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Zacher, G. (GE Sensing & Inspection Technologies GmbH) | Kaufhold, A. (Federal Office for Radiation Protection (BFS)) | Halisch, M. (Leibniz Institute for Applied Geophysics (LIAG)) | Patel, A. (GE Inspection Technologies)
ABSTRACT: In recent years use of X-ray Computed Tomography (CT) has become more common for geoscientific applications and is used from the μm-scale (e.g. for investigations of micro-fossils or pore scale structures) up to the dm-scale (full drill cores or soil columns). In this paper we present results from CT imaging and mineralogical investigations of an Opalinus Clay core on different scales and different regions of interest, emphasizing especially the 3D evaluation and distribution of cracks and fractures and their impact upon mechanical testing of such material. Enhanced knowledge of the behaviour of the Opalinus Clay as a result of these tests is of great interest, especially since this material is considered for a long term radioactive waste disposal and storage facility in Switzerland. Hence, results are compared regarding the mineral (i.e. phase) contrast resolution, the spatial resolution, and the overall scanning speed.
With this extensive interdisciplinary top-down approach it has been possible to characterize the general fracture propagation in comparison to mineralogical and textural features of the Opalinus Clay. Additionally, and to the best of our knowledge, a so called mylonitic zone has been observed for the first time in an experimentally deformed Opalinus sample. The multi-scale results are in good accordance with data from naturally deformed Opalinus Clay samples, which allows systematic analysis under controlled laboratory conditions. Accompanying 3D imaging greatly enhances the capability of data interpretation and assessment of such material.
In this study we present the investigations of an experimentally deformed Opalinus Clay (OPA, Kaufhold et al., 2016). The aim is the visualization of the shear failure at various scales to get more information about the deformation process. Enhanced knowledge of the testing behaviour of the OPA is of great interest, especially since this material is considered for a long term radioactive waste disposal and storage facility in Switzerland.
X-ray CT equipment are now frequently used for sample analysis because almost all geoscientific samples show 3D features which would be missed if analysing 2D sections only (e.g. by classical microscopy). These features include, for example, the abundance of minerals, location of particular particles towards bedding (or texture in general), pore system, cracks, and veins. The 3D distribution of these features can be extracted and used for a variety of numerical modelling purposes (Andrä et al., 2013).
ABSTRACT: The Vaca Muerta formation has been under study as a potential shale reservoir since approximately 2007; however, the first well, targeting this formation, was drilled in 2010. During the last few years, many works showing different discoveries about Vaca Muerta mechanical rock properties have been published. All of them are an invaluable piece of work; nevertheless, they do not give a full characterization of Vaca Muerta mechanical rock properties. Until today, many meters of core samples were recovered from Vaca Muerta formation and a complete set of rock mechanics laboratory tests have been performed on core plugs to accurately characterize all the parameters that governs the mechanical behavior of the rock. In addition, to further understand the variability in the mechanical response of the rock, changes in the setting up of the laboratory tests were done in similar plugs on a same core sample (sister samples). It is the aim of this paper to present an overview of recent Vaca Muerta research works which include mechanical laboratory characterization of the rock and discuss about how different laboratory test parameters such as temperature, confining pressure, time, and fluid saturation among others affects the mechanical response of the rock. To complete the understanding of the Vaca Muerta formation mechanical behavior, special laboratory studies like Biot's coefficient and creep test were performed in some core shale gas samples; nevertheless, in this work, the quality and validity of these results are left as an open discussion. The most important conclusion of the analysis done in this work is that some elastic properties are not mainly controlled by the “rock families or facies”. As a result, a unique correlation between many elastic properties has been found and a simple workflow to fully characterize the rock mechanical properties of Vaca Muerta formation is proposed.
During the exploration, delineation and development of the principal shale oil and gas reservoirs in Argentina, several studies like petrophysics, geochemistry, biostratigraphy and geomechanics haves been done. In order to calibrate the parameters of different models many meters of core has been recovered from the Vaca Muerta formation in the Neuquén basin, Argentina. This core acquisition was fundamental for the correct characterization of the mechanical rock properties. These properties play a fundamental role in every stage in the life of a well i.e. drilling process (wellbore stability), fracturing process and finally production forecasting.
