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Fourth EAGE Shale Workshop
- Conference date: April 6-9, 2014
- Location: Porto, Portugal
- Published: 06 April 2014
32 results
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Better Understanding Shales by Accurate Qualitative and Quantitative Mineralogy
Authors R. Adriaens, G. Mertens, E. Zeelmaekers, N. Vandenberghe and J. ElsenSummaryA thorough characterization of the mineralogy and detailed clay mineralogy is an important tool in scientific research and industrial applications of shales. It helps to understand the geological history of the deposits and significantly contributes to solving specific geological problems. In three separate examples, it is demonstrated how the clay mineralogy of fine-grained deposits during different stages of diagenesis is characterized in detail.
In a first example, clay minerals are used as provenance indicators in recent muds of the Belgian North Sea. A second example handles on the detailed clay mineralogy of early diagenetic Ypresian clays which are currently being researched as possible host rock for radioactive waste disposal. A third example involves the mineralogical characterization of a later diagenetic potential shale gas play in Belgium.
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The Role of Specific Surface Area and Cation Exchange Capacity in Determining Shale Rock Properties
Authors M. Josh, A. Bunger, J. Kear, J. Sarout, D. Dewhurst, M.D. Raven, C. Delle Piane, L. Esteban and M.B. ClennellSummarySurface area, surface charge and the exchangeability of cations have long been known as factors in determining the physical and mechanical properties of shales. Such properties are related to the fine grain size of the various clay minerals and also the particle shapes, edge-face microstructural arrangements and are impacted by the likes of salinity and depositional environment. In general, the siltier a given clay or shale, the lower their cation exchange capacity (CEC) and specific surface area (SSA).
CEC and SSA have direct impacts on mechanical and flow properties. Wellbore instability can be caused by the development of osmotic pressures resulting from differences between pore fluid composition and drilling fluid composition and the resultant time dependent effects based on permeability. Such instability can be mitigated by the use of oil-based muds or for example high KCl water-based muds. Electrical and dielectric properties are also dependent on mineral surface charge and related directly to the CEC of a given shale, especially properties such as resistivity (or conductivity) and permittivity (dielectric constant). This paper will detail SSA and CEC values of shales from multiple basins worldwide and also from individual boreholes to demonstrate their role for shale rock properties.
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Caprock and Shale Gas Characterization - Appropriate Petrophysical Methods and Integration with Log Data
Authors M. Fleury, S. Youssef and P. BoulinSummaryThe characterization of caprocks and gas shales is very similar and cannot be performed using conventional petrophysical techniques. Even for basic properties such as porosity and permeability, appropriate protocols and instrumentation are required. We present a summary of the experience accumulated during several years on samples for which measurements were believed too inaccurate or too long and costly before the new techniques described below were used. For porosity, tortuosity and connectivity of the pore space, we heavily rely on single and multidimensional NMR techniques with an appropriate instrument. For permeability, we developed an efficient steady-state technique for both water and gas effective permeability. For the visualisation of the pore space, we rely on recent microscopy techniques (FIB) that are appropriate at nanometre scale. Finally, at larger scale, standard log data should also be considered very carefully and improved existing methods for estimating porosity by NMR should be used systematically.
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Permeability and Resistivity Anisotropy of Resedimented Boston Blue Clay
Authors A.L. Adams, J.T. Germaine and P.B. FlemingsSummaryChanging mudrock permeability in sedimentary basins affects fluid migration, consolidation rates and overpressure generation. However, obtaining quality specimens to measure the permeability of mudrocks is costly and difficult. To solve this dilemma, we measure the permeability anisotropy and resistivity anisotropy of resedimented mudrocks in the laboratory. We find that the permeability anisotropy of mechanically compressed Resedimented Boston Blue Clay (RBBC) mudrock increases from 1.2 to 1.9 as the applied stress increases from 0.4 to 10 MPa and the porosity decreases from 0.5 to 0.36. Further increases in applied stress result in a decrease in permeability anisotropy, likely due to the progressive cutoff of horizontal flow paths under an axial stress regime. Resistivity measurements reveal that the electrical conductivity anisotropy follows the same trend, indicating that the conductivity anisotropy is directly proportional to the permeability anisotropy of uniform RBBC.
