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Sixth EAGE Shale Workshop
- Conference date: April-May 28-01, 2019
- Location: Bordeaux, France
- Published: 28 April 2019
41 - 60 of 67 results
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Breakout Stabilization in Shales - Evidence and Imaging from Large Scale Laboratory Simulation
SummaryThe paper intends to provide an insight into a large scale experimental campaign incorporating numerical simulations to develop understanding and to, refine the shale failure geometry stabilization mechanism and criteria. The paper also discusses the technical details, considerations and observations of the large scale mechanical rock testing campaign (including triaxial and polyaxial block test) which were carried out on 2 shales. The results of the induced onset of breakout and stabilized breakout geometry were laser scanned, measured and compared with the 3D finite element numerical simulation predictions. The outcome of the research is applied in well design considerations, predicts wellbore stability in 3D and offers valuable insights to the well design team. The results provide visualization of the extent and magnitude of the mechanical wellbore damage whilst drilling in shale formations.
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Fluid Flow Pathways in Shales: Damage Induced Permeability Change
Authors M. Houben, J. Van Eeden and S. HangxSummaryShale reservoir potential is largely determined by the connected pore network in the rock and the connection between the pore network and the naturally present or mechanically induced fracture network. These together determine the total permeability of the rock. We have used a number of different techniques to investigate the microstructure and permeability of Early Jurassic Shales from the UK (Whitby mudstone) when intact and with a mechanical induced fracture network. Permeability changes in the shales due to mechanically induced fractures are not straightforward, depending on the bedding orientation with respect to fluid flow and bedding orientation with respect to induced fractures permeability of the samples either increase by orders of magnitude, increase slightly or are very similar before and after fracturing.
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Organic Hosted Porosity in the Wufeng-Longmaxi Shale: A Combined Electron Microscopy and Neutron Scattering Approach
Authors C. Delle Piane, H. Ansari, W. Rickard, M. Saunders, J. Mata and D.N. DewhurstSummaryThe upper Ordovician Wufeng shale and lower Silurian Longmaxi shale are part of the Fuling shale gas play located in the south-eastern part of the Sichuan Basin, southern China, representing the first commercial shale gas production project outside North America. We studied the occurrence of porosity at micro- and nano-scale in samples of contrasting organic richness from the post-mature part of the Wufeng-Longmaxi gas play. Using a combination of high resolution scanning and transmission electron microscopy and small angle neutron scattering we highlight the impact of different types of organic matter (primary versus migrated) on the development of organic matter (OM)-hosted porosity. The results indicate that the overall porosity in the samples is proportional to the organic richness, although the nanoscale imaging revealed that OM-hosted porosity is preferentially present in the migrated bitumen and not in the primary detrital particles. Distinguishing between primary and migrated OM is therefore important for understanding the creation of an interconnected network of OM during hydrocarbon migration. This may have an important control on the estimation of gas in place and the transport properties of the shale.
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Quantitative Analysis of the Pore Structure of Premature-To-Postmature Organic Rich Mudrocks Using Small Angle Neutron Scattering
Authors A. Rezaeyan, V. Pipich, P. Bertier, T. Seemann, L. Leu, N. Kampman, A. Feoktystov, L. Barnsley and A. BuschSummaryThe pore structure of organic rich mudrocks is associated with both inorganic and organic constituents. The contribution of organic matter to the pore structure has been investigated on Posidonia and Bossier Shale samples having different organic carbon content and thermal maturity. Development and distribution of organic matter pores were studied by using small angle neutron scattering technique at a broad pore scale size investigation, from 2 nm to 2 µm. The pore structure of the mudrocks studied is highly complicated at which total pore volume and specific surface area are not significantly affected by thermal maturation, however, the maturity attribute contributes to different pore size distribution on meso- and macro-pores. Thermal maturation is likely to be the factor of amalgamating small organic matter pores into larger pores in overmature organic rich mudrocks, potentially causing an increase in pore volume at macroscale pores. Although not considerably, the increased macroporosity can enhance the permeability of pore network for viscous gas flow in organic rich mudrocks.
