Pavel Golodoniuc

Parameterization of elastic stress sensitivity in shales

Stress dependency and anisotropy of dynamic elastic properties of shales is important for a number of geophysical applications, including seismic interpretation, fluid identification, and 4D seismic monitoring. Using SayersKachanov formalism, we developed a new model for transversely isotropic (TI) media that describes stress sensitivity behavior of all five elastic coefficients using four physically meaningful parameters. The model is used to parameterize elastic properties of about 20 shales obtained from laboratory measurements and the literature. The four fitting parameters, namely, specific tangential compliance of a single crack, ratio of normal to tangential compliances, characteristic pressure, and crack orientation anisotropy parameter, show moderate to good correlations with the depth from which the shale was extracted. With increasing depth, the tangential compliance exponentially decreases. The crack orientation anisotropy parameter broadly increases with depth for most of the shales, indicating that cracks are getting more aligned in the bedding plane. The ratio of normal to shear compliance and characteristic pressure decreases with depth to 2500 m and then increases below this to 3600 m. The suggested model allows us to evaluate the stress dependency of all five elastic compliances of a TI medium, even if only some of them are known. This may allow the reconstruction of the stress dependency of all five elastic compliances of a shale from log data, for example.

Representing and publishing physical sample descriptions

Abstract This paper presents a metadata model for physical samples, developed by CSIRO for its role as an allocating agent. The model is essential for connecting various samples to the Web in a systematic manner. It serves as a basis for registering and publishing samples from researchers and laboratories in CSIRO with the International Geo Sample Number (IGSN). The model is simple, extensible and publicly available. We specify how existing controlled vocabularies are incorporated into the model development, and discuss their relevance and limitations. We also describe the mappings between the developed model and existing standards. This is necessary to extend the models adoption across various science domains. The model has been implemented and tested in the context of two large sample repositories in CSIRO. The results demonstrate the effectiveness of the metadata model while maintaining its flexibility to adapt to various sample types.

Geospatial Information Modelling for Interoperable Data Exchange - Application Schema Modelling: From Concept to Implementation

Geographic information is inherent to many application domains in various disciplines and constitutes an integral part of Earth sciences, including geology, geophysics, meteorology, hydrology, oceanography, and soil science. Communication of sophisticated geographical data requires the use of complex technologies that enable interoperable geospatial information exchange channels. The primary authorities in geographic information standardization are ISO Technical Committee 211 (ISO/TC 211) and the Open Geospatial Consortium (OGC), which govern the abstract standards including information models for cross-domain concerns, architectures for distribution of geospatial services and implementation of ISO standards through service interfaces, data models and encodings. The “language” that all participating parties should understand in the communication process is defined by GML Application Schema, which will conform to best practice guidelines and international standards if it is developed using the Hollow World modelling environment. Solid Ground toolkit may be especially useful during the modelling process in accomplishing some routine operations and refactorings. Application schema defined in conceptual terms of a particular domain may be then easily transformed into its physical representation – a set of W3C XML Schemas, which is achieved by the use of Full Moon XML Processing framework. This modelling approach, which places the structural definition of the information at the centre of the design process, is known as Model Driven Architecture, which makes the information model the only artefact that has to be maintained by the governing body. These technologies provide a complete set of tools required to design and implement an Application Schema using the ISO 19100 series of international standards.

An Open Source Web Service For Registering And Managing Environmental Samples

Records of environmental samples, such as minerals, soil, rocks, water, air and plants, are distributed across legacy databases, spreadsheets or other proprietary data systems. Sharing and integration of the sample records across the Web requires globally unique identifiers. These identifiers are essential in order to locate samples unambiguously and to manage their associated metadata and data systematically. The International Geo Sample Number (IGSN) is a persistent, globally unique label for identifying environmental samples. IGSN can be resolved to a digital representation of the sample through the Handle system. IGSN names are registered by end-users through allocating agents, which are the institutions acting on behalf of the IGSN registration agency. As an IGSN allocating agent, our goal is to implement a web service based on existing open source tools to streamline the processes of registering IGSNs and for managing and disseminating sample metadata. In this paper, we present our ongoing work onthe design and development of the web service, and its data schema and database model for capturing key aspects of environmental samples. We show how existing controlled vocabularies can be incorporated into the service development to support the metadata registration of different types of samples. The proposed sample registration and curating approach has been trialed in the context of the Capricorn Distal Footprints project on a range of different sample types, varying from water to hard rock samples. The initial results demonstrate the effectiveness of the service while maintaining the flexibility to adapt to various media types, which is critical in the context of a multi-disciplinary project.