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Pore Space Relevant of Gas Transport in Opalinus Clay - Homogeneity, Percolation and Capillary Properties
By L. KellerSummaryThe application of percolation theory to geomaterials generally includes the study of simplified lattice models in order to derive information on pore connectivity. Here, and possibly applied for the first time to shales, we computed key percolation variables such as the percolation probability distribution from our reconstructed pore microstructures. Then, we used a finite-scaling approach to calculate the critical porosity (i.e. percolation threshold) of pore microstructures from sandy and shaley facies of Opalinus Clay. Both microstructures show anisotropic characteristics with respect to connectivity and percolation threshold. We found percolation thresholds with critical porosities 0.04 – 0.12 parallel to bedding and 0.11 – 0.19 perpendicular to bedding. The resolved porosity of the sandy facies (low clay content) is close to the percolation threshold, whereas the porosity of the shaley facies (high clay content) is below the percolation threshold. The approach yields similar results when compared to an approach that is based on morphological image techniques and involves the modification of the original pore voxel data. Therefore, we used modified pore structures in combination pore network modeling to discuss changes in gas transport properties in case porosity approaches the critical porosity from above.
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Quantifying Porosity, Compressibility and Permeability in Shale
Authors E.N. Mbia, I.L. Fabricius, P. Frykman, A. Krogsbøll and F. DalhoffSummaryThe Fjerritslev Formation in the Norwegian-Danish Basin forms the main seal to Upper Triassic-Lower Jurassic sandstone reservoirs. In order to estimate rock properties Jurassic shale samples from deep onshore wells in Danish basin were studied. Mineralogical analysis based on X-ray diffractometry (XRD) of shale samples show about 50% silt and high content of kaolinite in the clay fraction when compared with offshore samples from the Central Graben. Porosity measurements from helium porosimetry-mercury immersion (HPMI), mercury injection capillary pressure (MICP) and nuclear magnetic resonance (NMR) show that, the MICP porosity is 9–10% points lower than HPMI and NMR porosity. Compressibility result shows that deep shale is stiffer in situ than normally assumed in geotechnical modelling and that static compressibility corresponds with dynamic one only at the begining of unloading stress strain data. We found that Kozeny’s modelled permeability fall in the same order of magnitude with measured permeability for shale rich in kaolinite but overestimates permeability by two to three orders of magnitudes for shale with high content of smectite. The empirical Yang and Aplin model gives good permeability estimate comparable to the measured one for shale rich in smectite. This is probably because Yang and Aplin model was calibrated in London clay which is rich in smectite.
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Hydromechanical Characterization of Shales on multiple Scales
Authors A. Papafotiou, R. Senger and L. KellerSummaryMicrostructural analyses by means of micro-scale imaging, laboratory tests of core samples, and formation hydraulic tests in boreholes are used to establish correlations between mineralogical, hydraulic and geomechanical properties of four investigated shale and claystone formations in Switzerland. The analyses spanning scales ranging from the micro- to the macro-scale indicate that the mechanical behaviour associated with clay content is a key factor determining on the one hand transport properties of the intact rock matrix and on the other hand the mechanism of self-sealing that controls the transport capacity of fractures in shales. Clay content from geophysical logs is additionally used to derive continuous profiles of transport properties such as porosity and permeability. The profiles are analysed statistically incorporating the borehole lithostratigraphic information to derive representative variogram descriptions of the formation facies, depicting structural bedding features of the stratigraphic and mineralogical profiles. The geostatistical descriptions are used to perform conditioned 3D simulations of porosity and permeability for the investigated formations. The described methodology thus establishes clay content as the leading property in order to characterize the transport mechanisms in each formation and accordingly derive quantitative models of transport property distributions.
