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

MULTI-PHASE FLOW IN THE PRESENCE OF A FRACTURE TIP: EXPERIMENTS AND MODELING

ABSTRACT This paper focuses on multi-phase flow in the presence of a fracture tip. This work studies two-phase water-oil displacements in layered Berea Sandstones that have been artificially fractured with single extensional fracture perpendicular to the natural layers. Two experiments are considered in this paper. In the first experiment the fracture was induced at the inlet end of the sample and it spanned the first third of the core. Thus, the diverging flow at the tip of the fracture was studied. In the second experiment, the fracture was induced at the outlet end of the sample spanning about one third of the core, and multi-phase fluid convergence to the fracture tip was studied. The temporal and spatial saturation distributions of the two cases were determined using x-ray computed tomography, CT. The experimental data were simulated. At the tip of the fracture, the two experiments showed different fluid flow patterns. The presence of the tip of the fracture in both experiments illustrated the displacement path in each layer. The presence of the fracture caused by-passing during the displacement process. From an experimental point of view, the presence of the fracture tip is essential for highlighting the property contrast between the natural layers in the sample, much more than in a displacement process without a fracture. Matches of the simulation results to the experimental data showed that when the fracture is at the inlet end, fluid diverges from the fracture to the matrix along the entire length of the fracture. The movement of the displaced phase was delayed in the regions neighboring the fracture and it preferentially flowed in the outer regions of the core. Understanding multi-phase fluid flow in fractured rocks is essential for designing and optimizing hydrocarbon recovery processes. The fluid flow interactions between the fractures and the matrix have a significant impact on displacement processes. This work provides modeling results and experimental observations that explain some of the displacement processes around a fracture tip

Four-Dimensional X-ray Microtomography Study of Water Movement during Internal Curing

While the effectiveness of internal curing has been verified via a variety of experimental measurements, including internal relative humidity, autogenous shrinkage, restrained shrinkage, strength development, and degree of hydration, a direct observation of water movement during internal curing in four dimensions (three spatial dimensions and time) has been lacking. X-ray microtomography offers the possibility to dynamically monitor density changes in a material, during its curing process, for example. In this paper, this technique is applied to monitoring water movement from saturated lightweight aggregate particles to the surrounding hydrating cement paste in a high performance mortar mixture over the course of the first 2 d of hydration at 30 o C. A four-dimensional data set is created by obtaining three-dimensional image sets on a single specimen after various hydration times, from just after mixing to after 47 h of hydration, with a voxel dimension of less than 20 μm, allowing a clear delineation of individual lightweight aggregate particles and much of their internal porosity. Many of the changes in local density, corresponding to water movement, occur during the first 24 h of hydration, during the acceleratory period of the cement hydration reactions. The four-dimensional data set i

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

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