Rafeh, F. (Polytech'Lille University Lille 1 Sciences and Technologies) | Mroueh, H. (Polytech'Lille University Lille 1 Sciences and Technologies) | Burlon, S. (Université Paris-Est, Institut Français des Sciences et Technologies des Transports)
Unexploited underground cavities may represent a potential hazard for the surrounding environment when the occurring consequential instabilities augment and propagate upwards toward the surface causing serious damage. Nowadays, North France and its region face a similar risk where large constructed areas, and due to urban expansion, have been extended to surfaces underlain by shallow cavities. Therefore, it is necessary to anticipate and control the circumstances . This work aims to provide an understanding of the deformational behavior of underground cavities on long term process by carrying out a numerical creep analysis of the failure mechanisms. This is performed based using a coupled constitutive law developed to account for the long term degradation phenomenon of the chalk continuum in the presence of joints. Numerical analysis of the time dependant behavior of the underground cavity considering different joint orientations is performed. In addition to visual inspections which remain the most important for the cavity risk assessment, this approach provides comprehensive elements for a better understanding of the progressive failure and thus the temporal aspect of the deformational behavior and failure of underground cavities.
Natural and anthropogenic underground cavities may represent a potential hazard for the surrounding environment due to consequential instabilities that might end up with serious damage. Particularly, when these cavities are at shallow depths, the risk that the damage reaches the ground surface is increased due to the upward propagation of localized rupture that might happen. North France and its region are nowadays threatened by such a risk where large built-up areas have been developed in zones underlain by unexploited shallow cavities excavated several decades ago in the existing chalk layers. Hence, this contribution aims to analyze by numerical means, the behavior of these underground cavities and the corresponding failure mechanisms, taking into consideration the effect of time. This latter is represented by the presence of induced fractures which might appear in short term or in long term, and degradation of the chalk which is a time dependent behavior.
Guo, L. (Imperial College London) | Xiang, J. (Imperial College London) | Latham, J.-P. (Imperial College London) | Viré, A. (Delft University of Technology) | Pavlidis, D. (Imperial College London) | Pain, C. C. (Imperial College London)
A three-dimensional fracture model developed in the context of the combined finite-discrete element method is incorporated into a two-way fluid-solid coupling model. The fracture model is capable of simulating the whole fracturing process. It includes pre-peak hardening deformation, post-peak strain softening, transition from continuum to discontinuum, and the explicit interaction between discrete fracture surfaces, for both tensile and shear fracture initiation and propagation. The fluid-solid coupling model can simulate the interactions between moving fluids and multi-body solids. By incorporating the fracture model into the coupling model, a methodology of using the new coupling model to capture fracturing behaviour of solids in fluid-solid coupling simulations is proposed. To solve the problem in the coupling model of having adaptive continuous meshes being used by the fluid code and discontinuous meshes in the solid code, a scheme to convert different meshes is developed. A single fracture propagation driven by fluid pressures is simulated and the results show that the modelling obtains the correct critical load and propagation direction for fluid-driven fracturing. Several important phenomena, such as stress concentration ahead of the fracture tip, adaptive refinement of fluid mesh as a response to the fracture propagation and fluids flowing into fractures, are properly captured.
Hydraulic fracturing has been used in the oil and gas industry for more than half a century. It is particularly important for the extraction from unconventional reservoirs, which otherwise would be considered as uneconomical. In the research field of hydraulic fracturing, an increasing amount of effort has been put into the development of novel numerical models to simulate realistic scenarios. However, the numerical modelling of hydraulic fractures still remains a challenge for computational mechanics because of the complexities in fluid-solid coupling, fracture characterisation, and particularly in three dimensions, the complicated geometry and topology update when fractures propagate. In recently years, many attempts have been made and significant progress achieved in three-dimensional hydraulic fracturing simulations. Carter et al. (2000)  proposed a fully threedimensional hydraulic fracture model, but they neglected the fluid continuity equation in the area around the fracture. Secchi and Schrefler (2012)  developed a method to simulate three-dimensional hydraulic fractures in porous media and presented an example of a concrete dam. In this paper, a three-dimensional fracture model is incorporated into a two-way fluid-solid coupling model for hydraulic fracture simulations. The three-dimensional fracture model is capable of simulating the whole fracturing process. It includes prepeak hardening deformation, post-peak strain softening, transition from continuum to discontinuum, and the explicit interaction between discrete fracture surfaces, for both tensile and shear fracture initiation and propagation. After incorporating it into the fluid-solid coupling model, where the interaction between fluids and solids can be explicitly simulated, the initiation and propagation of fractures can be driven by forces generated both from solids loading and transferred from fluids loading, e.g. fluid pressure, which significantly extends the application of this fracture model into some very important areas that would not be possible with only solids modelling, e.g. the simulation of hydraulic fractures.