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Microstructure of the Shaly Facies of Opalinus Clay on the Mm-nm Scale
Authors M.E. Houben, G. Desbois, J.L. Urai, D.A.M. de Winter, M.R. Drury and J.O. SchwarzSummaryOpalinus Clay is one of the fine-grained sedimentary formations investigated as a possible geological repository for the long-term storage of radioactive waste. Porosity, pore size, pore shape and connectivity of pores will help to define sealing capacities of a host rock. Here a combination of FIB-SEM, BIB-SEM and μ-CT has been used in order to investigate the nm-mm scale microstructure of the Shaly facies of Opalinus Clay in 2D and 3D. The μ-CT measurements gave a 3D overview of the microstructure of a 2 mm core down to a 2.6 μm voxel size. Which showed cracks, larger bivalve fragments and pyrite nodules embedded in a matrix, which was investigated down to the nm scale (enabling to image the pores) in 2D using the BIB-SEM method. FIB-SEM measurements were afterwards performed on selected areas to investigate the pore shape and connectivity in 3D down to the nm scale.
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Clay Mineral Preferred Orientation - How to Predict It and What It Might Control
More LessSummaryThis paper systematically explores the controls on the preferred orientation of clay minerals. Fabric development in mudstones occurs as a result of mechanical consolidation and mineral reactions. The magnitude of the effect of these actions on fabric is variable across temperature/depth space. It is possible to be partly predictive concerning fabric development when some geological information is available. This has implications for predictive modeling, e.g. permeability anisotropy in basin modeling packages, velocity modeling in seismic inversion.
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Upscaling in Anisotropic Layers with Tilt in a Symmetry Axis
By A. StovasSummaryShales are known as the typical rocks with TI anisotropy. The method for upscaling in tilted TI media is proposed. By using the Schoenberg-Muir theory for upscaling in general anisotropic media, I analyze the effect of intrinsic tilt on effective anisotropy parameters defined in different symmetry planes.
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The Paleozoic Source Rock of NW Gondwana
Authors F. Dartora and I. MorettiSummaryDuring the Paleozoic, South America and Africa was part of the same continent, the Gondwana. The land mass was close to the south pole for a while and then migrated northward after the Carboniferous. Due to this common past geological setting, in both areas the Paleozoic formation have numerous similarities. The hot shale of north Africa, Silurian and Devonian in age are well known, and lead to large reserves in Algeria and Libya among others. The synthesis of the south American Paleozoic sources rock presented in this paper shows at the opposite that the richer organic shales are Carboniferous and Permian in age in South America. Silurian and Devonian source rocks exist, are sometime rather thick but never very rich (TOC ∼ 2%, initial HI around 500). At the opposite the Lower Carboniferous of the Madre de Dios basins (Peru and Bolivia) and the Permian of the South Peru, North Bolivia and Parana in Brazil are the best Paleozoic SR of SA, with initial S2 up to 80 mg HC/g. Central South America therefore differs from the general trend of the compilation of the source rock richness, worldwide, as in North Africa, Silurian and Devonian show the best values.
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Processes Governing Advective Gas Flow in the Callovo Oxfordian Claystone (COx)
Authors J.F. Harrington, R.C. Cuss, D.J. Noy and J. TalandierSummaryData from this study demonstrates advective movement of gas in COx is accompanied by dilation of the samples (i.e. the formation of pressure induced micro-fissures) at gas pressures significantly below that of the minimum principal stress. Flow occurs through a local network of unstable pathways, whose properties vary temporarily and spatially within the claystone. The coupling of variables results in the development of significant time-dependent effects, impacting many aspects of COx behaviour, from gas breakthrough time, to the control of deformation processes. Variations in gas entry, breakthrough and steady-state pressures may result from the arbitrary nature of the flow pathways (which vary spatially and temporally), microstructural heterogeneity which may exert an important control on the movement of gas, or, a combination of the two. Examination of other clays, mudrocks and shales indicate gas flow is accompanied by an increase in sample volume associated with a change in porosity caused by the formation of gas pathways. Under these conditions, data suggests gas flow is along pressure-induced preferential pathways, where permeability is a dependent variable related to the number, width and aperture distributions of these features. This has important implications for modelling the migration and containment behaviour of these materials.
