Kerogen Swelling and Confinement: Its implication on Fluid Thermodynamic Properties in Shales

Abstract

Type I kerogen was isolated from Green River Shale and characterized using SEM, TGA, DSC, and nitrogen adsorption. The swelling behavior of this kerogen with decane was analyzed using traditional test-tube swelling experiment and Dynamic Light Scattering. The TGA and DSC were used to analyze the thermodynamic behavior of decane that was sorbed in the kerogen and show that kerogen suppresses the boiling point of decane due to the effect of confinement. However, the suppression is larger when oil (a multicomponent mixture) was used, possibly due to the combined effect of differential uptake of components by kerogen (kerogen prefers and sorbs polars and aromatics more than saturates, leading to splitting of oil into a sorbed and a free phase) and confinement in nano pores. Test-tube swelling, TGA, and DSC experiments were also performed on pyridine(polar-aromatic)-swelled kerogen. The combined and individual contributions from the two effects (the effect of confinement and differential uptake of hydrocarbon components) on properties of liquid in contact with kerogen, are studied in this work. Molecular Dynamics (MD) simulations revealed the variation in the swelling of type II kerogen in the presence of same amount of different liquids (differential swelling of kerogen).Kerogen is a complex heterogeneous carbonaceous material, and it is the precursor of oil and gas found in the sub-surface. It is the polymeric material formed by the biogeochemical alteration of detrital and dissolved organic matter that was deposited together with inorganic sediment, and it is the primary source of oil and gas accumulations throughout the world. Kerogen is a key component of shale rocks and plays a key role in the storage and recovery of hydrocarbons from them. Current techniques for the recovery of hydrocarbons from shales leave over 80% of hydrocarbons in the subsurface 1 . Kerogen is the fraction of the organic matter in buried sediments that is insoluble in common solvents. Although kerogen does not dissolve in organic solvents, like insoluble synthetic polymers such as elastomers and other porous materials 2 , it sorbs and is swollen by them. Kerogen is usually classified into four types based on the depositional environment, the biological source of the organic matter from which it was derived and its elemental composition, particularly the H/C and O/C ratios. Type I (predominantly lacustrine) and type II (predominantly marine) kerogens have high H/C and low O/C ratios, and they are capable of generating oil and gas as progressive burial in the subsurface increases their temperature and pressure. Type III (predominately terrestrial) humic kerogen has lower H/C and higher O/C ratios than types I and II. It generates natural gas but little or no oil. Type IV kerogen is the recalcitrant organic matter that has been pyrolyzed, oxidized and/or recycled. It has the lowest H/C and highest O/C ratios of any type of kerogen, and it cannot generate significant amounts of oil or gas. A fifth class, type IIS, sulfur-rich type II kerogen,...

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