Abraham S. Grader

Bubble growth and rise in soft sediments

The mechanics of uncemented soft sediments during bubble growth are not widely understood and no rheological model has found wide acceptance. We offer definitive evidence on the mode of bubble formation in the form of X-ray computed tomographic images and comparison with theory. Natural and injected bubbles in muddy cohesive sediments are shown to be highly eccentric oblate spheroids (disks) that grow either by fracturing the sediment or by reopening preexisting fractures. In contrast, bubbles in soft sandy sediment tend to be spherical, suggesting that sand acts fluidly or plastically in response to growth stresses. We also present bubble-rise results from gelatin, a mechanically similar but transparent medium, that suggest that initial rise is also accomplished by fracture. Given that muddy sediments are elastic and yield by fracture, it becomes much easier to explain physically related phenomena such as seafloor pockmark formation, animal burrowing, and gas buildup during methane hydrate melting.

Tracer diffusion from a horizontal fracture into the surrounding matrix: measurement by computed tomography

The vertical diffusion of NaI solution from a horizontal fracture into and within the surrounding matrix was tracked and quantified over time using an artificially fractured chalk core (30x5 cm) and a second-generation X-ray computed tomography (CT) scanner. The different tracer-penetration distances imaged in the matrix above and below the horizontal fracture are indicative of a greater tracer mass penetrating into the lower matrix. The enhanced transport in the matrix below the fracture was related to the Rayleigh-Darcy instability induced by the density differences between the heavier tracer solution in the fracture (1.038) and the distilled water that had initially resided in the matrix. Our observations suggest that below the fracture, the tracer is propagated by an advection-diffusion process that is characterized by both higher rates and higher concentrations relative to its propagation by diffusion above the fracture. The experimental results suggest that the prediction of contaminant migration in a rock intersected by both vertical and horizontal (e.g. along bedding planes) fractures is difficult because of density effects that result in different solute-penetration rates.

Simulation of three-phase displacement experiments

Three-phase displacement experiments for a water-benzyl alcohol-decane system are simulated. Literature experimental three-phase relative permeabilities for the system are used to describe the relative permeabilities in the three-phase region for different three-phase relative permeability models. Saturation trajectories and elliptical regions are mapped in the three-phase region. Simulations are performed to model displacement experiments including breakthrough and the formation of multiple shocks. The model can be used to predict the results for other displacements. In an experiment where significant gravity segregation is present, the displacement is more accurately modeled by assuming a uniform initial condition than by using the actual vertical saturation and assuming no cross flow.It is shown how different residual saturation values can be measured in the laboratory depending on the initial saturation conditions in the core. The experimental residual saturations can be significantly different than the ‘theoretical’ or model values.

Mapping of Permeability Damage Around Perforation Tunnels

We have investigated porosity arid permeability damage around perforations using a combination of transient analysis and X-ray CT. The method applied allowed us to perform the entire experiments on samples under simulated in-situ stress conditions and to map variations in permeability along the length of the core as well as with radial distance from the perforation. Berea (10.2-cm (4-in.) dia) cores saturated with low-viscosity silicone oil were perforated using conventional-shaped charges (6-g HMX) and API RP43 procedures by using 6.88-MPa (1000-psi) effective stress and 5.16-MPa (750-psi) and 2.61-MPa (350-psi) underbalance. Low-permeability Torrey Buff Sandstone was also perforated using 5.16-MPa (750-psi) underbalance. After sufficiently flowing the perforations, higher-viscosity silicone oil was injected. The movement of fluids was tracked using X-ray CT to measure the local velocity of the viscous fluid front at different locations along the perforation. Results of these tests were compared in terms of permeability and porosity damage. Quantitative analysis on Berea cores show, for the specific charge and test conditions used, that damage extends approximately 2 cm (0. 78 in.) from the center of the perforation. Comparison of tests performed with 2.41-MPa (350-psi) and 5.16-MPa (750-psi) underbalance show a clear increase in permeability near the tunnel wall at the higher underbalance. A zone of somewhat-reduced permeability exists at approximately 1.7 cm from the perforation center in the latter case. Porosity profiles calculated show that porosity is almost uniform out from the tunnel and there is no compacted zone near the tunnel wall in liquid-saturated cores. However there is a high-porosity zone from the tunnel wall out about 2 mm. This may be due to a region of circumferential partings and small cracks that lead to high porosity or due to the possible artifacts discussed in the paper. Qualitative results have also been obtained for a tight sandstone for which underbalance was insufficient to remove debris from the perforation tunnel. CT images reveal that the plugged tunnel acts as a conduit for fluid flow, showing that the plugging material has significantly higher permeability than the surrounding rock.

