Abstract
Hydraulic fracturing, which is realized by injecting fluid and proppant materials with high pressure to open the formation, has been successfully employed for unconventional resource recovery for decades. During flowback and early-time production, the fracture closure may exhibit with a pressure drop of fracturing fluid dewatering and hydrocarbon withdrawal. This paper presented a model fully coupling flow and geomechanics for flowback and early-time production, which can comprehensively capture the dynamic behavior of fracture key parameters and optimize the strategies of resource development. In the coupled model, the control-volume method is used to numerically model fracture flow, with sufficient flexibility to consider geometries and conductivity distributions of propped and unpropped fractures. For the fracture geomechanics, the fracture aperture of unpropped fractures is characterized using the joint-closure relation. And the fracture conductivity is described using an empirical formula associated with effective normal stress and parameters of proppants for the propped fracture, which closes on the proppant. The discontinuous displacement method (DDM) is adopted to calculate fractures aperture and dynamically coupled with flow equations by updating of fracture parameters, with consideration of the matrix transient linear flow. The model is simple, but rigorous enough to consider the flow and geomechanics physics of the propped and unpropped fracture system. The ease of model setup and improved computational performance make it convenient for practical application. Detailed flow behavior analysis shows that the unpropped fractures, which are connected with the wellbore through propped fractures, experience a relative slow aperture throughout the closure process. And the fracture segments that open against lower normal stress have a larger initial aperture and faster aperture decline, compared to the segments that open against higher normal stress. The fractures key parameters, including the fracture permeability and fracture length are well interpreted by matching the field flowback and production data. Specially, the relationship between fracture conductivity and effective stress for both propped and unpropped fractures can be further correlated in a similar form with that of pressure-dependent fracture conductivity, which can be well used in the analytical, semi-analytical and fully numerical simulation models. The dynamic behavior of propped and unpropped fractures are investigated using a novel model, which fully couples fracture flow and stress with consideration of the fracture deformation. The new findings are also applied to the field data of a fractured shale gas well from the ChangNing shale in China. The results demonstrate the importance of accounting for dynamic behavior for deriving reservoir/fracture properties from flowback and early-time production data and for forecasting.
