Chemical interactions between silicates and their pore fluids [electronic resource] : how they affect rock physics properties from atomic to reservoir scales
Keywords: Geophysics.

This thesis focuses on physico-chemical interactions between rocks and fluids that lead to changes in acoustic and transport properties. The goal was to improve the predictive power of rock physics models and seismic interpretation by including the effects of chemically-induced changes in rock properties. This thesis explores three fluid-rock interactions to understand their effects from the subatomic level to basin and reservoir scales. The first fluid-rock interaction is the dissolution of opal-CT and its precipitation in the more stable quartz phase. Marine diatoms deposit biogenic silica as amorphous opal-A. These deposits interact with saturating aqueous solutions, transforming to microcrystalline opal-CT and eventually quartz. Deposits undergoing these mineralogical changes display corresponding changes in acoustic, transport, and storage properties. Enhanced permeability and preserved porosity during these transitions may result in the formation of diagenetic hydrocarbon traps, even in the absence of structural traps. Successful exploitation of diagenetic traps in oil and gas exploration requires an understanding of how quickly the opal-CT to quartz phase transition occurs and a method for predicting trap locations. In this study, the kinetics of the opal-CT to quartz phase transition were determined using a series of hydrous pyrolysis experiments designed to approximate subsurface conditions. The acquired data were fit well by a nucleation and growth model with one- to two-dimensional crystal growth. The zero-order kinetics parameters were then utilized in a basin and petroleum system model to predict the location of the opal-CT to quartz transition along a cross section of the southern San Joaquin Basin, California. Predicted transition depths were within 1200 ft of observed transition depths in nearby oil fields, a significant improvement over predictions based on published kinetics. The second fluid-rock interaction is the adsorption of carbon dioxide in zeolitic tuff samples. Zeolites are aluminosilicates with large, cage-like structures and electrically charged frames, which make many of them strong adsorbents of carbon dioxide. This study examined the effects of carbon dioxide adsorption on the acoustic properties and strain behavior of zeolite-rich tuff samples. A tuff containing chabazite, one containing clinoptilolite, and one that had not undergone zeolitization were measured in a hydrostatic pressure vessel during exposure to pressurized helium and carbon dioxide. Ultrasonic acoustic velocities exhibited classical dependence on differential pressure. However, the zeolitic samples exhibited significantly reduced strains when saturated with carbon dioxide. The tuff without zeolite showed no anomalous strains when saturated with carbon dioxide. The interaction between the carbon dioxide molecule and the chabazite frame was modeled using electronic Density Functional Theory (DFT). The third fluid-rock interaction is the induced precipitation of salt in sandstone samples. Ionic salt precipitation in reservoir rocks can lead to formation damage and impermeable zones. Successful seismic monitoring of salt precipitation requires knowledge of how the salt deposition alters the acoustic and transport properties of the rock. In this study, Fontainebleau Sandstone samples were saturated with brine and subjected to evaporative drying to induce salt precipitation. Acoustic velocities, porosity, and permeability were measured before and after salt precipitation. The changes in porosity and permeability resulting from salt precipitation mimicked the natural diagenetic trend for Fontainebleau Sandstone.