Detecting sub‐wavelength layers and interfaces in synthetic sediments using seismic wave transmission

The interface between two different homogeneous sediment layers, each composed of uniform grain sizes, is a region of heterogeneity comprising a thin layer with a thickness typically much smaller than a seismic wavelength. Seismic waves propagating parallel to the interface experience a reduction in both their amplitude and frequency content but the effect on group velocity is unresolvable. In the case of a single homogeneous layer bounded on both sides by homogeneous halfspaces, the ability to spatially resolve the layer thickness from amplitude or frequency information is limited by diffraction when the layer thickness is smaller than a seismic wavelength. However, the presence of a subwavelength interface or layer is always marked by a decrease in amplitude and in some cases by a decrease in frequency.

Effect of Pore Fluid Type on Perforation Damage and Flow Characteristics

Perforation tests were performed to investigate porosity and permeability damage caused by perforating in gas-saturated vs. liquid-saturated conditions. The liquid-saturated core had a cleaner tunnel with a larger diameter and deeper penetration than the gas-saturated core.

Microstructure and Materials Science of Fire Resistive Materials

Fire resistive materials (FRMs) are a critical component in the design of safe buildings. Current performance testing is strongly based on the ability of the FRM to adhere to and to control the temperature rise of its substrate. A fundamental understanding of the microstructure and performance properties of these materials is sorely needed to model their performance in real world systems and scenarios. While room temperature properties are more easily evaluated, it is the high temperature properties of the materials that are critical to performance during an actual fire. This paper will describe preliminary efforts in an experimental/computer modeling program being conducted at NIST to apply a materials science approach to characterizing the microstructure and properties of these materials. Three-dimensional x-ray microtomography is applied to obtain a representation of the microstructure of the materials. These microstructures can then be analyzed quantitatively to characterize critical parameters such as porosity and pore sizes, and the effects of these parameters on properties such as thermal conductivity. This analysis, along with characterization of the density and heat capacity of the FRM as a function of temperature, will provide the inputs needed for thermal performance models.

Stress- and Chemistry-Mediated Permeability Enhancement/Degradation in Stimulated Critically-Stressed Fractures

This work has investigated the interactions between stress and chemistry in controlling the evolution of permeability in stimulated fractured reservoirs through an integrated program of experimentation and modeling. Flow-through experiments on natural and artificial fractures in Coso diorite have examined the evolution of permeability under paths of mean and deviatoric stresses, including the role of dissolution and precipitation. Models accommodating these behaviors have examined the importance of incorporating the complex couplings between stress and chemistry in examining the evolution of permeability in EGS reservoirs. This document reports the findings of experiment [1,2] and analysis [3,4], in four sequential chapters.

Critical Chemical-Mechanical Couplings that Define Permeability Modifications in Pressure-Sensitive Rock Fractures

This work examined and quantified processes controlling changes in the transport characteristics of natural fractures, subjected to coupled thermal-mechanical-chemical (TMC) effects. Specifically, it examined the effects of mineral dissolution and precipitation mediated by mechanical effects, using laboratory through-flow experiments concurrently imaged by X-ray CT. These were conducted on natural and artificial fractures in cores using water as the permeant. Fluid and mineral mass balances are recorded and are correlated with in-sample saturation, porosity and fracture aperture maps, acquired in real-time by X-ray CT-imaging at a maximum spatial resolution of 15-50 microns per pixel. Post-test, the samples were resin-impregnated, thin-sectioned, and examined by microscopy to define the characteristics of dissolution and precipitation. The test-concurrent X-ray imaging, mass balances, and measurements of permeability, together with the post-test microscopy, were used to define dissolution/precipitation processes, and to constrain process-based models. These models define and quantify key processes of pressure solution, free-face dissolution, and shear-dilation, and the influence of temperature, stress level, and chemistry on the rate of dissolution, its distribution in space and time, and its influence on the mechanical and transport properties of the fracture.

ANOMOLOUS TWO-PHASE FLOW BEHAVIOR IN FRACTURED SANDSTONE EXPLAINED USING X-RAY COMPUTED TOMOGRAPHY

Understanding fracture morphology in terms of a porous media is necessary for accurate simulation of multiphase transport in fractured rocks. Although ambient- stress methods for obtaining fracture morphology exist, previous research lacks the ability to map fracture closure as a function of stress or the distribution of immiscible phases in the fracture. A twenty-five-millimeter cylindrical sandstone sample was artificially fractured in tension and placed under confining stress in an x-ray transparent vessel. The fracture morphology was characterized under dry conditions using high-resolution x- ray computed tomography. Multi-phase fluid distributions in the fracture were mapped between limits of the mobile saturation range using controlled fractional flows. These distributions were correlated with flow rate and pressure drop measurements. We observed order of magnitude differences in effective permeabilities under conditions of nearly constant overall fracture saturations. These differences in permeability are associated with re-arrangement of the physical distribution of the phases. Distributions associated with low permeability are unstable on a time frame of several hours, much longer of the time frame associated with snap-off phenomena. This phenomenon may be responsible for similar field observations reported in the literature.