Numerical Simulation of Proppant Heterogeneous Distribution in Shale Gas Reservoirs

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Abstract Slickwater fracturing with high injection rate and large volume has been widely used in the development of shale reservoirs, while the proppants cannot distribute uniformly in fracture networks, usually in the forms of sand-bed, separated pillars and local displacement. Thus, there are still some key issues need to be figured out for better fracturing design, such as the proppant placement status in complex fractures, and how to achieve effectively propped fractures, etc. In this paper, a three-dimensional proppant transport model is established by Dense Discrete Phase Model (DDPM). The flow field and concentration distribution of the proppants were analyzed, by tracking the trajectory of proppants in different types of complex fractures. Besides, the placement status of particles in the primary and secondary fractures was analyzed. In addition, we also conducted the research on proppant transportations in perf clusters. Finally, according to actual fracturing treatment schedule, three different sizes of proppants were gradually injected and the distribution law of the proppants was studied. Based on the concentration distribution of proppants in the complex fractures, fractures can be divided into four zones: pure fluid zone, critical suspension zone, flowing slurry zone and settling bed zone, while the critical suspension zone and flowing slurry zone of secondary fracture are smaller than primary fracture. Besides, the proppants concentration entering into the fractures far from the entrance is more than that entering the fractures near the entrance. The ratio of proppant concentration into each perf cluster (3 clusters per stage) is about 1:1.5:2, while that concentration ratio is 1:1.25:1.6:2 in four clusters per stage scenario, while the proppant concentration is inversely proportional to the amount of proppant and liquid injected. The proppants of 100 mesh injected in the early stage migrates to the far end of the fracture, and some enters into the branch fractures, while most of them settled in the sand bed of the main fracture. Meanwhile, the following 40/70 mesh proppant injected is mainly distributed in the main fracture at flowing slurry zone. The 30/50 mesh proppant injected in the later stage is mostly located at the fracture entrance and at the top of the sand bed. Finally, it was found that high injection rate, small particle size, and low-density proppant could improve proppants preferentially entering branch fractures. This paper provides a clear insight of proppant distribution law in complex fractures under field fracturing treatment schedule. And our model can be used to optimize the fracturing treatment design.