Numerical Investigation on Propagation Behaviors of a Three-Dimensional Fracture Network Coupled with Microseismicity in Fractured Shale Reservoirs

The formation mechanism and propagation behaviors of a three-dimensional hydraulic fracture network in fractured shale reservoirs remain unclear, especially when the scale of hydraulic fractures is much larger than that of natural fractures. In this study, taking the well XH in the Longmaxi shale reservoir in the Sichuan Basin, China as an example, we develop a fully three-dimensional numerical model for hydraulic fracturing coupled with microseismicity based on the discrete lattice method. We introduce a randomly generated discrete fracture network into the proposed model and explore the formation mechanism of the hydraulic fracture network under the condition that the hydraulic fractures are much larger than natural fractures in scale. Moreover, microseismic events are inversely synthesized in the numerical model, which allows the evolution of the fracture network to be monitored and evaluated quantitatively. In addition, we analyze the effects of injection rate, horizontal stress difference, and fluid viscosity on fracture propagation. Our results show that when the scale of hydraulic fractures is much larger than that of natural fractures, the fracture morphology of “main hydraulic fractures + complex secondary fractures” is mainly formed. We find that a high injection rate can not only create a complex fracture network, but also improve the uniform propagation of multi-cluster fractures and enhance far-field stimulation efficiency. Optimizing the horizontal wellbore intervals with low horizontal stress differences as the sweet spots of hydraulic fracturing is also beneficial to improve the stimulation efficiency. For zones with a large number of natural fractures, it is recommended to use an injection schedule with high viscosity fluid early and low viscosity fluid late to allow the hydraulic fractures to propagate to the far-field to maximize the stimulation effect.

Identification of Shale Bedding Layers from Micromechanical Evaluation

Bedding layers are well-developed in shale and recognized to enhance the complexity of the hydraulic fracture network. A micromechanical evaluation approach of shale bedding layers based on indentation testing techniques was proposed. Rock thin section identification and scanning electron microscope were utilized to describe the shale laminations. To capture high-resolution image with large area and ensure abundant morphology and mineral information, Atlas 5 system, which could create large mosaic images, was applied. Combined with FIB, a method for positioning the laminations in SEM and accurately navigate to it in the micromechanics experiment was established. Fractures were emphasized to be initiated and extend in the clay mineral layers. It can be explained that the flaky structure of clay mineral is beneficial for the extension of fracture. It will become more potential to connect with the fluid-driven fractures. Nanoindentation was conducted to obtain micromechanical properties in different lamination layers. Results verify the feasibility of this scheme and demonstrate fine-grained, transition and coarse-grained layers. Indentation results show variation in micromechanical properties of coarse-grained layer, the transition layer and the fine-grained layers. The present work is helpful to identify shale bedding layers from micromechanical evalutions.

Monitoring the Geometry Morphology of Complex Hydraulic Fracture Network by Using a Multiobjective Inversion Algorithm Based on Decomposition

The fracturing technique is widely used in many fields. Fracture has a greater impact on the movement of fluids in formations. Knowing information about a fracture is key to judging its effect, but detailed information about complex fracture networks is difficult to obtain. In this paper, we propose a new method to describe the shape of a complex fracture network. This method is based on microseismic results and uses the L-system to establish a method for characterizing a complex fracture network. The method also combines with decomposition to construct a new method called the multiobjective fracture network inversion algorithm based on decomposition (MOFNIAD). The coverage of microseismic monitoring results and the degree of fitting of production data are the two objective functions of the inversion fracture network. The multiobjective fracture network inversion algorithm can be optimized to obtain multiple optimal solutions that meet different target weights. Therefore, this paper established a multischeme decision method that approached the ideal solution, sorting technology and AHP to provide theoretical guidance for finding a more ideal fracture network. According to the error of microseismic monitoring results, we established two cases of fracture to verify the proposed method. Judging from the results of the examples, the fracture network finally obtained was similar to actual fractures.