Development Characteristics and Distribution Patterns of Fractures in the Wufeng-Longmaxi Formation Shale in the Southwestern Sichuan Basin, China

AbstractAs the important storage space and main seepage channel of the shale gas reservoirs, fractures control the migration, enrichment, and preservation of shale gas. Therefore, studying the development characteristics of fractures within shale is the key to the exploration and development of shale gas reservoirs. Based on core observation, microscopic thin section examination, field-emission scanning electron microscopy (FE-SEM) analysis, and three-dimensional seismic attribute analysis, this paper studies the development characteristics and distribution patterns of fractures in the Wufeng-Longmaxi Formation shale in the southwestern Sichuan Basin. The results show the following: (1) Both macrofractures and microfractures are developed in the Wufeng-Longmaxi Formation shale in the study area. On the macroscale, horizontal bedding fractures, low-angle slip fractures, vertical shear fractures, and high-angle fractures are mainly developed; on the microscale, intergranular fractures, intragranular fractures, abnormally high-pressure fractures, hydrocarbon generation shrinkage fractures, bedding fractures, etc. are mainly developed. These fractures of different scales work together to create an intricate fracture system within the shale. (2) Based on the 3D seismic attribute analysis, the distribution patterns of fractures was studied in the research area. It was found that nearly east-west-trending and nearly south-north-trending large faults were mainly developed in the Wufeng Formation and the Long-11 submember, with accompanying small faults and secondary fractures. From the bottom boundary of the Wufeng Formation to the top boundary of the Long-11 submember, the fault development shape is unchanged, and the secondary fractures are developed gradually densely, showing the characteristics of a zonal distribution in the entire study area. From the bottom boundary of the Wufeng Formation to the top boundary of the Long-11 submember, the development of microfractures shows variation in different zones. Microfractures are mainly developed around large faults, and the development range of microfractures decreases from bottom to top, but the development of microfractures is gradually intensive. The microfractures are densely distributed from the central to the southern part of the study area (gradually from convergence to divergence), are sparsely distributed in the western part of the study area, and are densely and widely distributed in the northern, northeastern, and northwestern parts of the study area. The research results can provide some guidance for the prediction of shale gas sweet spots in this area, which is beneficial to the further exploration and development of marine shale gas in this area

Development of a hybrid Lagrangian-Eulerian model to describe spark-ignition processes at engine-like turbulent flow conditions

With the engine technology moving towards more challenging (highly dilute and boosted) operation, spark-ignition processes play a key role in determining flame propagation and completeness of the combustion process. On the computational side, there is plenty of spark-ignition models available in literature and validated under conventional, stoichiometric SI operation. Nevertheless, these models need to be expanded and developed on more physical grounds since at challenging operation they are not truly predictive. This paper reports on the development of a dedicated model for the spark-ignition event at non-quiescent, engine-like conditions, performed in the commercial CFD code CONVERGE. The developed methodology leverages previous findings that have expanded the use and improved the accuracy of Eulerian-type energy deposition models. In this work, the Eulerian energy deposition is coupled at every computational time-step with a Lagrangian-type evolution of the spark channel. Typical features such as spark channel elongation, stretch, attachment to the electrodes are properly described to deliver realistic energy deposition along the channel during the entire ignition process. The numerical results are validated against schlieren images from an optical constant volume chamber and show the improvement in the simulation of the spark channel during the entire ignition event, with respect to the most commonly used energy deposition approach. Further development pathways are discussed to provide more physics-based features from the developed ignition model in the future

Development of a hybrid Lagrangian–Eulerian model to describe spark-ignition processes at engine-like turbulent flow conditions

With the engine technology moving toward more challenging (highly dilute and boosted) operation, spark-ignition processes play a key role in determining flame propagation and completeness of the combustion process. On the computational side, there is plenty of spark-ignition models available in literature and validated under conventional, stoichiometric spark ignition (SI) operation. Nevertheless, these models need to be expanded and developed on more physical grounds since at challenging operation they are not truly predictive. This paper reports on the development of a dedicated model for the spark-ignition event at nonquiescent, engine-like conditions, performed in the commercial CFD code converge. The developed methodology leverages previous findings that have expanded the use and improved the accuracy of Eulerian-type energy deposition models. In this work, the Eulerian energy deposition is coupled at every computational time-step with a Lagrangian-type evolution of the spark channel. Typical features such as spark channel elongation, stretch, and attachment to the electrodes are properly described to deliver realistic energy deposition along the channel during the entire ignition process. The numerical results are validated against schlieren images from an optical constant volume chamber and show the improvement in the simulation of the spark channel during the entire ignition event, with respect to the most commonly used energy deposition approach. Further developmental pathways are discussed to provide more physics-based features from the developed ignition model in the future