Browsing by Subject "Cements"

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Opening history and porosity evolution of fractures in sandstone, Triassic to Jurassic La Boca Formation, Northeast Mexico
(2006-12) Ward, Meghan Elizabeth, 1981-; Laubach, Stephen E. (Stephen Ernest), 1955-
Sandstone outcrops of the Triassic to Lower Jurassic La Boca Formation, NE Mexico, offer an unusually well preserved example of cements within opening-mode fractures (veins and joints). Although all fractures in these outcrops that can be reliably attributed to subsurface deformation contain cements, some retain open pore space whereas others have been sealed. Quartz cement within otherwise open fractures contains crack-seal textures that record fracture opening history. Elsewhere otherwise identical fractures are sealed with calcite. Outcrops contain fractures that range over three orders of magnitude in size quantified by kinematic aperture. Kinematic apertures of sealed microfractures and open and sealed macrofractures have distributions that can be described by power law scaling. Fracture size and cements deposited during and after fracture opening govern fracture porosity evolution. Although similar patterns have been found in many cored fractures from low-porosity sandstones, my study is the first to document these patterns in outcrop. I used aperture measurements along scanlines, petrography, fluid-inclusion analysis and high-resolution scanning electron microscope-based cathodoluminescence (CL) mosaics to characterize cement and fracture populations. CL imaging of quartz cement reveals crack-seal textures that show cement precipitation while fractures were opening. Using recent cement precipitation models and maps of cement deposit patterns within crack-seal cement bridges, I infer that cement deposition proceeded at rates comparable to overall fracture pattern formation but slower than individual cracking episodes. My observations are consistent with a model of quartz precipitation in fractures proposed by Lander et al. (2002) in which thermal exposure history and rock surface area govern cement accumulation rate and patterns.
Understanding fluid flow in rough-walled fractures using x-ray microtomography images
(2015-08) Tokan-Lawal, Adenike O.; Eichhubl, Peter; Prodanović, Maša; Cardenas, M. Bayani; Fisher, William L
Natural fractures provide fluid flow pathways in otherwise low permeability reservoirs. These fractures are usually lined or completely filled with mineral cements. The presence of these cements causes very rough fracture walls that can constrict flow and hinder the connectivity between the fracture and matrix/fracture pores thereby reducing porosity, permeability and matrix/fracture transfer. In order to accurately predict fluid transport in the unconventional reservoirs, I study the influence of diagenesis (cementation and compaction in particular) and fracture roughness on flow in artificial (fractured polyethylene) and naturally fractured carbonate (Niobrara formation outcrop) and tight gas sandstones (Torridonian outcrop and Travis Peak reservoir in particular). X-ray microtomography imaging provides information on fracture geometry. Image analysis and characterization of the connectivity and geometric tortuosity of the pore space and individual fluid phases at different saturations, is performed via ImageJ and 3DMA Rock software. I also use a combination of the level-set-method-based progressive-quasistatic algorithm (LSMPQS software), and lattice Boltzmann simulation (Palabos software) to characterize the capillary dominated displacement properties and the relative permeability of the naturally cemented fractures within. Finally, I numerically investigate the effect of (uniform) cementation on the fracture permeability as well as the tortuosity of the pore space and the capillary pressure-water saturation (Pc-Sw) relationship in the Niobrara. Permeability estimates in the different formations vary by several orders of magnitude with the different correlations that currently exist in the literature for all samples studied. The presence of cements increases the geometric tortuosity of the pore space and capillary pressure while reducing the permeability and porosity. Contrary to our expectation, the tortuosity of either wetting or non-wetting phase and their respective relative permeabilities show no clear correlation. Overall, pore scale methods provide an insight to flow characteristics in rough walled fractures at micron scale that are not well represented by existing correlations. The measured properties can be used as input in reservoir simulators.