An Innovative Approach to Model Two-Phase Flowback of Shale Gas Wells with Complex Fracture Networks

Abstract

AbstractTwo-phase flow has generally been of more concern in the hydraulic treatment design of shale gas reservoir, especially, during the flowback period. Investigating the gas and water production data is important to evaluate the stimulation effectiveness. We develop a semianalytical model for multi-fractured horizontal wells by incorporating the two-phase flow in both matrix and fracture of the shale-gas wells. We employ the node-analysis approach to discretize the complex fracture networks into a given number of fracture segments, depending on the complexity of fracture system. The two-phase flow is incorporated by iteratively correcting the relative permeability to gas and water phase and capillary pressure for each fracture segment with the fracture depletion. The model is validated by numerical model and field observation. A good match between them was obtained. Then, the early-time gas and water production performance is analyzed using various fracture properties and geometries. A systematic type curves are obtained with the fracture system from simple to complex geometries. The flow regimes that were identified could assist in constraining the fracture geometry and complexity. Additionally, the gas and water decline rates highly depend on the fracture properties such as initial gas saturation in fractures, fracture conductivity, fracture spacing, fracture geometry and connections with natural fracture networks. The improved network fracture conductivity and complexity especially the connections between hydraulic fracture and natural fractures can enhance the gas production and shorten the dewatering time, illustrating that the effective stimulation could facilitate the fractures to clean up more quickly. The gas/water supply from natural fractures and their flow dynamics controlled by two-phase relative permeability effects could be the major reasons for the formation of "V-shape" behavior on the plot of gas/ water ratio vs. cumulative gas production. This work, for the first time, extends the semianalytical model from single-phase flow to two-phase flow in shale gas reservoir with complex fracture networks. The method is simple and gridless, but is capable of capturing the complex fracture system and gas/water transport mechanisms. Also, it provides an efficient technique to evaluate the hydraulic fracture treatment design in multi-fractured horizontal wells for shale gas reservoirs at early production times.