The elasto-plastic computational models commonly in use predict that lining loading increases with increasing ground strength under certain conditions. This is contrary to the behavior that might be expected on the basis of intuition or tunneling experience. The present paper investigates the conditions under which this paradox occurs and shows that it can be traced back to a combination of large deformations ahead of the face and small deformations of the support system. Additionally, the paper investigates why such behavior does not occur in reality. It shows that the decisive simplifying modeling assumptions are, alone or in combination with each other, (i) that ground behavior is time-independent (whereas in reality overstressed ground generally creeps) and, (ii) that the support operates with full stiffness close to the face (which is not feasible in most cases due to the nature of the excavation and support installation procedures). When these two effects are taken into account in the design computations, the paradoxical model behavior is eliminated.
Under certain conditions which are frequently encountered in tunnel design, the computational models commonly in use predict that poor-quality ground will be more favorable for tunnel construction than high-quality ground. More specifically, the models suggest that ground of higher strength develops a greater load upon the lining than the load developed by low-strength ground (all of the other parameters being equal). This is clearly contrary to the behavior that might be expected both intuitively and on the basis of tunneling experience, which is that overstressing of the lining or severe convergences are associated with ground of poor quality . The model behavior deserves to be called a paradox, i.e. “a seemingly absurd or contradictory statement or proposition which when investigated may prove to be well founded or true” (Oxford Dictionary). The paradox has been mentioned in passing in a number of older works dealing with the elasto-plastic analysis of tunnels. (A listing of the works can be found in ). More recently, it has been noted by Boldini et al.  and Graziani et al. , who obtained “unforeseen results” from axisymmetric elasto-plastic numerical analyses of advancing tunnels, and explained them by means of the convergence–confinement method (“The decrease in the loading in the plastic case is caused by the increased convergence before the installation of the lining, which overshadows the negative effect of the flattening of the convergence curve in the plastic range”). Also, Mair  drew basically the same conclusion when discussing the results of plane strain analyses (“This is because the weaker ground leads to higher deformations occurring ahead of the face prior to installation of the lining; the consequence of more ground deformation before installation is a smaller pressure induced on the lining”). Although the paradox has been noted by a number of authors, it is, interestingly, neither widely appreciated nor well understood in the broader engineering and scientific community. It may therefore perplex the tunnel engineer and raise doubts as to the predictive power of standard tunnel design calculations, and this makes it deserving of closer investigation.
The Stability Graph Method is widely used in Canadian underground hard rock mines for open stope support design and is frequently used in the mine planning phase to assess the viability of stope geometries and to determine maximum permissible spans. The Method is well accepted due to its simplicity and suitability to a wide range of mining methods requiring the use of sublevels to access and extract the mining blocks. While this Method is reliable in situations where the maximum induced tangential stress creates adequate compressive stress keeping the face in a state of confinement, the Method fails to reflect the effect of low confinement caused by lower compressive stresses and instances where tensile stresses are developed. In this paper, a new Rock Stress Factor, A’, an update to the original Rock Stress Factor, A, is proposed, which will reflect the impact of low confinement or overstress regime on critical face stability. Two case studies, based on two Canadian operating gold mine shows that using the new Rock Stress Factor A’ to predict the stability of stope hanging-wall and footwall gives better agreement with the CMS survey results, compared with the prediction using the original rock stress factor.
The Stability Graph Method is widely used in Canadian underground hard rock mines as a basis for open stope support design and is frequently used in the mine planning phase as a tool to assess the viability of stope geometries and to determine maximum permissible spans. The Method is well accepted due to its simplicity and suitability to a wide range of hard rock mining methods requiring the use of sublevels to access and extract the mining blocks. Building on the work of Mathews et al. , Potvin  revised the Stability Graph Method using case studies from Canadian underground mines. Nickson  added further case studies and extended the Method to address cablebolt support requirements in hard rock mining methods.
