Application of a state variable description of inelastic deformation to geological materials

The work reported in this thesis is an investigation of the potential for employing non-steady-state constitutive relations to describe the inelastic deformation properties of rock forming minerals. Attempts to determine generally applicable constitutive equations for inelastic deformation are complicated by the severe path dependence of the deformation response and by the wide range of mechanisms by which that deformation is accomplished. In the geological literature it has been normal practice to circumvent these problems by approximating the deformation as occurring at steady-state. This presents difficulties for the description of deformation by inherently transient mechanisms, for the description of small strain deformation, and for the reliability of extrapolating laboratory mechanical properties to geological deformation conditions. In contrast, in the materials science literature several systems of inelastic constitutive equations which do not make the steady-state approximation have been proposed. One of the oldest and most widely applied of these i.e. that due to Hart and coworkers, was chosen for investigation here to determine its potential for geological applications. To be successful any deformation constitutive equation must satisfy three criteria. Firstly it must provide an adequate description of the material behaviour, secondly it must be analytically / numerically integrable for deformation modelling purposes, and thirdly it must contain material parameters which can be evaluated from the results of deformation experiments. In the first half of this thesis the descriptive capacity and integrability of Hart's equations are investigated by attempting to provide a comprehensive description of inelastic deformation from the perspective offered by Hart's analysis. Hart bracketed from consideration several aspects of inelastic deformation which are of considerable geological importance i.e. in particular the influence of nominally deformation independent recovery processes (active at high temperatures), of solute impurities, of finely dispersed inclusions and of grain-size. Procedures for identifying the effect of these factors on the results of deformation experiments, and possible strategies for extending the analysis to accommodate them are outlined. The second half of the thesis describes an experimental programme designed to apply Hart's description to the inelastic deformation of Carrara marble at 200 MPa confining pressure in the temperature range 120 to 700°C. The primary aim of the experimental programme was to determine whether the material parameters in Hart's description can be determined with sufficient accuracy at elevated confining pressures (given the technical limitations on the quality of the data obtained from such tests) for the approach to be of interest for the characterization of geological materials. From the results of several hundred experiments it is found that those material parameters can be evaluated sufficiently accurately. Furthermore, the proposed strategy for extending Hart's analysis to high temperatures is shown to have considerable potential. In so doing the first description of the inelastic deformation properties of calcite at temperatures below 400°C is presented, and an experimental programme is outlined which can be applied to materials of even greater geological significance

Geomechanical characterization of mud volcanoes using P-wave velocity datasets

Mud volcanoes occur in many petroliferous basins and are associated with significant drilling hazards. To illustrate the type of information that can be extracted from limited petrophysical datasets in such geomechanically complex settings, we use P-wave velocity data to calculate the mechanical properties and stresses on a two-dimensional vertical section across a mud volcano in the Azeri-Chirag-Guneshly field, South Caspian Basin. We find that: (1) the values of the properties and stresses calculated in this way have realistic magnitudes; (2) the calculated pore fluid pressures show spatial variations around the mud volcano that potentially highlight areas of fluid recharging after the most recent eruption; and (3) the information obtained is sufficient to provide helpful indications of the width of the drilling window. Although calculations of this kind may be readily improved with more sophisticated petrophysical datasets, the simplicity of this approach makes it attractive for reconnaissance surveys designed to identify targets worthy of further investigation in developing our understanding of mud volcano geomechanics, or which could be used to help formulate drilling strategies