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Novel Apparatus and Methodology to Permanently Monitor Formation Pore Pressure in a Wellbore
Authors J. Park, I. Viken, J.C. Choi, B. Bohloli, Ø Godager, K. Borgersen, E. Skomedal and A.G. CasseresSummaryWe introduce a novel approach to measure directly the formation pore pressure and temperature during injection and production operations. We analyze a set of acquired data and compare with conventional injection operation data. Through a system identification algorithm, we process the raw data and remove noise from the data. The overall result shows that the novel sensor technology applied provide the direct and reliable measurements of formation pore pressure and temperature.
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Capillary Seal Capacity of Cenozoic Mudstone Caprocks of Shallow Gas Occurrences, Dutch Offshore
Authors J.M. Verweij, V. Daza Cajigal, G. De Bruin and K. GeelSummaryThe seals of the shallow gas occurrences in the northern offshore of the Netherlands are formed by siliciclastic intra-delta mudstones of Plio-Pleistocene age. There are no publicly available measured values of the seal capacity of these Plio-Pleistocene caprock mudstones. We developed, tested and applied a systematic approach to evaluate the permeability and capillary seal capacity of mudstones at shallow depth (400–900 m) using new grain size analysis data of offshore and onshore Cenozoic mudstones and publicly available pressure data. The calculations showed that the vertical permeability of the mudstones varies between 2.8E-20 and 1.1E-18 m2. Most of the pore throat radii of the mudstones vary between 0.5 and 1.5 μm. The gas column heights were also calculated for 10 offshore wells and all samples from these wells. The calculated capillary seal capacities of the mudstones vary between approximately 10 and 24 m. Overall, the systematic approach using grain size analysis data was found to be a promising methodology for estimating petrophysical properties (porosity, permeability, pore throat) and capillary seal capacities of Plio-Pleistocene mudstone caprocks.
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Shear Strength of Two Gulf of Mexico Mudrocks and a Comparison with Other Sediments
Authors B. Casey, B.P. Fahy, P.B. Flemings and J.T. GermaineSummaryThis study examines the shear strength behavior of two mudrocks from the Gulf of Mexico through a series of KO-consolidated triaxial compression tests. The tests are performed over a very wide range of effective stresses from 0.1 up to 10 MPa. Samples were prepared for triaxial testing by laboratory resedimentation of the core material. There is a consistent decrease in the critical state friction angle of both mudrocks with increasing stress level, and this corresponds with an increase in the value of KO (the ratio of horizontal to vertical effective stresses for one-dimensional consolidation). The strength behavior of the Gulf of Mexico mudrocks is consistent with that observed for other materials from a wide variety of geologic backgrounds. It is shown that variations in critical state friction angle with stress level can be correlated to a mudrock’s liquid limit, an easily measured index property which reflects the clay mineralogy and clay fraction of a mudrock. This finding has great practical value as it allows for a reasonable estimate of the critical state friction angle of a mudrock without having to obtain intact field samples, which is frequently infeasible in the case of deep sediments or sediments from the deep ocean.
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Mechanical Anisotropies of Shale
Authors R.M. Holt, A. Bauer, E. Fjær, P. Horsrud, O.M. Nes and J.F. StenebråtenSummaryLaboratory experiments have been performed with a number of shale samples, including commonly studied outcrops like Pierre and Mancos Shale. The angular dependences of unconfined strength, friction angle, static E-modulus and P-wave velocity have been measured and modelled. The anisotropy of the ratio between dynamic and static moduli is found to represent plastic / brittleness anisotropy. Stress and stress path dependent velocity anisotropy will be shown and related to possible 4D seismic applications. Implications for field applications to borehole stability and fracturing will be discussed.