While the Stability Graph Method is reliable in situations where the maximum induced tangential stress creates adequate compressive stress keeping the face in a state of confinement, the Method fails to reflect the effect of low confinement caused by lower compressive stresses or stress relaxation, and instances where tensile stresses are developed. These low compressive and tensile stresses in the critical face are common along the hanging-wall and footwall of relatively tall stope created when mining with VRM, longhole open stope, AVOCA, and other similar methods. This paper discusses a newly proposed Rock Stress Factor, A’, which will reflect the impact of low confinement on critical face stability. 2. STABILITY GRAPH METHOD This method uses the hydraulic radius of the critical face and the Modified Stability Number, N'', proposed by Potvin , to estimate the stability of unsupported (Fig. 1) and supported underground openings.
Hassani, H. (Amirkabir University of Technology, Faculty of Mining, Metallurgical and Petroleum Eng.) | Sarkheil, H. (Amirkabir University of Technology, Faculty of Mining, Metallurgical and Petroleum Eng.) | Foroud, T. (Amirkabir University of Technology, Faculty of Mining, Metallurgical and Petroleum Eng.) | Karimpooli, S. (Amirkabir University of Technology, Faculty of Mining, Metallurgical and Petroleum Eng.)
An effective way to enhance production from mature oil reservoirs is drilling horizontal wells. However there are many advantages for this kind of wells, they require extra investment for drilling and completion. The objective of this research is to build and optimize response surface based on quadratic, multiplicative and radial basis functions to express accumulative oil production over a planning horizon as a function of location, direction and length of a new horizontal well. Investigating the impacts of horizontal well parameters on the production using repetitive runs of reservoir flow simulation would involve enormous computation time for evaluating possible well placement scenarios. Instead, we employ these proxy models in order to build a good approximation of accumulated outflow from a reservoir when a new horizontal well is to be drilled. This leads to a significant reduction in the computation time. After setting up the models, the best model was introduced to a global optimization search using a Genetic algorithm. Numerical results showed that more than 3 million barrels of more oil could be produced from the designated well as compared with those of the initial scenarios which is a significant improvement..
Choosing an appropriate method to enhance production from a mature oil reservoir is a critical decision making process. A reservoir is called mature when its production rate is decreasing after hitting a peak in the production history. This stage of a reservoir life is very important due to approaching the profitability limit of the exploitation project. World statistics reveal that most of the important oil fields are in their maturity stage or becoming close to it (Babadagli, 2005), which shows the importance of finding new ways to improve recovery rate of such reservoirs. An effective way which has been used in many situations is drilling horizontal well. Horizontal wells can improve recovery factor due to enlarging contact area with the oil reservoir and possibility of reaching trapped oil packs in remote areas. Although there are many advantages associated with horizontal wells, high investment required for drilling and completion is a hindering factor. Thus, it is very important to examine the feasibility and features of this method. There is no doubt that it is necessary to find the optimal location, length and direction of horizontal wells before making any decision on drilling such wells. There has been some research on this subject during last two decades. Guo et al. (1993) developed an economic model to assess feasibility of drilling horizontal wells in naturally fractured carbonate reservoirs. Aanonsen et al. (1995) suggested a method for well placement optimization while considering geological uncertainties. They employed a response surface method, an experimental design using a Kriging model to reduce the required simulation runs. Wagenhofer et al. (1996) tried to optimize horizontal well depth using water and gas coning concepts and developing a surrogate model to use instead of the original simulator. The concept of “quality map”, which is a bi-dimensional representation of reservoir performance, was introduced by Cruz et al. (1999).
Understanding borehole collapse mechanisms during underbalanced drilling (UBD) in shale is becoming increasingly important for the petroleum industry, due in particular to the implementation of the UBD technique in operational practices. However, it is challenging to determine the proper constitutive material model to describe the UBD borehole environment sufficiently and accurately. Moreover, it is difficult to define how and where "borehole failure" occurs during simulation or even in laboratory conditions. In an attempt to minimize borehole instability problems, detailed and careful analysis of the excavation process is often performed during the planning stage. However, the accuracy of these analyses is highly dependent on the constitutive model adopted for the shale. Three important features of the constitutive model for shale are the dissipation of the pore pressure, the strength and the plasticity after yielding. This paper presents rock strength and borehole deformation results from a hollow cylinder (HC) testing program on Pierre-1 shale. Furthermore, a calibration was performed on a virtual borehole model  against HC laboratory data. The HC tests at underbalanced conditions were performed on samples drilled parallel and perpendicular to the bedding to properly quantify the effects of anisotropy and to determine possible scaling/geometry effects. Laboratory observations of material failure were compared with numerical outcome and showed promising similarities.
