Sixth EAGE Shale Workshop

We have performed a comparative study of undrained triaxial testing with five different laboratories, to explore the reproducibility of test results. Opalinus Clay was sourced as testing material from a borehole at the Mont Terri URL. Cores were vacuum-sealed, and resin impregnated immediately after recovery. Systematic determination of basic properties such as water content, grain density and bulk mineralogy of specimens after testing assisted in diagnostic test evaluation. A detailed testing protocol was requested to avoid specimen damage during initial loading («swelling») and to verify specimen saturation. A balanced pore fluid was used for testing, and a consolidation phase was performed to reach specific target effective stress levels prior to the shear phase. One laboratory deviated from these protocols, as it did not use an external pore fluid. Instead, specimens were brought to variable saturation levels in a desiccator prior to assembling them into the rig. For specimens with almost identical basic properties, the test results were indeed found to be in very good agreement, despite the different procedures applied. Differences in test results can be attributed to material heterogeneity. The study provides compelling evidence that robust triaxial testing can be achieved with shales.

We present an evaluation of shale dominated cap rocks relevant for Middle Jurassic sandstone reservoirs in the Central North Sea, based on well log data from the Norwegian Continental Shelf. Previously established indicators for brittleness and seal quality, E (Young's modulus) and ν (Poisson's ratio), are utilized in the analysis. Similar ductile to fairly ductile behaviour is found in different formations for five analysed wells, of which two are oil discoveries, one contains only oil shows, and two are dry. Cap rocks in the discovery wells are comparatively most brittle, compared to a published E–ν template. Uplift of ~500 m in one of the discovery wells is not found to have compromised the sealing capability. We also investigate how organic content influence an organic-rich shale interval in terms of cap rock properties by using kerogen substitution and comparing to the other more organic-lean shales, which does not support a direct correlation between TOC and ductility. Finally, we consider how observed properties of different shales relate to different mineralogical composition.

Black shales of the Bazhenov Formation are characterized by a high heterogeneity of the composition — a complex combination of carbonate, siliceous, clay, and organic phases, which alter laterally and in the section. Ash beds occur in the lower unit (Tithonian stage, Late Jurassic). Based on a detailed description of the core and complex analytical data (petrographic analysis, scanning electron microscopy with energy dispersive spectrometry, X-ray fluorescence and X-ray phase analyzes, ICP-MS, etc.) the composition, structure and genetic features of ash layers are given. The paper also discusses the carbonate microbialites of the Bazhenov Formation and the impact of hydrothermal processes on their genesis. Volcanogenic and hydrothermal phenomena could have a fundamental impact on the water transparency and other environmental factors, affect the marine biota and its mass extinction and finaly could lead to the formation of stratas with highly increased content of organic matter. Jurassic- Cretaceous global geological processes in the Western Siberia and in adjacent areas confirm the suggested assumptions.

The Namurian Chokier Formation of late Mississippian has been studied in two shallow boreholes in southern Belgium which addresses to its shale gas potential. To help with the global assessment of this shale gas succession, the research focuses on the detailed characterization of the Chokier mudstones in Belgium which distribute in the Campine Basin and Wallonian Basin. After the description of cores and quarries from 6 sampling locations, the samples have been analysed with X-ray diffraction (XRD), microscopy, cold cathodoluminescence for petrographical and mineralogical study. It reveals that Chokier Formation is composed of successive mudstones in the Campine Basin while mudstones with more interbedded sandstones in the Wallonian Basin. The mudstones consist of the mixture of clay minerals (including kaolinite, illite, chlorite), quartz, feldspars, mica (including muscovite, biotite, phlogopite), carbonates (including calcite, dolomite, siderite, ankerite), sulfides (mainly pyrite), organic matter and amorphous material. In consideration of variable sedimentary structures and relative proportions of clay minerals, carbonates, quartz, feldspar and mica, 3 lithologies and 8 lithofacies have been classified in the Chokier mudstones. The deposition setting of Chokier Formation changes from marine, prodelta to delta front from the Campine Basin in northern Belgium to the Wallonian Basin in southern Belgium.

% We present methodology that allows measuring vertical hydraulic permeability and a full stiffness tensor of transversely isotropic shale samples simultaneously during mechanical compaction experiments. In this study we investigated artificial shales composed of quartz, smectite, and kaolinite. For the first time, compaction trends of these properties are obtained, at a 15--40~\% porosity range. The hydraulic permeability values of these shales exhibit exponential compaction trends and lay within the bounds of compaction trends found in natural shales. Strong correlations between the hydraulic permeability and elastic properties of shales are observed.