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Seismic Characterisation of Mud-rich Sediments - Application to Seal Risk
Authors S. Karimi, A.C. Aplin, K.D. Kurtev and F.B. KetsSummaryThis study demonstrates the possibility of using seismic attributes for texture detection and leakage risk assessment in mud-rich seal units. Single attributes are insufficient to generate comprehensive insights to leakage risk. The proposed multiple attribute approach significantly increases seal characterisation efficiency by taking into account several important factors which increase leakage risk, and also using conditioning attributes as a platform for the calculation of other attributes. In addition, seismically-derived risk attribute (so-called Seal Risk Factor “SRF”) showed robust correspondence to the lithologies within the seismic volume. High seismic SRFs were often a good approximation for volumes containing a higher percentage of coarser-grained and distorted sediments, and vice versa. We believe that this is the first attempt at quantitative 3D seal risk characterisation in mud-rich sediments. Its application on modern seismic surveys can save days of processing/mapping time and can reduce exploration risk by basing decisions on seal texture and their proven link to leakage elements.
Our approach has been applied to regional seals in a case study from West Africa.
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Creeping Behavior of Thermally and Chemically Destabilized Shale
Authors F.B. Kets, L.M. Duffy and A.C. AplinSummaryThe results of a triaxial measurements on drained smectite rich shale have been analyzed. Static and dynamic properties of the shale were found to depend on shale matrix alteration due to differential thermal expansion and chemical alteration effects. Strong creep has been observed, which seems to originate from poroelastic effects, notably pore fluid drainage, and from rearrangement of matrix and pore space.
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Thermally Induced Compaction of Shales and Its Impact on Acoustic Velocities
Authors A. Bauer, R.M. Holt, L. Marøen, J. Stenebråten and E.F. SønstebøSummaryThermoplastic effects in consolidated, rather competent subsurface shales is often ignored in geomechanical modeling. However, in laboratory tests with several subsurface and outcrop shales, we have found sizable, non-reversible compaction upon heating to temperatures up to 120 degrees C, in some cases exceeding 1% volumetric strain. Such large strains could potentially result in rock failure, compromising the integrity of the shale formation, and would have to be accounted for in, e.g., caprock integrity assessments for thermal enhanced oil recovery or CO2 storage projects. Seismic monitoring could help to map thermal strains and stresses in shale formations but the interpretation of seismic data would require calibrated thermal rock-physics models that are not available yet. In order to get a better understanding of thermally induced compaction in shales and its impact on acoustic velocities, we carried out some dedicated laboratory thermal compaction tests with Pierre shale. Heating resulted in timedependent volumetric strains of up to 5%, accompanied by a large increase in ultrasonic p-wave velocity that was explained by the reduction in porosity.
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Compaction and Rock Properties of Cenozoic Mudstones in the Quadrant 15, Offshore Norway
Authors M. Koochak Zadeh, N.H. Mondol and J. JahrenSummaryCompaction and evolution of rock properties of Cenozoic mudstones are investigated in the Quadrant 15, offshore Norway. The study area is a normally subsided basin located in the southern Viking Graben of the Norwegian North Sea. A suite of well log data from 41 exploration and appraisal wells is utilised to study the compaction behaviour of Cenozoic mudstones. The detailed petrophysical analyses suggest the occurrence of high quantities of smectite in the Eocene and Oligocene sediments, in general agreement with previous publications. The transition from mechanical to chemical compaction takes place between 70–90oC corresponding to depth 2–2.5 km. The geothermal gradient is higher in the south of the study area but, temperature alone is not sufficient to explain chemical diagenesis in the area. Initial mineralogy seems to be as important as temperature with respect to cementation. The degree of chemical compaction and cementation reflects the initial smectite content and the availability of potassium for the reaction of smectite to illite and quartz to take place. The overall results of this study contribute to delineate compaction processes controlled by geological processes (e.g. provenance and depositional environments) in a highly prosperous region in terms of hydrocarbon exploration and production.