Closure of rough surfaces under normal closure stress is investigated in this study. Rough surface closure model presented in this paper is based on surface asperity deformation. The main components of deformation are asperity compression and half-space deformation. Mechanical interaction among asperities which is a consequence of half-space deformation is considered in the model and its impact on the closure behavior is analyzed. Asperities are assumed to be elastic-perfectly-plastic materials and therefore may experience inelastic deformation under closure stress. Modeling results indicate that a significant portion of closure takes place earlier on at low stress levels because there are fewer asperities in contact initially. Asperity inelastic deformation is found to influence rough surface closure with its degree of impact depending on surface profile. A mechanical interaction sensitivity analysis indicates that neglecting interaction among asperities may lead to erroneous results particularly in surfaces with closely spaced asperities. By conducting an analysis on the elastic properties of asperity and half-space we found that the normal stiffness is much more influenced by Young’s modulus of half-space rather than that of asperity.
Acid fracturing is a stimulation technique which is being used in carbonate reservoirs. This technique is considered as an alternative to the well-known propped hydraulic fracturing. Fractures tend to close due to the in-situ stresses acting normal to the plane of fracture. Fracture closure has detrimental effect on the conductivity and therefore, should be prevented. Proppant is widely used in the hydraulic fracturing process and this material serves to keep the fracture open against closure stress. However, the mechanism by which the fracture is being held open is essentially different in acid fracturing technique.
Acid fracturing is a complex process in which acid reacts with rock and removes some parts of it, resulting in two random rough surfaces. Asperities on these surfaces act as pillars to keep the fracture open. Fracture surfaces come into contact after the pump pressure is dissipated. The success of an acid fracturing job depends on how well the asperities withstand the closure stress. Increasing effective stress often reduces the fracture aperture and its conductivity.
Salt is usually added to oil well cement systems to overcome compatibility issues between well cements and salt containing formations as well as salt related durability issues with cement sheath. Several studies on the impact of adding salt to oil well cement have been documented in the literature; however, the impact of compression on salt cement sheath such as during hydraulic fracturing has not been fully investigated. This study focuses on investigating the impact of compression (cement’s compaction) on the petrophysical and mechanical properties of wellbore cement containing salt and their potential impact on long term wellbore integrity issues. A unique bench-scale physical model, which utilizes expandable tubulars, was used to simulate the compaction of a previously cemented casing under field-like conditions. The impact of the compression on the cement’s petrophysical and mechanical properties were quantified by measuring the porosity, permeability and hardness of 1in x 2in cores drilled parallel to the orientation of the pipe from the compacted cement sheath. The acquired results indicate that the compaction of the cement sheath resulted in a reduction in porosity and permeability of the cement sheath and an increase in the hardness of the cement sheath after compaction. Furthermore, the results confirm reduction in the detrimental effect of salt on the strength and stiffness of the cement after compaction..
Cementing forms an integral part and is often regarded as one of the most critical steps in oil well completions. It is used to fill the annulus between the casing and the formation and between two consecutive casing strings. The main objectives for cementing oil-wells are to isolate the well from its surrounding in order to prevent fluid migration, protect the casing from corrosion and provide mechanical support for the casing string. The properties of the cement slurry and its behavior depend on the components and additives of the cement slurry design . As a result of the development of offshore fields and drilling activities through salt formations, seawater has become widely used for cementing purposes especially in the US Gulf of Mexico with well-known massive salt formations often in excess of 10, 000ft thickness . There are several reasons for adding salt to oil well cement such as more desirable mechanical properties, better compatibility with salt formations and offsetting of bulk hydration shrinkage. However, salt could adversely affect admixture performance and lower the ultimate compressive strength of the cement . Calcium chlorides in particular, have been used to shorten cements’ setting time and calcium chloride and sodium chloride to a lesser extent have been shown to leach calcium hydroxide leading to chemical changes in the cement resulting in loss of strength and corrosion of the casing . The presence of salt in oil well cements presents a danger to the cement. Although the exact mechanism of salt damage to oil well cement have not been fully understood, it has been shown that salt, which is a mild acid, lowers the pH of the cement and attracts more water into the pore structure of the cement leaving room for expansion leading to fracture initiation and propagation .