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Resolving the Pore Structure and Sorption Properties of Methane in Mudrocks - A Small Angle Neutron Scattering Study
Authors T. Seemann, P. Bertier, N. Maes, A. Rezaeyan, V. Pipich, L. Barnsley, A. Busch and V. CnuddeSummarySmall angle neutron scattering has been combined with supercritical methane sorption resulting in a collection of isothermal scattering curves for Opalinus Clay, Posidonia Shale and Eagleford Shale. Scattering data have been analyzed with respect to sorbed phase behavior for which a two- and three-phase model have been used. The two-phase model clearly indicates the formation of a sorbed phase of which properties like density and volume fraction change with pore size. Application of the three-phase model yields sorbed phase densities higher than the bulk density of methane as predicted by the equation of state provided by NIST. Current work focuses on implementing localized density calculations in order to quantify the effect of pore size on the sorbed phase properties.
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SAXS and WAXS Microscopy Applied to Mudrocks: A New Method for Systematic Multiscale Studies of Porosity, Pore Orientation and Mineralogy
Authors L. Leu, P. Bertier, A. Georgiadis, A. Busch, A. Diaz, J. Klaver, J. Schmatz, V. Lutz-Bueno, J. Ihli, H. Ott and M. BluntSummaryWe apply scanning SAXS and WAXS microscopy to different mudrock samples. The method characterizes the microstructure in terms of porosity and preferential pore alignment of small pores 6 −202 nm size. These small features are experimentally challenging to resolve for statistically relevant sample volumes with state of the art characterization techniques, such as imaging methods. A key novelty in this study is the quantification of the mineralogy and mineral phase content from the WAXS measurements. Thus, a detailed quantification and comparison of important microstructural parameters is achieved. The method is used in a raster scanning mode, where thousands of consecutive measurements are performed, with a high micrometric spatial resolution, over mm sized sample areas. Therefore, simultaneously the variation of the microstructure is resolved on the pore and lamina scale. We propose to use scanning SAXS-WAXS microcopy in future studies for investigations of the systematic relationships between mineralogy and the pore network.
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Improved Visualization of Heterogeneity within Shales: Colour Contoured Maps Constructed from Large Area High-Resolution SEM Montages
Authors J. Buckman and A. BuschSummaryLarge area high-resolution scanning electron microscopy montages are often taken of shales to characterise variability in composition. Although resolution can be superb in such montages, details can be hard to ascertain from the full images, and observations made from single high-resolution tiles are difficult to place within the context of the whole sample. This paper presents a new method that utilizes the individual images collected from such montages, to produce contoured coloured maps to better visualise heterogeneous distributions within shales. The method repurposes previously collected backscattered electron images, through batch image analysis processing, to quickly extract numerical data and produce coloured maps of a range of important parameters. Examples of maps for mean gray value, pyrite, quartz, calcite, porosity and permeability distribution are illustrated and discussed. Such maps allow variability in distribution to be graphically displayed in a fashion that clearly displays any heterogeneity at the millimetre to centimetre scale, as well as numerically defining any variation present.
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On the Evidence of Multiscale Anisotropy of Shales
By L. DurantiSummaryA heuristic and empirical model of elastic anisotropy of shales derived from experimental studies is applied to real geophysical conditions in the attempt to predict elastic constants and anisotropy of shales. Results are then compared to other expressions of elastic properties and anisotropy obtained at different scales. In general, the overall approach is found to be valuable, however there is evidence that further tuning of the initial model, mostly linked to the impact of temperature on elastic and anisotropic properties, will likely reduce the misfit between predictions and real data.
