ExxonMobil

About: ExxonMobil is a company organization based out in Irving, Texas, United States. It is known for research contribution in the topics: Catalysis & Polymerization. The organization has 16969 authors who have published 23758 publications receiving 535713 citations. The organization is also known as: Exxon Mobil Corporation & Exxon Mobil Corp..

Dolomitization, anhydrite cementation, and porosity evolution in a reflux system: Insights from reactive transport models

Abstract: Significant volumes of world hydrocarbon reserves occur in dolostones, and the majority of these reservoirs are interpreted to be of reflux origin. We used a two-dimensional numerical reactive transport model to investigate systematically the temporal and spatial distribution of replacement dolomitization, dolomite cementation, anhydrite cementation, and porosity evolution in a reflux system. We tested the sensitivity of reflux dolomitization to brine concentration (mesohaline to near-halite-saturated brines), near-surface temperature, flow rate, porosity-permeability feedback relationships, reactive surface area, and the effect of initial porosity-permeability heterogeneity. Simulations support the contention that hypersaline reflux is capable of extensive pervasive dolomitization, and that mesohaline reflux dolomitization is viable. Reflux generated a tabular dolomite body that is thickest close to the brine source and thins basinward. Incorporation of initial porosity and permeability heterogeneity resulted in a dolomite body with pronounced lateral fingers. Replacement dolomitization increased porosity (up to 8%), but postreplacement reflux resulted in the precipitation of minor dolomite cements (overdolomitization). Anhydrite cements that were spatially and temporally associated with replacement dolomitization caused a significant porosity reduction of up to 25%. Simulated rates of replacement dolomitization by reflux are fast, up to three orders of magnitude faster than seawater dolomitization. The rate of dolomitization and anhydrite mineralization proved to be critically sensitive to the flow rate and the brine chemistry. Temperature and reactive surface area were important controls on the rate of dolomitization, whereas the feedback relationship between porosity and permeability was a relatively moderate control. Our reactive transport models predict the general spatial and temporal trends in dolomite, porosity, and anhydrite observed in some major dolomite reservoirs.

Control of Metal Dispersion and Structure by Changes in the Solid-State Chemistry of Supported Cobalt Fischer–Tropsch Catalysts

Abstract: Controlling preparation variables in supported cobalt Fischer–Tropsch catalysts has a dramatic effect on the dispersion and distribution of cobalt, and determines how active and selective the resulting catalyst will be. We detail specific examples of catalyst synthesis strategies for modifying interactions between the support and the cobalt precursor, promoting reduction, stabilizing catalysts to high-temperature treatments, minimizing deleterious support metal interactions, and controlling the distribution of cobalt on large support particles. It is important to optimize the support and precursor interaction strength, so that it is strong enough to obtain good dispersion but not too strong to prevent low temperature reduction. We show examples in which formation of surface complexes and epitaxial matching of precursor and support structures improves dispersion dramatically. Reduction promoters can help in those cases where support–precursor interactions are too strong. We show how substitutions of silicon into a titania lattice stabilizes surface area and retards formation at high oxidation temperatures of cobalt ternary oxides that reduce only at very high temperatures—an important consideration if oxidative coke removal is necessary. In addition, surface treatment of TiO2 with an irreducible oxide like ZrO2 can inhibit deleterious support interactions that can block surface cobalt sites. Selectivity can also be dramatically altered by catalyst synthesis. We illustrate a case of large (2 mm) SiO2 particles onto which cobalt can be added either uniformly or in discrete eggshells, with the eggshell catalysts having substantially higher C5+ selectivity. These approaches can lead to optimal Fischer–Tropsch catalysts with high activity and C5+ selectivity, good physical integrity, and a long life.