Impact of Flowback Dynamics on Fracture Conductivity

Abstract

We aim at the development of an integrated modeling workflow for design and optimization of the flowback operation on a multistage fractured well within the concept of a managed pressure flowback, where the process is controlled by the choke on surface. In this paper, we focus on the impact of flowback dynamics on the conductivity of a single fracture in a multi-stage fractured completion. In our methodology, we employ a coupled modeling approach, where geomechanics phenomena are considered together in the framework of a global model for fluids displacement in a fracture. Among the solid mechanics effects considered are: proppant embedment with plastic deformations and creep of the rock near the fracture surface, proppant crushing, proppant pack compaction under the action of the closure stress with elastic compaction of proppant grains and frictional rearrangement of the grains, tensile rock failure at high filtration rates from the reservoir to the fracture (when drawdown pressure drop is excessively high). These geomechanics effects are implemented into the global fluid flow model in a propped fracture, which takes into account: displacement of fracturing fluid by oil, two-phase effects, suspension filtration with particle trapping and mobilization resulting in permeability damage, colmatation and skin buildup inside the fracture in the near-wellbore zone (where particles are mainly formation fines and crushed proppant).In our numerical experiments and parametric study, we made the following observations, structured by the discipline. Geomechanics: proppant pack compaction and proppant embedment due to rock plasticity and creep may reduce the fracture width by tens percent from the original (ideal) estimates, thus having a profound impact on fracture conductivity. Tensile rock failure in the near-wellbore zone may result in fracture being disconnected hydraulically from the wellbore. Fluid mechanics: colmatation of the proppant pack by small particles (formation fines or crushed proppant) may result in further conductivity decrease.We developed a modeling workflow for the transient fracture flowback to evaluate two scenarios of managed pressure flowback: "fastback" vs. "slowback". Based on the results of parametric studies, we provide evidence that a “smooth” scenario of piece-wise constant choke opening ("slowback") is preferable in terms of the preserved nondimensional fracture conductivity and increased cumulative well production, as compared to an “aggressive” scenario wtih rapid choke opening and dramatic decrease in bottomhole pressure. Implications for field operations are discussed along with modeling-based scenarios to be evaluated in a field-testing campaign. The novelty of our approach stems from the ambition to construct an integrated framework for a coupled geomechanics-fluid mechanics model of flowback, accurately equipped with proper submodels for each important physics phenomenon peculiar to the flowback operation. This effort will continue to scale up from a single fracture to the system of fractures connected to a near-horizontal wellbore with the choke on surface, with the ultimate goal to solve an inverse problem in terms of the optimum choke size sequence to provide the drawdown strategy for the managed pressure flowback.