ABSTRACT: While rock strength is an important input parameter in geotechnical engineering, the size effect of intact rock samples on strength has long been recognised as an issue. Conventionally, it has been believed that increase in size leads to decrease in strength. A few investigations have reported different trend to that consider conventionally where the uniaxial compressive strength (UCS) follows ascending and descending behaviour. Masoumi et al.  investigated this behaviour from analytical and experimental viewpoints that resulted in the formulation of the unified size effect law (USEL) which accounts for the reverse size effect behaviour of intact rocks. A limitation of the current USEL is that at very large sizes, the UCS prediction by USEL tends to zero. This paper outlines an improvement to USEL to overcome this limitation based on the multifractal theory. Attributes of the improved USEL model were highlighted and verified using UCS data from different rock types. Finally, it was demonstrated that there is a good agreement between the improved USEL prediction and the experimental results.
Size effect has been an important research issue in the general area of geomechanics where some investigations have focused on the mechanical properties of intact rock at different sizes. In this paper, the influence of three dimensional volumetric changes on the mechanical characteristics of intact rock is defined as the size effect.
Uniaxial compressive strength (UCS) is one of the most important mechanical properties of an intact rock which has long been studied by different researchers from the size effect viewpoint ([2-18]). Generally, it is believed that increase in size leads to decrease in strength. Initially, the size effect was explained by Weibull  as on a statistic phenomenon, whereby larger samples contain more micro-cracks than a smaller sample and thus the probability of failure at lower stress is higher with a larger sample. In other words, increase in size causes reduction in strength. This trend has been confirmed by other theories including fracture energy  and multifractals .
ABSTRACT: We compare different algorithms and boundary conditions for Digital Rock Physics modeling. We report results of Simpleware, Comsol and NIST for elasticity and electrical conductivity and Simpleware and Lattice-Boltzmann for permeability. Overall there is good consistency, though there are sample-to-sample differences. The comparison shows a good agreement between Simpleware and Comsol with a difference of ˜1% for effective elastic properties, ˜1-3% for permeability and 5% for effective conductivity. The Simpleware Physics Modules allow a streamlined workflow to compute effective properties and is especially suitable for processing a large number of digital samples. Comsol Multiphysics on the other hand allows more multiphysics coupling and access to internal field variables useful for research purposes. The NIST codes for elasticity and electrical conductivity are very efficient but are specifically only for those two physical properties.
Computation of physical properties on 3-D CT-scanned digital rocks is used in geosciences applications, and a number of different workflows and algorithms have become available. We discuss a comparative study on elasticity and electrical conductivity in different rock types (Figure 1) using Simpleware, NIST (National Institute of Standards and Technology) and Comsol Multiphysics codes. Permeability is also investigated using Simpleware and Lattice-Boltzmann permeability simulators. The 3D CT-scan images of different rock types are Sandstone (S2), Berea Sandstone, Fontainebleau Sandstone, Carbonate (C1) and Carbonate (C2), as well as digitized version of Finney pack with identical spheres (Figure 1). The materials for the grains and pore-fill assumed for the study were water, quartz, and calcite (for the carbonate rocks), with the following elastic properties and conductivities:
2. VOLUME MESHING
2.1. Simpleware (SW)
We import the 3D scan volumes of each sample in Simpleware ScanIP (Synopsys, Mountain View, USA) for image processing and segmentation. Since all of our 3D scan volumes are 100×100×100, each stack has 100 images of 100×100 size. We perform meshing using the Simpleware +FE-Free meshing algorithm with tetrahedral elements. For permeability, the Simpleware Physics Modules +SOLID, +FLOW and +LAPLACE are used with different mesh coarseness Since the rock model is based on a pre-segmented image (a binary image without any greyscale information reflecting the sub-voxel geometry of the sample), it is best to select the “Binarise before smoothing” option. In this way the meshing algorithm in Simpleware does not try to use any spurious greyscale information that might have been introduced into masks by thresholding, and the generated mesh has porosity closer to that of the original image.