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Insights into Fractures and Fabric of Shales from Microseismicity
By A.F. BairdSummaryShales typically have low permeabilities due to their fine grain size, unless fractures are present to provide fluid conduits. Both aligned fractures and the preferred orientation of phyllosilicate minerals in shales are effective mechanisms to produce seismic anisotropy. Microseismicity recorded during hydraulic fracture stimulation can be used to provide excellent estimates of anisotropy through the observation of shear wave splitting. Here I illustrate through examples how these measurements can be used to infer the development and evolution of vertical fracture parameters during stimulation as well as provide improved estimates of the in situ rock fabric. A key challenge is untangling intrinsic anisotropy due to mineral alignment from extrinsic anisotropy due to cracks and pores aligned parallel to the dominate petrofabric of the shales. This may be improved by incorporating petrofabric and microstructural analyses of shale samples from the reservoir to better constrain the orientations and proportions of the constituent minerals.
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Rapid Workflow for Quantitative Interpretation of Shale Mineralogical Composition From Seismic Data
Authors R. Beloborodov, M. Pervukhina, J. Hauser, M. Josh, V. Shulakova, D. Chagalov, M.B. Clennell, G. Ward and M. WaughSummaryHere we propose a workflow for the quantitative interpretation of clay mineral composition in shales that includes a combination of statistical learning methods and seismic inversion. The workflow is validated on a case study from the Northern Carnarvon Basin, the North West Shelf of Australia, where the thick smectite rich shales seal the source rock. To overcome the issues of rock physics modelling for shales we establish the relationship between mineral composition and elastic properties of shales by applying clustering and regression analysis to available petrophysical and laboratory data. Consequently, Amplitude Variation with Offset/Angle (AVO) inversion is applied to seismic data to estimate elastic properties of shales using linearised Zoeppritz reflectivity equations for anisotropic medium. Finally, the smectite probability maps are obtained by translating the elastic properties into smectite content using the regression relationship established earlier.
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Modelling Advective Gas Flow in a Compact Clay: Application and Assessment of Different Numerical Approaches
Authors J. Harrington, E. Tamayo-Mas, H. Shao, E.E. Dagher, J. Lee, K. Kim, S.H. Lai, N. Chittenden, Y. Wang, I.P. Damians and S. OlivellaSummaryIn a repository for radioactive waste, corrosion of metallic materials leads to the formation of hydrogen and other gases. If the gas production rate exceeds the gas diffusion rate within the pores of the surrounding material, a discrete gas phase forms. Gas pressure will then continue to accumulate until its pressure becomes sufficient to exceed the entry pressure of the surrounding material.
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Borehole Imaging of Natural Fractures and Stress Indicators in the Jurassic Carbonate Shale Plays, Eastern Saudi Arabia
By M.S. AmeenSummaryAn operational geomechanics study has been underway to facilitate the unconventional exploration and appraisal of the Jurassic carbonate shale (mudstone) pay zones of Eastern Saudi Arabia. Part of the study involved the multi-scale imaging of natural fractures and stress indicators. Natural fractures are mainly of microscopic and mesocopic scale (individual fracture length ≤ several ft). No major faults occur in any of the tens of imaged long reach horizontal wells, and the imaged and cored vertical wells. Therefore, the natural fractures facilitate hydrofracturing. The absence of major fault zones or fracture clusters reduces the risk of pressure and fluid thieves during hydrofracturing, and minimizes potential damage associated with fracture hits. The natural fractures and current day stress regime is persistent regionally, and has been actively driven by the Late Cretaceous to present Arabian Plate tectonics. The current day stresses within the pay zones are characteristic of thrust to strike slip tectonics with the maximum horizontal in-situ stress trending ENE-WSW parallel to the current day Zagros stresses. These regional stress and fracture patterns facilitate well planning, completion and stimulation design with horizontal wells designed perpendicular to the maximum horizontal in situ stress.