Fundamental borehole stability models for underbalanced drilling (UBD) wells are directly associated with the mechanical properties, formation anisotropy and heterogeneity, true pore pressure, mud weight, well trajectory and formation temperature of the rock. The mud weight (MW) and the well trajectory are controllable parameters and are considered key factors in the borehole design models, whereas rock strength and formation pore pressure (pf) are the main unmanageable parameters directly involved in the borehole stability models. In practice, rock strength is described by the collapse pressure (CP) of UBD wells. Determining the true rock strength is of prime concern in the drilling industry to obtain trustworthy borehole collapse simulation models. Moreover, accurate knowledge of the rock strength is essential for drilling optimization, rate of penetration (ROP) prediction  and also for sand production prediction models . Conventional drilling simulators provide a tool to determine the rock strength for the drilling engineer to further model and study the effect of different drilling parameters, where the overall drilling process performance can then be optimized. The rock strength is therefore a key element in borehole stability modeling as well as in the optimization of the drilling process. Several challenges appear when creating borehole stability analysis models. The main challenge is fitting an existing constitutive material model using the mechanical parameters of the rock for the borehole stability model to a reliable and realistic state. Another important consideration is to evaluate and to model the borehole stability in shale. The lack of relevant test data to describe shale properties accurately is due to the shale’s heterogeneity and its anisotropic behavior. Field observations indicate that most of the drilling problems occur when drilling in shale sections.
Coal and coal seam reservoirs have unique properties and therefore pose unique reservoir engineering challenges. Coal reservoir permeability is due primarily to fracturing where gas is stored primarily through sorption in the intact matrix. Coal permeability is stress dependent and changes in gas contents and composition induce volumetric strain. Coal strength and deformation behavior is a function of fracture spacing (cleating) and is anisotropic and nonlinear. An integrated laboratory testing facility capable of observing these effects at reservoir conditions is presented. The facility is designed to capture expected coal seam reservoir conditions under primary CBM production and ECBM/CO2 storage operations. The facility is designed to measure multi-component gas and liquid permeability at independent axial and radial stress conditions. Two types of testing cells are used in the facility: one with isotropic stress control and one with triaxial stress control. The isotropic cell is designed to measure diffusion and swelling characteristics of coal. The triaxial cell is designed to measuring axial strain and radial strain as well as axial P and S waves. Both cells are capable of measuring permeability. The cells are enclosed in an isothermal air bath with a medium temperature fluid pump and time controlled gas sampling system.
Canada’s Clean Coal and CO2 Capture and Storage Strategic Plan formulated in 2005 identified key knowledge gaps that need to be addressed for post capture injection, long term reliability, and monitoring of sequestered CO2 in coalseam, or more commonly, coalbed methane (CBM) reservoirs. For each of these issues, understanding the unique geomechanical responses of the CBM formation under CO2 storage conditions have been labeled as high priorities by Canada as well the international community . Therefore there is a need not only to identify the controlling hydromechanical processes in coalseams during the sequestration life cycle, including preinjection CBM production, but there is a critical need to characterize and link these processes for use in simulations and field monitoring for performance and risk assessment. An important component to address these needs is the development of a robust experimental facility encompassing in situ CO2 injection and storage, as well CBM production, conditions found in Canada, specifically in the Alberta Basin (Figure 1). This work presents a flexible facility design enabling measurement of the unique hydromechanical properties of coal including deformation, strength, acoustic velocity, permeability and relative permeability all at variable stress and temperature conditions.
CBM production operations in the Alberta Basin have been capricious with success in shallower (<600m) Horseshoe Canyon formations, using a single vertical well to complete and production multiple low gas content coal seams. However, in the deeper Upper Manville formation (>900m) high gas content seams, limited production success has been achieved using vertical or horizontal wells. One approach to increase production in the Upper Manville formation may be to use enhanced coalbed methane (ECBM) recovery techniques. ECBM involves injecting a secondary gas, such as CO2 or a gas mixture (flue gas), into the formation to displace the methane .