ABSTRACT: Rock fracture toughness testing has been widely studied in the past forty years. Most previous testing used pillar-shaped rock specimen as a result of the inconvenience and difficulties in processing rock into other geometry. The development of diamond-impregnated wire saw for rock processing provides us the opportunity to get large quantities of notch-precisely-machined specimens, whose shape are Double-Cantilever-Beam planks. The study here used Double Cantilever Beam (DCB) testing to obtain highly repeatable values of rock fracture toughness. Particle Flow Code 5.0 2D (PFC 2D) was used to simulate the whole testing process, on the other hand, the testing results were used to calibrate the PFC parameters in turn. Conclusion shows that model I fracture toughness of Longmaxi formation outcrop is around 1.55MNm-3/2. Critical tensile force has positive correlation with width of notch/crack tip. The study also indicates that in PFC simulation, micro tensile strength among balls (pb_ten) determines both model I fracture toughness and uniaxial compression strength. The study may guide the drilling process in recognizing the strength of rock fracture and provide experiment-based fracture toughness for the simulation of fracturing process.
Fracture toughness of rock was scholars’ highly concern as most issues of rock mechanics are closely related to failure process, such as mining process and hydraulic fracturing, etc.
Lots of work has been done since the first suggested method of International Society for Rock Mechanics for rock fracture toughness testing1 was published. Since then, the testing method mentioned above is regarded as the standard method for static fracture toughness testing of rock. As the availability of pillar-shaped rock specimen, almost all the suggested methods used rock pillar to conduct fracture toughness testing. The machined specimen is shown in Fig. 1 (Atkinson 1987). In order to obtain the intrinsic fracturing property of rock, the notch of the specimen is machined to the shape of V or chevron. Displacement and force used to calculate fracture toughness is recorded after the fracture propagates a small distance to form a natural sharp crack, and that is what the shape of notch used to. This elaborate design eliminates the influence of machined notch width on fracture toughness because that the ‘natural sharp crack’ is thought to be a parameter merely related to rock property.
ABSTRACT: Unintentional releases of natural gas to the surface or subsurface environment are recognized as a challenge to the safe and reliable operation of underground fuel storage facilities, including for natural gas. Previous studies have documented some of the occurrences at such facilities and assessed leakage or failure mechanisms along with their degree of severity, in part to inform risk-based assessments of subsurface geologic storage of fuels and wastes (such as carbon dioxide). This paper summarizes the past occurrences and several hundred others of varying severity (from nuisance/non-hazardous to catastrophic), that have occurred or earlier ones that have come to light since a previous study of Evans  was undertaken. Worldwide, these have occurred at facilities developed in salt cavern (~320), porous rocks (aquifer and depleted hydrocarbon field – ~40 and ~600 respectively), mined hard-rock cavern (~50) and storage facilities of as-yet to be determined type (~7). The present work includes categorizing the fuel type (natural gas, LPG, NGL’s, crude oil), type, nature and severity of any leakage for both above ground and subsurface incidents, all of which provide key parameters to risk probability assessments.
Worldwide, a total of 1023 occurrences at underground fuel storage facilities are documented, of which 706 involve natural gas facilities. Of these, 63% can be attributed to subsurface causes (38% to well integrity, 25% to geological or subsurface integrity causes) and 36% to surface causes including pipeline and wellhead issues. It is important to note that not all involve product loss: only 588 occurrences are linked with migration or leakage of product, of which only 428 occurred to, or at, the surface. Together, the 1023 occurrences provide evidence of problems that do occur and which could contribute to a more serious occurrence and thus their recognition informs risk assessments. In the US, a total of 817 occurrences are documented, of which 538 are linked to migration/leakage of product, with 397 occurrences to, or at, the surface. Some 599 occurrences involved natural gas facilities. Of these, 59% can be attributed to subsurface causes (33% to well integrity, 26% to geological or subsurface integrity causes) and 38% to surface causes. In the US, the highest numbers of occurrences are found in California, Pennsylvania, and Texas, with product losses and related issues occurring in depleted oil and gas fields and in solution-mined salt cavern facilities.
