Characteristics of Gaseous/Liquid Hydrocarbon Adsorption Based on Numerical Simulation and Experimental Testing.

Hydrocarbon vapor adsorption experiments (HVAs) are one of the most prevalent methods used to evaluate the proportion of adsorbed state oil, critical in understanding the recoverable resources of shale oil. HVAs have some limitations, which cannot be directly used to evaluate the proportion of adsorbed state oil. The proportion of adsorbed state oil from HVA is always smaller than that in shale oil reservoirs, which is caused by the difference in adsorption characteristics of liquid and gaseous hydrocarbons. The results of HVA need to be corrected. In this paper, HVA was conducted with kaolinite, an important component of shale. A new method is reported here to evaluate the proportion of adsorbed state oil. Molecular dynamics simulations (MDs) of gaseous/liquid hydrocarbons with the same temperature and pressure as the HVAs were used as a reference to reveal the errors in the HVAs evaluation from the molecular scale. We determine the amount of free state of hydrocarbons by HVAs, and then calculate the proportion of adsorbed state oil by the liquid hydrocarbon MD simulation under the same conditions. The results show that gaseous hydrocarbons adsorptions are monolayer at low relative pressures and bilayer at high relative pressures. The liquid hydrocarbons adsorption is multilayer adsorption. The adsorption capacity of liquid hydrocarbons is over 2.7 times higher than gaseous hydrocarbons. The new method will be more effective and accurate to evaluate the proportion of adsorbed state oil.

Classification and Control Factors of Reservoir Types in Hybrid Sedimentary Rocks: A Case Study on Lucaogou Formation in the Jimusaer Sag, Junggar Basin.

Hybrid sedimentary rocks (HSR) represent a significant reservoir type in fine-grained sediments. However, the classification and understanding of HSR reservoirs, including their storage mechanisms and identification of optimal "sweet spots," have been limited due to the lack of clarity regarding the multiple sources of components and their mixing processes. This study focuses on the Lucaogou formation of Jimusaer Sag and aims to highlight the reservoir classification principles, controlling factors, and evolutionary patterns associated with the components of HSR, beginning with examining the microscopic pore structure. The analysis of the microscopic pore structure characteristics reveals the presence of five distinct reservoir types within the HSR. The quality of these reservoirs is governed by various factors, including the composition and support mode of particles, diagenesis, provenance, and sedimentary microfacies. In regions near a provenance with strong hydrodynamic conditions, the HSR predominantly exhibits type I and type II reservoirs, characterized by numerous coarse-grained components and a granular-support mode. As the distance from the provenance increases, transitioning into medium hydrodynamic conditions, the HSR shifts to an interbedded-support mode, primarily developing type III reservoirs. In areas far from the provenance with weak hydrodynamic conditions, HSR reservoir types primarily consist of type IV and type V. Additionally, diagenetic effects such as compaction and calcite cementation further deteriorate intergranular and dissolution pores, consequently diminishing reservoir quality. Notably, during the mixing deposition processes of sand and dolomite, the developmental mode of HSR shifts from type I to type II and type III. Likewise, in the mixing deposition of mud and sand, the HSR transitions from type II to type III and type IV. Similarly, the mixing deposition of dolomite and mud leads to a change in the developmental mode of HSR from type III to type IV and type V. Moreover, this study effectively predicts the occurrence of "sweet spots" using reservoir classification, which reveals their continuous distribution. These findings provide a geological foundation for evaluating "sweet spots" and testing the oil production in HSR reservoirs.

Molecular Dynamics Simulations of Oil-Water Wetting Models of Organic Matter and Minerals in Shale at the Nanometer Scale.

Wettability is an important physical property of shale. This parameter is related to the shale material composition and the fluid properties in the shale pores and plays an important role in the exploration and development of shale oil. Wettability is affected by the scale and roughness. The contact angle at the nanoscale on a smooth surface can better reflect the wettability of shale than the contact angle at higher scales. Molecular dynamics simulations can be used to measure the contact angle on a smooth surface at the nanoscale. This paper focuses on the effects of organic matter and minerals in shale and different components of shale oil on shale wettability. Wetting models of "organic matter-oil component-water," "quartz-oil component-water" and "kaolinite-oil component-water" at the nanoscale were constructed. Molecular dynamics simulation was used to study the morphological changes of different oil components and water on different surfaces. Studies have shown that organic matter is strongly oleophilic and hydrophobic. Polar components in shale oil can make organic matter slightly hydrophilic. It was recognized by quartz wettability experiments and simulation methods at the nanoscale that the cohesive energy of a liquid has a significant influence on the degree of spreading of the liquid on the surface. The "liquid-liquid-solid" wettability experiment is an effective method for determining mineral oleophilic or hydrophilic properties. The nanoquartz in the shale is strongly hydrophilic. The water wetting angle is related to the crude oil component. Nanokaolinite can have a tetrahedral or an octahedral surface; the tetrahedral surface is oleophilic and hydrophobic, and the octahedral surface exhibits strong hydrophilicity. The wettabilities of both surfaces are related to the crude oil component.

Simulation of Oil-Water Rock Wettability of Different Constituent Alkanes on Kaolinite Surfaces at the Nanometer Scale.

Kaolinite is widely distributed in shale formations. Kaolinite has two surface types, Si-O and Al-OH, and the two surfaces have different chemical properties. The surface wettability of kaolinite minerals is closely related to the occurrence of crude oil, the migration process of crude oil, and the filling process of crude oil. In this paper, we focus on the oil-water rock wettability of different alkane hydrocarbons on the different surfaces of kaolinite and construct a model of oil and water with variation of the alkane components on the surface of tetrahedral and octahedral kaolinite. Molecular dynamics methods were used to study the morphological changes in water clusters in different alkanes on different surfaces of kaolinite and to calculate the wetting angles. Studies have shown that the octahedral kaolinite surface is strongly hydrophilic, and the water clusters become monolayers adsorbed on the surface. Water easily displaces the oil on the surface and preferentially drives low carbon number alkanes. The tetrahedral siloxane kaolinite surface is oleophilic, the water molecules in C6H14-C18H38 are clustered on the surface, and the wetting angle of the water cluster in the alkane increases with increasing carbon number. Water has difficulty displacing oil on this surface.