; Nordgren, 1972, Geertsma and de Klerk, 1969] based on linear elastic fracture mechanics (LEFM) have been proposed to analyze different mechanisms of fluid-driven fracturing. However, the complexity of hydraulic fractures brings challenge to classic model due to the constitution features of rock such as heterogeneity and nonlinearity. Therefore, to solve the complex fracturing problem, many numerical methods have been proposed. The Displacement Discontinuity Method (DDM) [Kumar, 2013; Weng et al., 2011; Sesetty and Ghassemi, 2012, 2013, 2017; Farmahini-Farahani and Ghassemi, 2015; Verde and Ghassemi, 2013] has been widely used in fracture mechanics, hydraulic fracturing and natural fracture networks interaction, especially in large scale problem and multiple complex fracture network simulation. Discrete Element Method (DEM) is an effective method of addressing engineering problems in granular materials. This particle-based method is an ideal tool to simulate rock failure and has been used to simulate the hydraulic fracture propagation [Damjanac et al., 2010; Deng, Podgorney and Huang, 2011]. Continuum based method associated with strain softening model has been widely used to capture the nonlinear mechanical behaviors of rocks. Coupled damage model have also been propsoed [Min et al., 2011; Min, 2013; Huang and Ghassemi, 2016] to simulate mixed-mode hydraulic fracture propagation. [Gao and Ghassemi, 2017] developed a 3D FEM model to analyze the pressurized fracture problem in heterogeneous rock. By introducing discontinuity into continuum Finite Element Method (FEM), the extended finite element method (XFEM) [Belytschko and Black, 1999; Moes et al., 1999] has also been implemented in hydraulic fracturing modeling [Gordeliy and Peirce, 2013, Dahi-Taleghani and Olson 2011, Wang 2015, Shi et al, 2017]. Phase field model [Moelans et al., 2008] is a versatile technique for solving interfacial problems at the mesoscale and has been employed to solve the fluid driven fracture propagation in porous media in small scale [Mikelic et al., 2015]. The Virtual Multidimensional Internal Bond (VMIB) is a particle based constitutive model that was proposed by [Zhang and Ge, 2005, 2006] to capture the macroscale material properties from the mechanical response of microscale particles and bonds. VMIB was used in [Huang et al., 2013] to simulate 3D mix-mode hydraulic fracture propagation. Recently, more hybrid methods have been proposed such as Finite- discrete element method [Zhao et al., 2014] and DDM-FEM [Kumar and Ghassemi, 2016] have been used for hydraulic fracturing simulation, which integrate the advance features of each method.