ABSTRACT: This work presents a probabilistic analysis of wellbore integrity using analytical and numerical (finite element method - FEM) approaches. The safety of the wellbore is evaluated through a casing integrity criterion. To evaluate the stresses considering the construction and production phases, the analytical solution considers the main steps of the wellbore lifespan: initial stress state in the formation, drilling operation, casing construction, initial stress state in the cement, pressure changes inside the casing and pore pressure variation. For both approaches, plane strain conditions, continuous homogeneous isotropic media and linear elastic materials are considered. Although the model hypotheses are similar, the FEM modeling allows the construction of a wellbore model with imperfections such as a cement channel. The uncertainties in the elastic parameters of casing, cement and formation are incorporated into the analysis using the AMV+ and LHS methods. The failure criterion to assess loss of wellbore integrity is given in terms of the cumulative distribution curve. In addition, the most important variables are identified. With this information, it is possible to define a risk scenario, mitigation strategies and procedures for better data acquisition and monitoring during wellbore operation lifetime.
1. INTRODUCTION AND LITERATURE REVIEW
In the oil industry, several hazards, such as gas migration (Ravi et al., 2006 and Martins et al., 1997), are related to wellbore construction and production. To overcome these problems, simple solutions considering the relevant aspects of the engineering systems are a good tool for a preliminary design for everyday scenarios.
Many studies have investigated the wellbore stresses during the wellbore lifespan. Analytical solutions can reach good results and can simplify the assessment of the interaction between variables and their impact on the results. Xu et al., 2015 present a solution considering the thick walled cylinder theory for casing, cement and formation with internal pressure acting inside the casing. For the contact between materials, a fully sticky condition is adopted. In addition, an instantaneous uniform temperature increment is applied to all materials.
ABSTRACT: Pit slope design relies on appropriate selection of design strength parameters. This selection is not trivial given the inherent heterogeneity of rock masses. In strong rocks, step-path failures may develop through combination of planar shear along joints and failure through intact rock. The presence of rock bridges between discontinuities increases shear strength along potential failure surfaces. However, estimating rock bridge percentages is challenging in the absence of mapping and joint persistence information. The lack of joint continuity data is common to many open pit projects until construction begins and the rock masses are exposed. This paper describes the rock bridge characterization approach used for the proposed Jay Pit area of the Ekati Diamond Mine, for which joint persistence data are not currently available. The approach involved developing a discrete fracture network (DFN) model to estimate the amount of intact rock between discontinuities as a percentage of the total failure surface length. The model used discontinuity orientation and spacing data from borehole drilling at the site, and discontinuity persistence data from wall mapping in a nearby pit. Possible stepped failure paths were determined for various DFN model realizations, and rock bridge percentages were calculated along these paths. Rock bridge statistics were applied in the stability assessment to steepen the recommended pit slopes by up to 6°.
As indicated by Call and Nicholas, 1978, a potential slope failure mode in jointed rock masses is a stepped path where the failure surface follows a combination of rock discontinuities. Where intact rock is present between discontinuities (i.e., rock bridges), the step-path surface must fail through the intact rock. While relatively small rock bridges of intact rock between otherwise continuous joints substantially increase strength, mapping of each joint and rock bridge is impossible on a practical basis (Einstein et al., 1983). When information on joint continuity is absent, before construction begins and the rock mass is exposed in the pit walls, alternative approaches are required to characterize rock bridging. One alternative approach consists in developing a discrete fracture network (DFN) model to estimate the percentage of intact rock between discontinuities. This method was used in the Feasibility Study for the Jay Pit at the Ekati Diamond Mine in Canada’s Northwest Territories.