Acoustic emission (AE) analyses have been used for decades for rock mechanics testing, but because AE systems are not typically calibrated, the absolute sizes of dynamic microcrack growth and other physical processes responsible for the generation of AEs are poorly constrained. We describe a calibration technique for the AE recording system as a whole (transducers + amplifiers + digitizers + sample + loading frame) that uses the impact of a 4.76 mm free-falling steel ball bearing as a reference source. We demonstrate the technique on a 76 mm diameter cylinder of westerly granite loaded in a triaxial deformation apparatus at 40 MPa confining pressure. In this case, the ball bearing is dropped inside a cavity within the sample while inside the pressure vessel. We compare this reference source to conventional AEs generated during shear loading of a saw-cut simulated fault in a second granite sample at confining pressures up to 120 MPa. All located AEs occur on the saw-cut surface and have moment magnitudes ranging from M -5.7 down to at least M -8. Dynamic events that rupture the entire simulated fault surface (stick-slip events) have measurable stress drop and macroscopic slip, and radiate seismic waves similar to those from a M -3.5 earthquake. The largest AE events that do not rupture the entire fault are M -5.7. For these events, we also estimate the corner frequency (200- 300 kHz), and we assume the Brune earthquake source model to estimate source dimensions of 4-6 mm. These AE sources are larger than the 0.2 mm grain size and smaller than the 76 × 152 mm fault surface. Finally, we compare our results to other calibrated AE studies performed on different loading machines and discuss reasons for the observed maximum AE magnitude.
Acoustic emissions (AEs) are tiny seismic events thought to be caused by microcracking or slip instability on the grain scale. They are sometimes recorded during rock mechanics experiments to monitor fracture and faulting processes . In slow loading experiments on rock samples containing pre-existing artificial faults, AEs tend to cluster around stick-slip instabilities (dynamic events that involve slip of the entire fault surface) in a manner reminiscent of foreshocks and aftershocks. It has long been assumed that AEs are in some sense small-scale versions of earthquakes and that they can provide insights into earthquake mechanics [2- 5]. Yet, while earthquakes are routinely quantified by their seismic moment, only rarely is the absolute size of an AE determined. This is because AE recording systems are not typically calibrated.
A method is presented to qualify the maximum horizontal stress direction on basis of dipole shear sonic anisotropy in near-vertical wellbores. The proposed scheme follows a similar qualification standard to that used for stress observations on the basis of image logs and four-arm caliper logs in the World Stress Map Project. Image log analysis and shear wave anisotropy analysis will often complement one another and add confidence when both are observed. The combination of geological setting and rock properties, together with drilling practices, does not always result in clear borehole failure, limiting the ability to quantify stress direction from images alone. Shear sonic anisotropy is often able to identify horizontal stress imbalance where borehole failure has yet not occurred. Herein, we review the methodology to determine stress direction on the basis of dipole borehole sonic data, including examination of the effect of hole ovality. The use of slowness frequency dispersion curves is particularly important, as dispersion curve analysis is essential for distinguishing shear sonic anisotropy due to horizontal differential stresses from that caused by lithological fabric and natural fractures.
Borehole dipole (flexural) sonic waves provide the opportunity to identify and quantify shear anisotropy. The dipole flexural waves polarize into a fast and slow shear in the presence of elastic anisotropy in the planes containing the borehole axis. Anisotropy may be the result of mechanical anisotropy, fractures, or stress. Slowness-dispersion analysis provides the ability to identify stress-induced azimuthal anisotropy. If anisotropy is confirmed to be a result of a differential stress, one can then determine the direction of maximum horizontal stress from the direction of the fast shear wave in a near-vertical wellbore [1, 2, 3, 4]. This technique is commonly used in the petroleum industry for complementing other methods, such as borehole failure measured from calipers and images, to deduce the direction and magnitude of the present-day horizontal stresses. Stress characterization methods that rely on the presence of borehole failure using images are limited to boreholes that exhibit failure. Similarly there are situations where high differential stresses are observed from borehole failure and yet shear wave anisotropy is absent. Theory and laboratory testing illustrate that all rocks have some degree of acoustic sensitivity to changes in stress, which is related to the compliance of the grain-to-grain contacts. In the borehole, this phenomenon is also limited by the accuracy of acoustic logging technology . These data are then used for future geomechanics-related studies for predicting sand production, planning stimulation treatments, reservoir engineering, and wellbore stability [6, 7, and 8].