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Polygonal Faults and Seal Integrity
More LessSummaryThis presentation reviews the impact polygonal faults exert on seal integrity. Often invoked as a class of seal bypass system, polygonal faults are embedded into many effective seals particularly in petroleum systems in which seal and reservoir were deposited in marine slope settings. Direct evidence for the role played by polygonal faults as conduits for fluid migration is sparse. However, there are numerous examples of seismic studies of leakage via polygonal fault networks. Two end member examples are reviewed here from the Ormen Lange Field, offshore Norway, and the Scarborough Field, NW Australia. In both cases, previously formed polygonal faults in the seal acted as fluid pathways during later leakage events, and this leakage occurred under radically different pressure and stress conditions than those prevailing during the formation and growth of the polygonal faults.
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Effect of the Orientation of Beddings on the Distribution of Stress around the Borehole in Anisotropic Shale Formation
Authors W. Li and D. SchmittSummaryThe far-field stresses will be concentrated near a borehole existing in any geological formations. If the magnitudes of the concentrated stresses are sufficiently large, the formation rocks near the borehole yields or even fails. In isotropic formations, the solution of the stress concentration near a borehole is solved in two-dimensions by Kirsch (1898) and three-dimensions by Hiramatsu and Oka (1962) (see also Schmitt et al., 2012). However, many formation rocks, for example shales, are elastically anisotropic due to their oriented mineralogical textures or micro-fractures at many scales. In anisotropic formations, a closed form of the solution is established by Lekhnitskij (1963) and Amadei (1983) . In isotropic cases, the magnitude of the concentrated σθθ on the contour of the borehole equals three times that of the far-field horizontal principal stress ( Kirsch, 1898 ). This conclusion is not suitable for anisotropic formations, such as shale formations. In this research, we study the effect of the orientation of beddings on the concentration of stress by a borehole in anisotropic shale formations.
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Forward Modeling of Evolution of Shale Properties in Geological Time
Authors I. Bekkevold and T. PetersenSummaryThe overburden is an essential part of the petroleum system. It provides the reservoir seal and host a significant part of the production facilities, and reservoir depletion might trigger severe deformations in the overburden compromising both seal and infrastructure integrity. In addition, the initial in-situ conditions in the reservoir (geo-stresses, pore pressure, fractures etc.) are result of the weight of the overlaying sediments. However, since the dawn of the O&G Industry, the main focus of the predictive, modelling and data acquisition efforts was the reservoir, thus, there is a massive lack of data (core samples, logs etc.) and significant gaps in our understanding of the medium and its reaction to our E&P activities.
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Domanik of the Russian Timan-Pechora Province - Structure and Development Prospects
Authors E. Grokhotov and O. PrischepaSummaryThe distribution area and conditions for sedimentation of the Domanik in the Timan-Pechora province are described. The structural features of shale strata and the degree of Domanic catagenesis are given. The method of mastering the Domanik in the Timan-Pechora province is determined.
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Characterization of the Organic Matter in the Bazhenov Formation Deposits From Northern and Central Parts of Western Siberia
Authors E. Leushina, E. Kozlova, A. Voropaev, A. Yurchenko, N. Bogdanovich and M. SpasennykhSummaryThe object of the study is the organic matter from the Bazhenov Formation. Rock-Eval pyrolysis, GC×GC-MS analysis of core extracts, IRMS of gases and kerogen in the Bazhenov Formation shale rocks were performed to characterize the organic matter from Northern and Central parts of Western Siberia. The studies revealed that variations in quality of the BF organic matter in central and peripheral regions are related to differences in both the thermal maturity and sedimentation conditions. More oxidative burial environment in South-Eastern and Northern regions resulted in lower generation potential of the Bazhenov Formation deposits when compared to in Central parts of Western Siberia.