Alshubbar, G. D. (Oklahoma State University) | Coryell, T. N. (Oklahoma State University) | Atashnezhad, A. (Oklahoma State University) | Akhtarmanesh, S. (Oklahoma State University) | Hareland, G. (Oklahoma State University)
ABSTRACT: Complex directional and horizontal well trajectories result in high torque and drag forces. The increase in these forces decreases the transfer efficiency of energy, which limits the potential depth the drill string and tubulars can reach. This paper illustrates the benefit of incorporating barite nanoparticles (NP) into Water Based Mud (WBM). Two formulations of 10 ppg API standard barite WBM with different rheologies were utilized as base cases. Chemical and mechanical methods of barite NP generation were used. This facilitated a better understanding of the effect of each NP generation mechanism on the friction coefficient (CoF). The experiment speculated that nano sized particles can generate a smooth film that coats the surfaces resulting in friction reduction. NP also increase mud rheology by providing more individual particles for a given concentration. This research aims to continue understanding barite NP performance, behavior, and their effect on the CoF and mud rheology for WBM systems. An application will be discussed in this paper to highlight the potential benefits.
WBM is gaining more popularity worldwide. It has started to overtake the Oil Based Mud (OBM) market share. This trend is influenced by the changing in environmental regulations, cost and more importantly the advancements in WBM technology. Developing WBM systems that match the performance of OBM is demanded now more than ever. Lubrication additives, borehole stabilizers, and enhanced high temperature polymers are all being sought (Sifferman et al. 2003).
The successful application of nanotechnology in medicine, electronics, paints, coatings, and many other fields motivated the oil industry to take the same path. Reservoir data has also been gathered utilizing nano-sensors and nano-markers (Hoelscher et al. 2013). These achievements have inspired the investigation of nano additives in drilling fluids in an attempt to address existing difficulties; namely shale inhibition, rheology modification, wellbore strengthening, and lubricity.
A clear illustration of the importance of developing such innovative NP WBM drilling fluids is the mitigation of the frictional forces in horizontal or extended reach wells. It can substantially increase the rig and equipment reach limit by efficiently transferring energy. A finite element analysis (FEA) approach, taken by Hareland, indicated the benefits gained by lowering CoF in complex directional wells. As a sample illustration, a 3,900 ft of lateral extension were obtained while keeping torque, buoyancy and downhole weight on bit constant (Hareland et al. 2012).
ABSTRACT: This paper presents a novel study on geomechanics of fluid injection from a fully penetrating vertical wellbore into a weakly consolidated formation confined with soft rocks. For the first time, impacts of vertical confinement are incorporated to evaluate: flow-induced poro-elasto-plastic stresses, failure mechanism/s, and failure planes. A new fully-coupled numerical model is developed where the response of the injection layer in the plane perpendicular to injection flow is simulated through adopting “interface” – a plane on which sliding or separation can occur – analogous to the Winkler model. An assessment of pore pressures, stresses, and failure planes confirms two types of induced behaviors: dilation in the well vicinity; and compaction, a main cause of physical clogging which impacts competence of the operation. Numerical results describe multiple distinct zones evolving with time around the injection well: (1) liquefied domain, (2) multiple plastic domains, (3) elastic region. Inner plastic domains are prone to occur.
The geo-environmental consequences of injecting large volumes of fluids into geological strata – a common practice in enhanced oil recovery, geothermal exploitation, aquifer storage and recovery, and deep waste disposal operations – remain a topic of dispute. The very essence of a safe and sustainable operation is to be able to quantify and control the induced subsurface flow and deformations, specifically to ensure preservation of sealing integrity of the confining rocks. This involves a realistic grasp of the interactions between the injection layer and the sealing strata, which entails a comprehensive understanding of the involved geomechanical processes.
Numerical computation of the injection problem in a confined weakly-consolidated formation is challenging due to the following: strong coupling between the diffusional-mechanical processes; high fluid-solid matrix stiffness contrast; plasticity and parting, where the strength and stiffness of the medium become effectively zero. This results in numerical instabilities and a considerably long run time of simulations. A common simplifying assumption in most available work in the literature is plane strain conditions perpendicular to the injection flow. This assumption is most appropriate for a case where a weakly consolidated reservoir formation is confined with stiff sealing rocks. In cases where the sealing rocks are of a softer nature, or when the stiffness of the reservoir layer increases over time during prolonged injection cycles, the plane strain assumption is no longer appropriate and the overall response of the injection layer is indeed governed by the behavior of the confining strata.