Fracture characteristics are important as they provide information about the mechanical integrity of its host rock. This study addresses the properties of fractured surfaces of Upper Jurassic organic-rich Shale (Draupne Formation) cored from an exploration well (16/8-3 S) in the Central North Sea. The characterization consists of two steps: i) petrographic studies of the fractured material at micro- and macroscale compared with the mechanical data, and ii) mapping of surfaces with photogrammetric method before and after the samples were sheared in a direct shear test (DST) to determine frictional properties. The preliminary results reveal complex structures, with the natural fractures appearing more diverse than the artificially reactivated surfaces produced from the direct shear test, which may affirm the importance of factors as e.g. temperature, displacement, slip rate and number of reactivations. Moreover, these small-scale fracture planes seem to link up with adjacent fractures to accumulate displacement over a wider area. Little attention has been paid to the details of fractures in shale, despite their importance in the energy industry. Therefore, objective of this study is to provide detailed descriptions of fracture surfaces in shale and the consequences this might induce.

Mudstones are of great interest in the petroleum industry, since they can act as a source rocks, reservoirs and seals. Understanding the mechanical behaviour of these clay-rich rocks is crucial for successful hydrocarbon exploration and production. This study examines the impact of water saturation on the mechanical behaviour of the Whitby Mudstone. Triaxial compression tests are performed on mudstone samples with different water saturations at effective confining pressures of 5 MPa, 15 MPa, 25 MPa, and 40 MPa. The results show that the degree of saturation significantly affects the rock strength and static elastic properties (Young's modulus and Poisson's ratio). The mechanical properties obtained from partially-saturated mudstones should be treated with care when extrapolating them to in situ conditions.

In this research the Haynesville-Bossier Shale play (USA), which is a clay-bearing siliceous, organic-rich, gas-mature shale play, was characterized over pore diameters ranging 2 nm to 3000 nm. Three advanced imaging techniques were utilized correlatively, including the application of Xe+ plasma focused ion beam scanning electron microscopy (plasma FIB or PFIB), complemented by the Ga+FIB method which is now frequently used to characterise porosity and organic/inorganic phases, together with transmission electron microscope tomography of the nano-scale pores. This is the first study to comprehensively develop pore geometric and network models in 3D based on experimental characterisation over these critical length scales

A dataset was assembled resulting from studies on ~35 preserved claystone cores from different locations around the world and burial depths from <200m to ~3700m. Combining all data together, within a small amount of scatter there appears to be a single common trend for bulk density (or porosity) vs. depth (or vs. effective stress), irrespective of location, clay type or the exact clay content, as long as the formation is clay-supported. High-pressure mercury injection porosimetry shows that the modal pore size reduces in a systematic way with increasing burial stress. This loss of porosity and closer packing of the clay aggregates, perhaps combined with geologic time and/or temperature, causes mechanical properties (e.g. stiffness, strength) to also change in a systematic way. Whether or not a claystone behaves in brittle vs. a ductile manner generally depends on the value of confining stress relative to the maximum past burial stress, with young high-porosity claystones behaving quite similar to soil mechanics critical-state theory and deeper, low-porosity claystones deviating from several aspects of this theory and having an apparent maximum past stress higher than actual. In some cases there is evidence of a clay-type effect (smectite vs. kaolinite) on mechanical behavior.

Existing exploration and research about Wufeng—Longmaxi shale gas reservoir show that the content of quartz is generally more than 50% and the American shale gas reservoir also share the same characteristics. The objective of this paper is to further these three question: what is the source of quartz? Why is quartz enriched? How does quartz affect the shale gas reservoir? In this study, the mineralogical analysis was conducted on a total of 202 samples collected from Longmaxi Formation. The samples were used for methane sorption analysis, the XRD, Microscope, SEM, TOC analysis. According to the final results, Wufeng—Longmaxi shale the quartz can be divided into 3 types according to their sources: Biogenic source quartz, terrigenous source quartz, Recrystallization quartz. Different sources of quartz have different effects on shale reservoirs: Biogenic quartz containing an abundance of organic matter and quartz critical to form high quality source rocks and shale reservoirs, Terrigenous quartz impedes organic matter accumulation but makes the rock brittle, Recrystallization of quartz is negative to the physical reservoir properties. Defining quartz sources should be a new key factor for the Exploration and Evaluation of shale gas reservoirs.