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Permeability and Compressibility of Resedimented Gulf of Mexico Mudrocks
Authors P.B. Flemings and W. BettsSummaryWe used resedimentation and constant rate of strain (CRS) compression tests to study the compaction and permeability evolution of material derived from Plio-Pleistocene aged mudrock from the Eugene Island Block 330 oilfield, offshore Louisiana. We compare our results to previously published in-situ data derived from well logs and to previous tests of intact core. We find remarkable agreement in both compression and permeability behaviour between our resedimentation experiments and intact measurements. Resedimentation provides a systematic approach to understanding mudrock behaviour at high effective stresses. The correlation between in-situ measurements and resedimented results allows us to derived characteristic compression and permability behaviour for Gulf of Mexico mudrocks. These results suggest that there has been little diagenesis or other ageing processes that have occurred in these mudrocks even though they have been buried to more than 3,000 m.
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On the Elastic Anisotropy of Shales
By L. DurantiSummaryThe elastic properties of shales, and their anisotropic character, have been tested more extensively with the introduction of the large group of lithologies which are collectively named gas shales. With respect to a traditional view of “shales = claystones”, gas shales have certainly introduced a more complex compositional mix, with a strong presence of carbonate material and, more limitedly organic matter. The direct impact is a distinct new trend in the relationship between elastic anisotropy and clay content, with probable implications on the constitutive equation of shales, as well as on the interplay between stiffness tensor and particle alignment. It is certainly noteworthy the fact that introducing isotropically (i.e. randomly) carbonate material in the architecture of an anisotropic composite has increased anisotropy significantly. In addition, we currently do not observe a direct relationship between the amount of organic material and elastic anisotropy. The implication is that the key to a general anisotropic “hardening” in gas shales relies in an additional parameter which is currently not understood.
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Mechanical Anisotropy of Gas Shales and Claystones
By R.T. EwySummaryThis abstract/presentation explores mechanical anisotropy of gas shales and claystones and compares the two. The types of anisotropy addressed are 1) static Young’s Modulus anisotropy, 2) static Poisson’s Ratio anisotropy and 3) compressive strength anisotropy. Claystones are found in many ways to be an ‘end member’ of the gas shale universe, and preliminary data suggest that carbonates might represent another logical end member. The dataset consists of mechanical property measurements on fifty different core intervals spread across six different shale gas plays, plus several different clay-rich mudstones. Gas shales mostly have Young’s Moduli values much higher than claystones, and often more anisotropic. Most gas shales have horizontal:vertical modulus ratios of ∼1.4:1 to 2.5:1 or higher. While modulus correlates with bulk density, anisotropy does not. Poisson’s Ratio is also anisotropic, with the vertical Poisson’s Ratio being less than the horizontal. Gas shales also have significant strength anisotropy, more so than most mudstones, due to a low relative shear strength of the bedding planes. Implications of the observed mechanical anisotropy for horizontal stress estimation and for wellbore stability are discussed.
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Thermal Well Stimulation in Gas Shales Through Oxygen Injection and Combustion
Authors G.C. Chapiro and J. BruiningSummaryThe amount of extracted natural gas from low permeable organic rich shale has increased rapidly over the past decade. The technologies used to improve the permeability of a shale gas reservoir are horizontal drilling and hydraulic fracturing. The potential environmental impact of hydraulic fracturing motivates research aiming at alternative permeability enhancement methods.
In this paper we investigate the possibility of in-situ combustion to improve permeability. We consider two possibilities for the in-situ fuel source, viz., methane or kerogen.
A one dimensional model is considered. Under simplifying hypotheses a quasi-analytical solution for the corresponding Riemann problem is obtained using advanced mathematical techniques. The solution was analysed with parameters that correspond to resonance conditions, i.e., conditions for which the speeds of the heat wave and the combustion wave are equal. We conclude that methane combustion cannot generate enough heat to enhance the permeability. However kerogen, if present in sufficient quantities, makes this possible. We present the set of parameters for which the combustion reaches the optimal temperature to improve the permeability.