Vargas, Pablo E. (Memorial University of Newfoundland) | Abugharara, A. N. (Memorial University of Newfoundland) | Molgaard, J. (Memorial University of Newfoundland) | Butt, S. D. (Memorial University of Newfoundland)
The passive-Vibration Assisted Rotary Drilling (p-VARD) technology was developed to enhance the drilling Rate of Penetration (ROP) for using Poly-crystalline Diamond Compact (PDC) drill bits by modulating the rock-bit interactions. A labscale p-VARD prototype was tested by drilling rock analogue fine-grained concrete samples with an Unconfined Compressive Strength (UCS) ~50 MPa. Results showed that within the operational range of the tool, defined by the range of WOB applied during drilling, ROP was consistently 50% or more greater as compared to conventional drilling without the p-VARD tool, as other drilling conditions being the same. A field-scale p-VARD prototype was developed and tested during field trials in September 2014. Evaluation of data from the field trials is ongoing; however, representative drilling results for drilling a red shale formation (with mechanical properties similar to the laboratory concrete material) showed similar results with ROP increased 50% to 100% when WOB was in the operational range of the tool.
By studying Vibration Assisted Rotary Drilling (VARD), the Advance Drilling Laboratories (ADL) of Memorial University of Newfoundland aims to introduce technologies that provide higher penetration rates and greater economic values in the process of drilling. Vibrations are mostly considered undesirable in the field of drilling and efforts are done to mitigate them. Vibrations are linked to whirl, stick-slip and nonuniform dynamic loading, which cause damage to bits and down-hole equipment. The Institute of Technical Mechanics, Ukraine tested devices that work on the principle of cavitation. A two to three times increase in ROP was reported . Another study was done on an Axial Oscillation Generator tool (AGT) and it was found that the AGT improves weight transfer to the bit and reduces torque on bit. Also it was found that it significantly reduces stick-slip . National Oil-well Varco Down-hole Ltd. (NOV) developed a small scale vibration test-rig, to simulate stick-slip and study stickslip mitigation methods. Axial friction reduction and axial load transfer can be achieved by introducing axial excitations in the oil-well, which results in an improvement in ROP and better Mechanical Specific Energy (MSE) . Heng Li et al reported that the combined effect of vibrations and rotation increases the rate of penetration for a coring bit ; also ROP improvement was reported as a function of amplitude of vibrations. Babatunde et al studied the effect of vibration frequency on penetration rates using natural diamond drag bits. Here again VARD improved the penetration rates . Both Heng Li et al and Babatunde et al used a shaker table under the sample as a source of vibrations. To further study the effect of vibrations on drilling performance, a prototype in-line tool (lab scale p-VARD tool) was designed and tested. Initial results were promising and significant increase in ROP using concrete specimens of medium strength was observed.
Fracture-permeability relations of shale are needed in hydraulic fracturing, CO2 sequestration, and nuclear waste disposal. In this study, we use triaxial coreflood experiments in combination with x-ray tomography to study fracture generation and permeability during injection of water. The experiments were conducted on Utica shale at temperatures from 25-50 °C and confining pressures from 3.4 to 13.8 MPa. Fractures were generated using direct shear methods with semi-circular anvils. Permeability was monitored following fracture events as a function of continuing deformation and hydrostatic pressure. Fractures propagating parallel to bedding were an order of magnitude more transmissive than fractures propagating perpendicular to layers. Maximum whole rock permeability ranged from 70-900 mD. Shale strength when fractures propagated parallel to bedding was a linear function of the angle between the fracture and the direct-shear plane and was Strength (MPa) = 0.8034 x Angle (°) + 26.259. This behavior appeared to be related to the amount of shear-plane deformation accommodated by bedding planes. Permeability was not a clear function of this angle but peaked sharply for fractures that crossed bedding planes at angle near 45°. Significant deformation (>1%The flow of fluids through fractured shale is important in hydraulic fracturing, CO2 sequestration, and nuclear waste disposal. In hydraulic fracturing, the goal is to produce hydrocarbon by generating (or reactivating) pervasive, permeable fracture systems. In CO2 sequestration, risk assessment studies must consider the possibility that pressurization of the storage reservoir could induce fractures in caprock (which may be shale) where the consequences are governed by the permeability of the damage zone. In nuclear waste disposal located in shale formations, risk assessment studies must consider the permeability of potential flow paths generated during excavation of the repository, within pre-existing fracture networks, and within any subsequent fracture events induced by seismicity.