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Marine Mudstone Attributes at Core, Well Log and Seismic Scales: Implications for Characterizing Fluid Flow in Thick Subsurface Shale Successions
By R. JonkSummaryThe recent increase in high-fidelity subsurface data (core, logs and seismic) from unconventional plays has shown that mudstone succesions are much more heterogeneous with regard to flow properties than typically assumed. Much of the flow heterogeneity is ultimately controlled by stratigraphic variations and dictated by the extremes of the lithofacies, which are often volumetrically insignificant, and therefore easily overlooked in a sampling and rock characterization strategy. Recognition of the flow extremes in cores and outcrop, appropriate upscaling to wireline log and seismic attributes, will significantly improve our abilities to predict patterns of fluid flow and retention within overall shale successions. This paper will demonstrate the nature of this variability and how to sample and upscale it from rock properties through seismic attributes in a number of subsurface case studies.
Fine-grained, clay-mineral rich sedimentary rocks deposited in the marine realm are the most common sedimentary rock type preserved in the stratigraphic record. Traditionally, marine mudstones were interpreted to be deposited by gravitational settling through the water column. The implications were that depositional conditions at the seafloor were quiet and steady, resulting in the interpretation of rather homogeneous deposits with fairly uniform attributes.
Within the petroleum industry, the study of mudstones was historically limited to two main disciplines: source rock and seal evaluation. Seal evaluation focused on defining the interface between reservoirs and overlying mudstones (seals). Typically, mechanical and capillary attributes of a limited set of samples are related to the bulk properties of the sealing interval. Some attempts were made to relate sealing attributes to bulk sequence stratigraphic packages ( Dawson & Almon, 2002 ), but limited sample measurements were still related directly to the bulk properties of thick, laterally extensive intervals.
Source evaluation focused on placing specific geologic periods in the context of regional or global anoxic events. With the engrained notion that mudstones recorded steady and quiet depositional conditions, heterogeneity in mudstone attributes with regard to preservation of organic matter are most easily explained with bottom water anoxia. Research on outcrops and well logs by Exxon workers in the late eighties and early nineties ( Creaney & Passey., 1990 ; Bohacs, 1998 ) demonstrated degrees of variability that could be tied to predictive stratigraphic frameworks (and hence, varying depositional conditions as fundamental controls on source rock preservation), but this work did not lead to a significant relook at how mudstone sequences were evaluated for rock and flow properties.
This attitude changed significantly during the rise of shale petroleum plays (“unconventional plays”) in the early 2000s. Suddenly, petroleum geologists were asked for detailed reservoir descriptions and predictions for shale sequences, much as they were used to doing for carbonate and sandstone reservoir intervals. This lead to a realization that (a) flow attributes of mudstone successions varied greatly at all scales, and (b) the industry was poorly equipped to provide meaningful characterizations and predictions for this variability. The necessity for reservoir characterization of mudstone successions, combined with a wealth of high-fidelity subsurface data (core, logs and seismic) coming available, has led to a significant improvement in our ability to characterize mudstone successions ( Passey et al., 2010 ), appreciation of the link between depositional processes and diagenesis ( Macquaker et al., 2014 ) and use modern oceanographic datasets to better understand the controls on transport and deposition of mudstones.
A number of the learnings from the unconventional revolution should be integrated back into evaluations of mudstone successions for applications related to the mechanical and capillary attributes important for understanding subsurface flow and retention of fluids. A key learning is the appreciation for very significant stratigraphic heterogeneity exerting a first-order control on subsurface fluid flow. Any characterization of subsurface shale successions has to include a robust sampling characterization within the stratigraphic framework. In addition, relating a sample measurement to critical and bulk properties of larger units (“upscaling”) has to be done within the understanding of the stratigraphic framework. Here, we will demonstrate examples of this process with a number of subsurface case studies.