Finally we validate the analytical approximation with direct numerical simulations. With the numerical model we can calculate the pressure distribution in the reservoir and the production improvement by the thermal treatment.
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On the Water Retention Behaviour of Shales
Authors V. Favero, A. Ferrari and L. LalouiSummaryThe involvement of shales in engineering fields such as the extraction of shale gas or the nuclear waste geological disposal is mainly driven by the low permeability and the high retention properties of these geomaterials. The high capillary forces developed in the matrix of the shales allow the fluid trapping with the consequence that the material remains saturated until significant values of suction. However, in the context of several engineering applications, the shale formations are exposed to relative humidity values which might induce changes in the degree of saturation. As a consequence, the investigation of the air entry value and of the retention behaviour of the material is of primary significance.The following paper presents an investigation on the water retention properties of shales. The experimental technique developed for such analysis is described in detail and selected results are presented. The testing methodology is based on the control of the water content and on the subsequent measurement of the total suction at equilibrium.The volumetric behaviour of the material is monitored in order to compute the retention curve in terms of degree of saturation. The results allow to identify a relationship between the air entry values and the porosity for Swiss shales.
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Hydraulic and Gas Transport Testing of Brauner Dogger and Opalinus Clay
Authors C.C. Graham, J.F. Harrington and R.J. CussSummaryWe present results from fluid flow experiments on samples of Brown Dogger and Opalinus Clay (OPA). Hydraulic testing was carried out on both samples, with flow perpendicular to bedding. Fitting numerical model outputs with the resulting laboratory data provides hydraulic permeabilities similar to those previously reported for Opalinus Clay. However, both samples exhibited slightly lower values, as might be expected given their burial history, whilst the Brown Dogger sample displayed a relatively higher value for specific storage than the OPA. We also present findings from gas injection testing of the OPA sample. Gas entry pressure is non-trivial to determine, but findings suggest that entry occurred close to a gas pressure of 4MPa above applied pore-water pressure. However, outflow was observed to be minimal until much higher excess gas pressures ∼10–11MPa. These findings indicate that, within the pressure window examined, the gas entry characteristics of OPA may be relatively insensitive to the applied boundary conditions and provide additional information on the hydraulic and gas transport properties of the Opalinus Clay/Brown Dogger formations.
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Experimental Observations of the Flow of Water and Gas along Fractures in Opalinus Clay
Authors R.J. Cuss and J.F. HarringtonSummaryThis study introduces experimental results of the flow of water and gas along fractures in Opalinus clay (OPA). The flow characteristics are far from simple. Initial swelling of the fractures reduced fracture flow by one order of magnitude. Shear reduced flow by a further order of magnitude and was an effective self-sealing mechanism. However, continued shear increased flow by over four orders of magnitude. The injection of gas had no detrimental effect on the hydraulic properties of the fracture. The gas entry pressure of OPA appeared to remain unchanged by shearing and this suggests shear was not an effective mechanism to reduce fracture gas transmissivity. The characteristics of gas pressure response post entry was nonrepeatable and suggests that the physics governing gas entry are reproducible, but the flow, which is dictated by the number and spatial distribution of dilatant pathways, was not consistent due to variation in the number of formed pathways.
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Gas Breakthrough and Flow Tests on Opalinus Clay - Ambiguities in the Interpretation of Experimental Data
Authors S. Amann-Hildenbrand, A. Busch and B.M. KroossSummaryWe present and discuss the results of a comparative study of two laboratory procedures for the determination of the capillary gas breakthrough pressures of low-permeable lithotypes with intrinsic permeability coefficients below 10–20 m2. Well-characterized and mineralogical homogeneous core section of the Opalinus Clay (Mont Terri) were used in this study. The experimental conditions corresponded to a depth of approximately 1500 m depth, representative of a typical CO2 storage scenario (30 MPa confining pressure, 45°C).
