Characterization of complex pore systems of tight rocks applying hydraulic and petrophysical methods - procedures and constraints -

Research output: ThesisDoctoral ThesisResearch

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Abstract

The objective of this thesis was to investigate the hydraulic, diffusive and electrical property of a complex sedimentary rock with pronounced anisotropic properties (immature source rock with high kerogen content). A detailed description of the structural property of a representative great volume (V ~ 0.1 m3) serving as unit cell with methods (e.g. CT, SEM, XRD) is a fundamental prerequisite. An adequate interpretation can only be given by an analysis of mineralogical and physical properties and of locally significant variations of clay, pyrite and kerogen content in detail. A procedure was developed for the collection of measuring data, so that a great number of investigation methods can be successively applied without irreversible alterations of the core material. The non-destructively obtained basic data (Φ, kg, CT, conductivity, NMR) are available for the assessment of the consecutive SCAL-investigations and provide a reliable fundament for the interpretation. Reliable porosity values can be obtained with water according to Archimedes principle under the injection pressure of 3 MPa. Specific attention is needed for the sample preparation and drying process to avoid irreversible alterations. The electrical conductivity measurements were preferentially applied because this method is well established and not too time-consuming. 2/4 electrode configurations yield matching results. For a proper assessment of the electrical conductivity, the impact of the three accompanying components is essential (clay minerals, pyrite, and kerogen). The high organic content turned out to be problematic. The CEC-measurements of clay result in specific surface areas, which are not matching with the BET and MICP-measurements. A partial covering of clay by kerogen is assumed to be responsible. The additional excess conductivity implicates that the porosity exponent of the Archie equation is no longer an intrinsic property of the core material. Models published by Waxman/Smits or others are not suitable any more. Therefore, three analytical models were generated to describe the core conductivity at different pore fluid conductivities. Preferentially, the apparent gas permeability was measured since the determination of the water permeability is very time consuming. The small pore size compared to the free path of molecules generates a flow regime at high Knudsen numbers. The flow behaviour is governed by a high ratio of diffusive flow, so that this non-Darcy flow regime cannot be described with a slip model approach like Klinkenberg. The distinct anisotropic effects of the Posidonia shale are visible due to the direction depending conductivity and the permeability as well. Dispersion measurements are an absolutely essential method. The effluent concentration curves deliver valuable results just as the determination of the temporary development of the tracer distribution inside the sample by CT. The mass transfer out of the low permeable zones into the high permeable layers by transversal diffusion due to high anisotropy governs the transport process inside the core material. A numerical simulation was executed with several thousand-grid blocks to separate the fractions of advection and diffusion by integrating appropriate terms in the transport equations. Applying an Archie approach, the electrical conductivity and the diffusion in relation to the advection can be described with a joint porosity exponent. The numerical simulation provides a deep insight into the transport behaviour and is a necessary step to perform a scale-up process into large-scale structures. NMR is a beneficial method to calculate pore sizes and surface relaxivities after fitting NMR spectra and MICP-data. These correlations fall short if the anisotropy is too high. The large number of the applied measuring methods can compensate constraints of one method by the results of other methods. By this approach,

Details

Translated title of the contributionCharakterisierung von komplexen Porensystemen (tight rocks) unter der Anwendung von hydraulischen und petrophysikalischen Methoden - Prozedere und Einschränkungen - procedures and constraints -
Original languageEnglish
QualificationDr.mont.
Supervisors/Advisors
  • Schön, Juergen, Assessor A (internal)
  • Blendinger, Wolfgang, Assessor B (external), External person
Publication statusPublished - 2016