Phenomenological Study of Seismic Anisotropy in Shales

Elastic anisotropy is important for a number of geophysical applications, including seismic interpretation, fluid identification and 4D seismic monitoring. Due to their strongly anisotropic properties, the presence of shales in the subsurface may cause significant errors in depth obtained from surface seismic data, in normal and dip moveout correction, migration and amplitude versus offset analysis. Here we analyze Thomsen’s anisotropy parameters and anellipticities of 37 shales with different mineralogical compositions, maximum overburden stress experienced, silt and clay fractions, porosities and other petrophysical parameters. The shales extracted from depths below 4000 m exhibit the largest values of P- and S-wave anisotropies, while negative delta values are mostly observed for shales extracted from shallow depths. The observed diversity of Thomsens anisotropy parameters and anellipticities can be explained with (1) intrinsic anisotropy of wet clay packs, (2) governed by compaction anisotropic distribution of discontinuities and (3) effects of aligned silt inclusions on shale elastic properties.

Stress dependency of elastic properties of shales: the effect of uniaxial stress

Understanding seismic anisotropy in shales is important for quantitative interpretation of seismic data, 4D monitoring and pore pressure prediction. Along with intrinsic anisotropy caused by preferred mineral orientation that is common in shales, anisotropic stress is an important factor that affects shale elastic response. While variations of elastic coefficients with anisotropic stress have been the subject of experimental studies, theoretical insight is still largely lacking. Here we suggest a new model that allows parameterization of the stress dependency of elastic coefficients of shales under anisotropic stress conditions. We show that the parameterization requires four parameters, namely, specific tangential compliance of a single crack, the ratio of normal to tangential compliances, characteristic pressure and a crack orientation anisotropy parameter. These parameters can be estimated from experimentally measured stress sensitivity of elastic coefficients in shales to isotropic stress.

Trends in micro-crack properties and reconstruction of TI elasticity tensors of shales

Stress dependency of the TI elastic tensor of shales is important for seismic interpretation, overpressure prediction, 4D monitoring, etc. Using Sayers-Kachanov formalism, we develop a new model for transversely isotropic (TI) media which predicts stress sensitivity behaviour of all five elastic coefficients. The model is used to parameterize elastic properties of about 20 shales obtained from our laboratory measurements and also from a literature survey. The four fitting parameters (namely, tangential and normal compliances of a single crack, characteristic pressure, and crack orientation anisotropy parameter) show correlations with the depth from which the shale was extracted. With increasing depth, the tangential compliance broadly decreases exponentially and the ratio of normal to tangential compliance generally increases linearly. The crack orientation anisotropy parameter exponentially increases with the depth for most of the shales indicating that cracks may be more aligned in the bedding plane. The characteristic pressure shows no simple correlation with depth. The suggested model allows prediction of stress dependency of all five elastic coefficients if only three of them are known. This can be useful, for instance, for the reconstruction of all five elastic coefficients of shale from log data.

An Estimation of Sonic Velocities in Shale Using Clay and Silt Fractions from the Elemental Capture Spectroscopy Log

Anisotropic differential effective medium approach is used to simulate elastic properties of shales from elastic properties and volume fractions of clay and silt constituents. Anisotropic elastic coefficients of the wet clay pack are assumed to be independent of mineralogy and to be linearly dependent on clay packing density (CPD), a fraction of clay in an individual wet clay pack. Simulated compressional and shear velocities normal to the bedding plane and are shown to be in a good agreement with measured sonic velocities. Further, elastic coefficients of shales, and , calculated from the log sonic velocities, calibrated porosity and clay fraction obtained from the mineralogy tool are used to invert for elastic constants of clays, C33 and C44. The obtained elastic coefficients of clays show lower scatter than the original elastic coefficients of shales. The noticeable increase of the clay elastic coefficients with the depth increase is shown to result from the positive trend of the CPD with depth. Being interpolated to the same CPD = 0.8, elastic coefficients of clays show no depth dependency. Our findings show that the CPD and silt fraction are the key parameters that can be used for successful modelling of elastic properties of shales.

Effects of isotropic and anisotropic stresses on elastic properties of shales

Understanding seismic anisotropy in shales is important for quantitative interpretation of seismic data, 4D monitoring and pore pressure prediction. Along with intrinsic anisotropy caused by preferred mineral orientation that is common in shales, anisotropic stress is an important factor that affects shale elastic response. The effect of stress on elastic properties of shales is also important for understanding of depositional trends especially at the upper 2000-3000 meters where the compaction is mostly mechanical. Despite the importance of the effects of isotropic and especially anisotropic stress on elastic properties of shales, little work has been done on theoretical understanding and predicting such properties.