A one-dimensional, liquid-gas two-phase flow with stochastic input of parameters system is analyzed, with intrinsic permeability, porosity of the formation and the parameters of capillary-water saturation relationship being the stochastic parameters. Several empirical mean and standard deviation values are adopted to generate spatially variational input parameters either independently or correlatively during the numerical simulation process. The simulation results are used to investigate the effects of stochastic input data on predicted response of CO2 geological sequestration through uncertainty propagation and to obtain probabilistic estimates of volumes of stored/injected CO2.
With the increasing emission of greenhouse gas, the reduction of emission of anthropogenic carbon dioxide has been actively studied. Carbon capture and sequestration in appropriate geologic formations (saline aquifers, coal seams, and gas reservoirs) is one of the promising methods to reduce the release of greenhouse gases in the atmosphere. There are three main processes involved during the CO2 injection and storage in the formations including hydrological, mechanical and geochemical process. A number of papers [4, 5, 9, 11, 22] have been published to discuss the three processes and their modeling and how they affect CO2 injection, storage and migration.
However, there is still more work needed to be done to account for the processes better. Geologic formations suitable for CO2 sequestration are inherently inhomogeneous and non-uniform. As a result, there is a natural variability of the measured parameters that are required to simulate fluid flow and transport in geologic formations. The analysis of subsurface flow of CO2 requires a stochastic analysis to account for the effects of variable parameters on saturation profiles, pressure drop and injection volume. The chemical reactions related to CO2 geological sequestration (i.e., solubility and mineral trapping) are long term processes that typically occur in decades or hundreds of years. Hence the focus of the paper is on short term processes related to the hydrodynamic trapping of CO2• The objectives of the paper are to: (1) account for the influence of the inhomogeneities of the formation on CO2 flow and transport by conducting a series of numerical modeling with the aid of Monte Carlo simulation, and (2) investigate the influence of the stochastic parameters on the corresponding results.
The gas-water two phase flow model is adopted in this paper. The basic continuity equations for gas and water are shown in Eqs. (1) and (2):
where t is time; f is porosity; k is intrinsic (absolute) permeability in m2 ; ∇D is the differential depth of the formation; Pg and Pw are the pore pressure of gas and water in Pa; vg and Vw are gas and water Darcy velocities; Pg and Pw are gas and water densities;
The reservoir model adopted in this paper is a horizontal and thin reservoir. Therefore the buoyancy effect results from the depth gradient and the gravity was not addressed here. However the buoyancy effect is an important mechanism for fluid aggregation and this effect will be addressed in the subsequent journal paper. With the horizontal reservoir model, Eqs. (1) and (2) can be expanded in the following forms:
Zhou, Y. (University of Pittsburgh) | Jaime, M.C. (University of Pittsburgh) | Gamwo, I.K. (U.S. Department of Energy, National Energy Technology Laboratory) | Zhang, W. (U.S. Department of Energy, National Energy Technology Laboratory) | Lin, J.S. (U.S. Department of Energy, National Energy Technology Laboratory, University of Pittsburgh)
This study follows up on our previous two dimensional discrete element study of rock cutting, which we have confirmed the adequacy of the discrete element method as a tool for estimating the cutting forces and for capturing the associated fragmentation process . From the rock cutting perspective, there are two levels of calibration that need to be addressed-which was one of our major focus. The first level of calibration is to obtain micro-parameters of the bonded-particle model based upon laboratory tests results. Often the void ratio was deemed as reflected in other mechanical properties and thus was not explicitly incorporated in the calibration. We looked into the potential impact of such consideration in a rock cutting application. The second level of calibration is to tune the parameters in the cutting model in view of available rock scratch test data. We investigated the issue of balancing the selection between the two intertwined parameters: the damping ratio and the cutter advancing speed. Finally, we carried out three-dimensional analyses in modeling groove cutting as a step toward building a tool to analyze the cutting action of a drilling bit.