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Hydraulic Conductance of the EDZ around Underground Structures of a Geological Repository for Radioactive Waste
Authors A. Alcolea Rodriguez and U KuhlmannSummaryThe present study focuses on the temporal evolution of hydraulic parameters of the Excavation Damage Zone (EDZ hereinafter). To that end, a hybrid finite/discrete element method (FEMDEM) was first used to simulate the geometry and geomechanical conditions of discrete fracture networks forming the EDZ, which develop in response to the excavation process in the rock mass around the underground structures of a radioactive waste repository ( NAGRA, 2013 ). The simulated geometry and geomechanical properties are mapped onto a finite element mesh, which allows us to solve the fluid motion equations at the near-field.
The resaturation of the EDZ causes a pore pressure increase at that zone. This leads to a decrease of the normal effective stress and, correspondingly, to the progressive closure of the fracture. This closure causes fracture hydraulic conductivity to drop in time, while matrix conductivity increases due to swelling.
The objectives of this study are:
- To quantify the temporal evolution of hydraulic properties in response to pressure variations caused by
- To quantify the temporal evolution of the specific axial flux through the EDZ.
- To quantify the total time required for a full resaturation of the EDZ.
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Modeling the Hydraulic and Two-phase Flow Behaviour of the Heterogeneous, Fractured EDZ in the Opalinus Clay During the HG-A Experiment at the Mont Terri URL
Authors R.K. Senger, G.W. Lanyon and P. MarschallSummaryThe characterization of gas migration through low-permeability clay formations has been a focus of R&D programs for radioactive waste disposal. Nagra has developed a comprehensive program to characterize gas flow in the Opalinus Clay including the excavation disturbed zone (EDZ) around the underground openings of the repository. The experiment (“Gas path through host rock and along seal sections / HG-A“) as part of the Mont Terri research programme was designed as a long-term water & gas injection experiment in a backfilled microtunnel, to investigate both water/gas leak-off rates and gas release paths from a sealed tunnel section in a low permeability host rock. The experiment combines field investigations with extensive hydro-mechanical instrumentation in and around the HG-A microtunnel with geotechnical laboratory studies on core samples and extensive modelling activities for the validation of coupled hydromechanical models. For the numerical modeling of the HG-A experiment, a 3D model was developed, taking into account the heterogeneity associated with the fractured EDZ. In addition, the relevant coupled hydro-mechanical phenomena were implemented which include (a) the gradual sealing of the fractures in the EDZ during resaturation, and (b) a pressure-dependent permeability associated the water- and gas injection tests.
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Geomechanical Characterization and Numerical Parametrization of Opalinus Clay with Application to Underground Structures
Authors S.B. Giger, P. Marschall, P. Nater, W. Shiu and D. BillauxSummaryResults of a new geomechanical testing program of Opalinus Clay core samples from a deep geothermal well in northeast Switzerland are presented. The experimental findings are in qualitative agreement with previously reported characteristics of the mechanical behaviour of Opalinus Clay. These characteristics comprise of a distinct strain softening irrespective of the degree of overconsolidation, anisotropy in elastic and strength properties, complex non-saturated and moderate swelling behaviour. Based on the relatively strong localization of deformation and pronounced brittleness, a bi-linear Mohr-Coulomb model with a tension cut-off was chosen to represent failure behaviour. Undrained triaxial deformation tests were then reproduced numerically using FLAC 3D software and a bilinear, strain softening ubiquitous joint constitutive model, capable of accounting for anisotropic strength but neither anisotropic nor stressdependent elastic properties. Comparison between laboratory and numerical results highlights that adequate reproduction of strength evolution can be achieved either parallel or perpendicular to the plane of anisotropy assuming isotropic elasticity, but not in both directions. Given this limitation the appropriate choice of modelling input parameters is discussed for application to cavern stability assessment for potential repositories in Opalinus Clay at a depth of 600m.
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