Search

The objective of this study was to establish a rock physics model of North Sea Paleogene greensand. The Hertz-Mindlin contact model is widely used to calculate elastic velocities of sandstone as well as to calculate the initial sand-pack modulus of the soft-sand, stiff-sand, and intermediate-stiff-sand models. When mixed minerals in rock are quite different, e.g., mixtures of quartz and glauconite in greensand, the Hertz-Mindlin contact model of single type of grain may not be enough to predict elastic velocity. Our approach is first to develop a Hertz-Mindlin contact model for a mixture of quartz and glauconite. Next, we use this Hertz-Mindlin contact model of two types of grains as the initial modulus for a soft-sand model and a stiff-sand model. By using these rock physics models, we examine the relationship between elastic modulus and porosity in laboratory and logging data and link rock-physics properties to greensand diagenesis. Calculated velocity for mixtures of quartz and glauconite from the Hertz-Mindlin contact model for two types of grains are higher than velocity calculated from the Hertz-Mindlin single mineral model using the effective mineral moduli predicted from the Hill's average. Results of rock-physics modeling and thin-section observations indicate that variations in the elastic properties of greensand can be explained by two main diagenetic phases: silica cementation and berthierine cementation. These diagenetic phases dominate the elastic properties of greensand reservoir. Initially, greensand is a mixture of mainly quartz and glauconite; when weakly cemented, it has relatively low elastic modulus and can be modeled by a Hertz-Mindlin contact model of two types of grains. Silica-cemented greensand has a relatively high elastic modulus and can be modeled by an intermediate-stiff-sand or a stiff-sand model. Berthierine cement has different growth patterns in different parts of the greensand, resulting in a soft-sand model and an intermediate-sti

The overall objective of the proposed research was to fundamentally and quantitatively understand, based on first physical principles, the micromechanics of penetration into and damage accumulation in cemented particulate targets subject to multiple impact. The approach was to integrate experiments, analytical micromechanics, and numerical modeling into a concerted effort. The project was to be conducted jointly with the University of Rhode Island. The Petrophysical Consulting part of the project was analytical micromechanics. To achieve this objective we provided an elastic description of completely cemented or partially cemented granular material where the elastic properties of the grains and cement differ from each other. This new solution gives a basis for investigating the plastic deformation and failure of cemented composite targets such as concrete and granite. We also analyzed natural rock materials using acoustic pulses and deriving the strength of such materials from their elastic moduli. These results are relevant to describing the strength and damage resistance of such target geomaterials as concrete and granite. The main relevance to the Air Force mission is through a quantitative description of elastic, non-elastic, and damage accumulation properties of cemented geomaterials subject to penetrator impact.

The overall objective of the proposed research was to provide a quantitative description of the microstructural, as well as macroscopic mechanical behavior of particulate materials with viscous and viscoelastic intergranular cement. By cement, a material is meant which fills the space between two or more particles at their contacts. The volumetric content of cement in the pore space of an aggregate may vary from zero to 100%. This objective was achieved by deriving microstructural contact laws for the combination of two elastic spherical grains with viscous cement. A simple and accurate viscoelastic approximation (Cole-Cole model) was found for the rigorous micromechanical solution. The contact law thus obtained was used to calculate the effective elastic moduli of a granular aggregate with viscous cement at a varying frequency of oscillations. The theoretical results were verified by experiments conducted on a pack of glass beads with viscous epoxy cement. Our second achievement was the extension of an exact solution for the elastic moduli of a cemented grain pack at a low cement content to a solution for the case where cement occupies the entire pore space or a large portion of it. The main relevance to the Air Force mission is through a quantitative description of cemented geomaterials such as asphalt cement. The results have been used at the Wright Laboratory to model asphalt concrete's behavior.

We interpret amplitude variation with offset (AVO) data from a bottom simulating reflector (BSR) offshore Florida by using rock‐physics‐based synthetic seismic models. A previously conducted velocity and AVO analysis of the in‐situ seismic data showed that the BSR separates hydrate‐bearing sediments from sediments containing free methane. The amplitude at the BSR are increasingly negative with increasing offset. This behavior was explained by P-wave velocity above the BSR being larger than that below the BSR, and S-wave velocity above the BSR being smaller than that below the BSR. We use these AVO and velocity results to infer the internal structure of the hydrated sediment. To do so, we examine two micromechanical models that correspond to the two extreme cases of hydrate deposition in the pore space: (1) the hydrate cements grain contacts and strongly reinforces the sediment, and (2) the hydrate is located away from grain contacts and does not affect the stiffness of the sediment frame. Only the second model can qualitatively reproduce the observed AVO response. Thus inferred internal structure of the hydrate‐bearing sediment means that (1) the sediment above the BSR is uncemented and, thereby, mechanically weak, and (2) its permeability is very low because the hydrate clogs large pore‐space conduits. The latter explains why free gas is trapped underneath the BSR. The seismic data also indicate the absence of strong reflections at the top of the hydrate layer. This fact suggests that the high concentration of hydrates in the sediment just above the BSR gradually decreases with decreasing depth. This effect is consistent with the fact that the low‐permeability hydrated sediments above the BSR prevent free methane from migrating upwards.

The overall objective of the proposed research was to provide a quantitative description of the microstructural, as well as macroscopic mechanical behavior of particulate materials with intergranular cementation in a wide range of strain amplitudes and for various types of the cementation material. By cementation, a material is meant which fills the space between two surfaces that are (a) separated or (b) contact directly. This objective was achieved by deriving microstructural contact laws for the combination of two cemented elastic spherical grains subject to normal, shear, or torsional loading. The cement was treated as an elastic, elastic-plastic, purely plastic, and viscous material. The macroscopic constitutive laws were derived from the above microstructural contact laws by using the formulas which relate the effective moduli of the random packing of identical spheres to the normal and tangential contact stiffnesses. The theoretical solutions were positively supported by experiments performed on artificial cemented aggregates, as well as by other experiments performed of real cemented rocks. The main relevance to the Air Force mission is through a quantitative description of cemented geomaterials such as asphalt cement. The results have been used at the Wright laboratory to model asphalt concrete's behavior.