Rai, A. R. (Barrick Gold Corporation) | Howell, R. S. (University of Nevada) | Weatherwax, T. (Barrick Gold Corporation) | Sandbak, L. (Barrick Gold Corporation) | Kallu, R. (Barrick Gold Corporation)
Turquoise Ridge Joint Venture underground mine (TRJV) consists of disseminated gold in very weak and altered limestones, mudstones, and carbon rich clays. These sediments are typically highly faulted and sheared, and are often bordered by highly altered dacite dikes. A more recent challenge has been mines going deeper and needing bigger and better single excavation .i.e. shafts to ventilate and provide ore skipping options. The stability concerns and shaft sinking method will be studied to evaluate the ground support needed and address with risk analysis to avoid costly and impractical decision made in the past.
The paper is an attempt to address the Geotechnical, hydrological, risk analysis to current technology and technical limits. Next five years several hundred millions dollars investment is expected for driving deeper shafts with innovative and sustainable technology to overcome current limitation. Also the author will address Engineering and support design criteria challenges to Northwest Nevada single excavation! Deep Shaft.
The Turquoise Ridge Joint Venture (TRJV) is located in northern central Nevada, and is situated within the Basin and Range province, near the northeast end of the Osgood Mountains (see Figure 1).
The TRJV mine consisted of two major segments; the Getchell Main Underground (GMU) mine and the Turquoise Ridge (TR) mine segment. The Getchell underground segment is currently shutdown. Newmont has a 25% stake in the joint venture with Barrick controlling the remaining 75%. The primary product is high grade (>0.3oz/ton) Carlin-style gold mineralization in combinations of altered mudstones, limestone, and calcareous sediments.
The Turquoise Ridge Mine is entirely an underground mine, and is accessed by two shafts used for ventilation, men and materials transportation, and ore hoisting. The TR mine utilizes the underhand cut-and-fill mining method due to the relatively low rock quality in the ore zones and the relatively shallow dipping 25-45 degree ore body geometries. The primary ground control uses bolting, mesh, and shotcrete. It includes the use of cemented rock fill and a quick mining sequence to minimize ground exposure time and unraveling ground.
Holderby, E. S. (Halliburton) | Dahl, J. (Devon Energy) | Eichler, C. (Halliburton) | Dusterhoft, R. (Halliburton) | Siddiqui, S. (Pinnacle–A Halliburton Service) | Yarus, J. M. (Halliburton–Landmark)
Advances in complex fracture modeling have provided multiple realizations of the development of fracture networks in unconventional reservoirs. This has allowed for calibrated fracture network geometry definition in reservoirs that are conducive to fracture network growth in multiple directions as a result of the presence of natural fractures and low horizontal stress anisotropy. The ability to predict fracture network creation has allowed for fracturing design that focuses on net pressure development to achieve the desired complexity. Increases in fracture complexity can result in increased production results in low permeability unconventional reservoirs with two-phase hydrocarbon production. While this capability has led to positive results, there is a desire within the unconventional arena to increase the collaborative development of assets for even greater improvement and increased understanding of optimized exploitation of these assets. For this reason, the ability to include the realizations of complex fracture networks in a three-dimensional (3D) asset description would prove invaluable to asset development and well planning and further increase understanding across various disciplines. The need to incorporate results of complex fracture modeling and geomechanical descriptions within an earth model led to the development of an analysis process combining the results of multi-disciplinary modeling techniques focused on unconventional assets.
1.1. Complex Fracture Modeling CapabilitiesWith advances in hydraulic fracture modeling aimed at describing the fracture development in unconventional plays, the ability to describe “non-typical” fracture network growth has improved significantly . With this type of modeling, it is possible to observe fracture network patterns that include hydraulically induced fractures, which are induced and propagated in the direction of the maximum horizontal stress, as has been understood for some time. Additionally, fracture network growth that can be leakoff induced or stress induced and includes the development of fractures oblique to the maximum horizontal stress is now possible. Understanding fracture network growth and these patterns is becoming more important in unconventional plays having a geological setting conducive to this behavior, which often results in fracture networks that are much more complex than a simple bi-wing fracture. This modeling tool’s ability to map and describe these types of fracture networks allows for better completions optimization and the development of workflows that collaborate across geoscience and engineering disciplines to help maximize asset value.