Capturing dynamic behavior of propped and unpropped fractures during flowback and early-time production of shale gas wells using a novel flow-geomechanics coupled model

Hydraulic fracturing has been widely used to exploit unconventional oil and gas resources for decades. The fracture closure during flowback and early production may be characterized by fluid dehydration and pressure drop in oil and gas production. In this paper, a fully coupled flow and geomechanics model is proposed to capture the dynamic behavior of key fracture parameters for flowback and early-time production. In this coupled model, the controlled volume method is used to numerically simulate the fracture flow, which can consider the geometry and conductivity distribution of the propped and unpropped fractures. For the fracture geomechanics, the joint-closure relationship is introduced to describe the fracture aperture of unpropped fracture. The empirical formula of effective normal stress and proppant parameters is applied to characterize fracture conductivity. The fracture aperture can be calculated coupled with the discontinuous displacement method (DDM) and the matrix transient linear flow. The coupled geomechanically model can consider the flow of the propped and unpropped fracture system, which is easy model-setup and convenient for practical application due to its excellent computational performance. Detailed flow behavior analysis shows that the aperture of the unpropped fracture is relatively slow during the fracture closure process. Compared with the fracture section opening under higher normal stress condition, the fracture section opening under lower normal stress condition has larger initial aperture and faster attenuation of aperture. The crucial fracture parameters, including fracture permeability and fracture length, can be well interpreted by matching flowback and production data. Furthermore, the relationship between fracture conductivity and effective stress for both propped and unpropped fractures can be explored. These new findings can also be applied to interpret reservoir/fracture properties from ChangNing shale fractured wells in China during flowback and early production.