Summary

Analysis of multi-fractured horizontal well (MFHW) production data completed in low-permeability (tight) oil reservoirs has traditionally focused on long-term (online) production after the initial flowback period. Recent studies, however, have demonstrated that important information about hydraulic fractures can be ascertained from flowback data and simulation studies are now being designed to model flowback along with the online production. However, numerical simulation model setup for this purpose is difficult and time-consuming, and therefore it is desirable to have simpler models that can capture the majority of the physics of the problem, and provide a reasonable "first pass" for matching flowback and online production data to constrain the setup of more rigorous numerical simulation.

In the current work, a new semi-analytical model is developed specifically for modeling water and hydrocarbon production during flowback and early-time production for tight oil wells. Two flow regions are assumed: a primary hydraulic fracture (PHF) and an enhanced fracture region (EFR) adjacent to the hydraulic fracture, where reservoir permeability has been enhanced due to stimulation. Alternatively, a non-stimulated matrix region (NSR), where reservoir permeability is not enhanced due to stimulation, may be placed adjacent to the PHF. The EFR is assumed to have a high fracture fluid saturation after stimulation. A coupled PHF-EFR model is created by assigning the average pressure in the PHF as the inner boundary condition of the EFR, and wellbore flowing pressure as the inner boundary-condition for PHF. If the initial fracture pressure is greater than reservoir pressure, the coupled model forecasts initial production to be single-phase flow of fracturing fluid, followed by two-phase flow of fracturing fluid and formation oil from the EFR to the PHF after breakthrough to the fracture. If the pressures in both regions are equal initially, then multi-phase flow occurs from the start of flowback. Transient flow of fluids through the PHF and EFR is modeled with the dynamic drainage area approach previously used by the authors. Equations of coupled flow/material balance are solved iteratively at each timestep. Stress-dependent properties of fractures and matrix are handled in the solution. The model can simulate flowback and early-time production period until the boundaries of the EFR have been reached. However, in future work, the model will be updated to forecast long-term production by adding a non-stimulated reservoir region (NSR) beyond the EFR, as well as multi-phase flow of gas, water and oil below saturation pressure.

The robustness of this innovative approach is tested through comparison with more rigorous numerical simulation, and its practicality demonstrated with a field example. The new technique should serve as a useful tool for petroleum engineers responsible for forecasting tight oil wells exhibiting these complexities.

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