With the development of the bonded particle model  that demonstrated rock can be well represented by cementing discrete particle together, the discrete element method (DEM) has been applied in analyzing various rock mechanics problems ranging from the stability of rock slopes  to the deformation analysis of tunneling . One of the defining features of DEM is that no special treatment or process is required in tracking failure progression or fragmentation evolution. They are just the natural results of an analysis. This strength of DEM makes it an attractive tool for analyzing problems such as rock cutting. Indeed, previous research has demonstrated that DEM was capable of reproducing two distinct failure modes as observed in laboratory rock scratch tests (RST): a brittle failure for deep cuts and a ductile failure for shallow cuts [5, 6]. However, to serve as a reliable tool it is essential that a DEM model be properly calibrated. From the rock cutting perspective, there are two levels of calibration that need to be considered. The first level of calibration is to determine the micro-parameters of the DEM rock material model. The second level of calibration is to tune the parameters of the cutting model. There are two bonded particle models in use; one is the parallel bond model, the other the contact bond model. A parallel bond can be viewed as a cylinder that glues particle together which is capable of transmitting moment. On the other hand, a contact bond can be viewed as a point of glue that has no size and cannot transmit moment. This study employed parallel bond model. For the parallel bond model, the model parameters include: The normal and shear stiffness of contact spring between rock particles, the shear and normal bond strength; the size of bond with respect to the size of particles in contact; the shear and normal stiffness of the bond;
A developing concrete material named CEM1 is thought to be a potential candidate as artificial seal for the underground storage for nuclear wastes. Noticing that the concrete always subjected to various destructive hydraulic and mechanical loading, an experimental research focused on the shear fracture of CEM1 under hydro-mechanical coupling was performed by using a pre-designed experimental set-up. Four series tests were carried out respectively to determine the closure of the shear fracture under different confining pressure; the effect of confining pressure on the ultimate shear capacities of the fracture; the effect of confining pressure combined interstitial pressure on the ultimate shear stress of the fracture; the influence of the relation between the hydraulic flow rate and the injection pressure on the fracture. The research results show that the fracture is closed primarily between 0 and 5MPa; the ultimate shear stress of the shear fracture is 12.1MPa under confining pressure of 5MPa, and it increases with the augmentation of the confining pressure; the confining pressure tightens the fracture and generates a diminution of permeability in material.
The concrete material can be used as the engineering background in many industrial and engineering applications. In recent years, it has been largely studied as being considered as potential engineering barrier for underground storages of high level radioactive wastes. Numerical studies and laboratory experiments  show that coupled hydromechanical processes will occur in the geological barrier for a very long time due to excavation/ventilation and subsequence backfilling/sealing. Moreover, the growth and the confinement of the concrete fracture during the loading is an essential issue for the perdition of the sealing function of the storages. The study presented in this paper is a part of a larger study on the hydro-mechanical behavior of the CEMI concrete for underground storages of radioactive wastes. In this paper, the studied concrete material, called CEMI is the potential candidate as artificial seal for the underground storage (Figure 1) for nuclear wastes requested by Andra (French National Agency for Nuclear Waste Management). This material is intended to be used as plug of the storage gallery and the sealer of the nuclear waste canister which will be located 500m deep at the Bure site (Meuse, France). From the excavation phase to the back filling phase, the stress redistribution of the rock layer can induce damage of the concrete. Furthermore, the mechanical and transport properties of concrete can be degraded due to various hydraulic and mechanical loading. Among these complex stress conditions, our study focuses on the shear stress and investigates the restorability of the shear fracture and its hydro-mechanical features submitted different confining pressure. Four series tests are carried out to determine respectively the closure of the shear fracture under different confining pressure; the effect of confining pressure on the ultimate shear capacities of the fracture; the effect of confining pressure combined interstitial pressure on the ultimate shear stress of the fracture; the relation between the hydraulic flow rate and the injection pressure of the fracture.
Controlled blasting techniques are used to control overbreak and to aid in the stability of the remaining rock formation. The less competent the rock mass itself is, the more care has to be taken in avoiding damage. Presplitting is one of the most common methods which is used in many open pit mining and surface blast design. The purpose of presplitting is to form a fracture plane across which the radial cracks from the production blast cannot travel. Presplitting should be thought of as a protective measure to keep the final wall from being damaged by the production blasting. The purpose of this study is to investigate of effect of presplitting on the generation of a smooth wall in a rock domain under a surface blast process. The 2D distinct element code was used for simulation of presplitting in a rock slope. The blast load history as a function of time applied to the inner wall of each blasthole. Important parameters that were considered in the analysis were stress tensor and fracturing pattern. The blast loading magnitude and blasthole spacing were found to be very significant in the final results.