Figure 1 shows an example of a detailed stratigraphic model of the Cretaceous Mowry Shale Formation (a mudstone succession containing source rocks, seals and hydrocarbon accumulations) populated with capillary properties to simulate the flow and retention of immiscible fluids (oil and gas) within a water-wet medium. The model was constructed using targeted sampling of identified lithofacies from outcrop and core, and using stratigraphic models to populate the succession with the appropriate lithofacies using upscaling of the attributes to wireline log properties. A key observation from this work shows the fundamental control of ash beds in controlling the flow and retention of hydrocarbon fluids within the succession. While volumetrically insignificant (and easily overlooked in a sampling strategy), these intervals with high capillary threshold pressures dominate the overall migration and retention patterns of hydrocarbon fluids within the succession.
While unconventional with regard to the required drilling and stimulation technology, study of numerous “shale gas” plays demonstrated that hydrocarbon liquids had migrated into certain mudstone reservoir facies and been retained through capillary mechanisms in subtle stratigraphic traps. Subsequent burial of these oil reservoirs led to secondary cracking of oil in conventional pore spaces and the development of solid pyrobitumen filled with gas. An important consequence of these observations is that certain mudstone facies possess pore geometries that allow for trapping of oil at relatively high oil saturations and relatively low capillary pressures. As such, while traditionally viewed as source rocks, seals and “unconventional” reservoirs, many of these mudstone successions contain attributes akin to conventional reservoirs with regard to the potential to retain significant amounts of hydrocarbon fluids through displacement of water in conventional pore systems.
In certain cases, unstimulated flow tests suggest significant permeability networks existing within mudstone successions, even in the absence of obvious fracture networks or sandier interbeds. In order to match reservoir simulations, facies have to be populated with relatively high horizontal permeabilities ( Fig. 2 ). Microscopic studies reveal the presence of mm-scale horizontal bitumen-filled seams, which may have originated as mechanical flow networks during hydrocarbon migration, and continue to provide subtle, yet effective pathways for efficient migration of fluids through mudstone successions.
In summary, mudstone successions are much more heterogeneous with regard to flow properties than typically assumed. Much of the flow heterogeneity is ultimately controlled by stratigraphic variations and dictated by the extremes of the lithofacies, which are often volumetrically insignificant, and therefore easily overlooked in a sampling and rock characterization strategy. Recognition of the flow extremes in cores and outcrop, appropriate upscaling to wireline log and seismic attributes, will significantly improve our abilities to predict patterns of fluid flow and retention within overall shale successions.
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On the Definition of an Effective Stress for Shales
Authors A. Tuttolomondo, A. Ferrari and L. LalouiSummaryEnergy-related engineering applications involving shales pose ever-increasing challenges for the constitutive modelling of these geomaterials. The present work proposes a new definition of effective stress developed by using a thermodynamic approach associated with the concepts of continuum mechanics. The developed expression makes explicit the dependence of the effective stress on the chemical composition of the pore water. The paper shows that the proposed expression is able to improve the interpretation of unsaturated shear strength envelopes that would be achieved by using other formulations.
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Strain-Rate Dependent Pre-Failure, Failure and Post-Failure Behavior of Shale: Experiments and Modeling
Authors M. Gutierrez and Z. HouSummaryThe strain rate dependent mechanical behavior of shale was extensively characterized using triaxial compression tests carried out at different axal strain rates. Based on the experimental results, a constitutive model for shale under different rates of loading was proposed. The model is based on a combination of viscoelasticity and damage mechanics and is formulated to predict the brittle behavior of shales from the pre-peak stage, peak and post peak strain softening regimes. Shear failure and strain softening are attributed to damage due to the growth of fractures in the shale, and de-bonding and decohesion mechanisms responsible for the fracture evolution. Damage is described by a scalar variable D and is assumed to commence when the stress-strain behavior deviates from linear elasticity. It was found that damage evolution during shearing in shale can be adequately represented probabilistically using a Weibull probability distribution function based on the axial strain level. An empirical axial strain rate dependent Young's modulus, together with the damage evolution law, completes the viscoelastic damage model. The model is shown to adequately represent the complete stress-strain response of shale at different axial strain rates and to predict the axial strain rate dependent shear strength of shale.
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