Drilling and blasting continues to be an important method of block production and block splitting. Drill and blast technique has a disadvantage that sometimes it produces cracks in uncontrolled manner and also produces micro cracks in the block as well as in remaining rock, if not carefully carried out. Recovery by this method is low as compared to other methods. Therefore, attempts have been made to develop controlled growth of crack in the desired direction. The control of fractures in undamaged brittle materials is of considerable interest in several practical applications including rock fragmentation and overbreak control in mining [1–3]. One way of achieving controlled crack growth along specific directions and inhibit growth along other directions is to generate stress concentrations along those preferred directions. Several researchers have suggested a number of methods for achieving fracture plane control by means of blasting. Fourney et al.  suggested a blasting method which utilizes a ligamented split-tube charge holder. Nakagawa et al.  examined the effectiveness of the guide hole technique by model experiments using acrylic resin plates and concrete blocks having a charge hole and circular guide holes. Katsuyama et al.  suggested a controlled blasting method using a sleeve with slits in a borehole. Mohanty [7,8] suggested a fracture plane control technique using satellite holes on either side of the central pressurized hole, and demonstrated its use through laboratory experiments and field trials in rock. Nakamura et al.  suggested a new blasting method for achieving crack control by utilizing a charge holder with two-wedge-shaped air cavities. Nakamura  performed model experiments to examine the effectiveness of the guide hole with notches. Cho et al.  performed experiments using a notched charge hole to visualize fracturing and gas flow due to detonation ofexplosives
Considerable faith is being put in the value of pre-conditioning for de-risking critical aspects of block caving mining such as cave initiation, propagation and fragmentation. However understanding how the hydraulic fractures develop within a naturally fractured rock mass is extremely difficult given the complexity of the system. To provide a framework to help address this, an approach has been developed within the Discrete Fracture Network (DFN) code FracMan that allows the simulation of hydraulic fractures to be carried out within a well constrained stochastic description of the fracture network system. These DFN based simulations have been supported by detailed geomechanical models using a hybrid FEM-DEM code that allowed the coupled stress-flow modelling of hydraulic frac interaction and pressure evolution, enabling certain stimulation design factors to be considered as well as testing the basis for the more stochastic modelling. A key learning from the scenarios modeling to date has been how limited the role of conventional hydraulic fractures are for rock breakage when compared to shear reactivation, fracture-fracture interaction and small scale rock mass damage. This work highlights the need for a better understanding of this process to help quantify its impact upon the caving process.
With ever-increasing global demand for mineral resources, mass mining methods for large lower grade deposits (e.g. block and panel caving) are becoming more attractive prospects. A large-scale block or panel cave mine constitutes an example of a high volume rockfactory, whose success and viability are dependent to a large extent on the caveability of the deposit and the fragmentation of the ore material. To help mitigate the risks associated with unfavourable cave propagation and fragmentation in stronger or less fractured rock masses, pre-conditioning through hydraulic fracture generation or blasting is increasingly been used. Considerable faith is being put in the value of these pre-conditioning methods for de-risking critical aspects of block caving mining such as cave initiation, propagation and fragmentation. Quantification of the impact of preconditioning is difficult and much of the data obtained to date is anecdotal. This paper focuses on hydraulic fracture preconditioning and in particular some initial steps at understanding how the hydraulic fractures develop within a naturally fractured rock mass. This is critical to assessing the impact of pre conditioning within the complexity of a naturally fractured rock system. This initial work has been addressed using a combination of stochastically generated Discrete Fracture Network (DFN) models and supported by numerical simulations of hydraulic fracture growth and interaction with the natural fracture system.
2. DFN ROCK MASS MODEL
The Discrete Fracture Network (DFN) approach is a modeling methodology that seeks to describe the rock mass fracture system in statistical ways by building a series of discrete fracture objects based upon field observations of such fracture properties as size, orientation and intensity. Initial interest in the DFN approach was largely associated with modelling of groundwater flow through natural fracture systems (largely as part of nuclear waste isolation programmes) and for modelling fractured hydrocarbon reservoirs.