A Simple Unified Pressure Transient Analysis Method for Fractured Waterflood Injectors and Minifracs in Hydraulic Fracture Stimulation

Abstract

AbstractPressure Transient Analysis (PTA) is commonly used in Industry to estimate parameters such as dimensions (length, height) and skin of induced (hydraulic) fractures, and reservoir permeability. PTA interpretation methods are well established for the limiting cases of ‘low’ leak-off [Mini-Frac Analysis (MFA) of stimulation hydraulic fractures] and ‘high’ leak-off [Injection Fall-Off (IFO) analysis of waterflood-induced fractures]. However, traditional methodologies used for these two limiting cases differ fundamentally and there exists no established approach for intermediate leak-off, in spite of many efforts to date.We present exact 3D simulations of hydraulic fracture propagation at arbitrary rate and for arbitrary leak-off (from very low to very high), followed by fracture closure as a result of shut-in and after-closure reservoir flow. Based on the trends resulting from these simulations, we propose a novel very simple analytical PTA method for estimating fracture dimensions, skin, leak-off coefficient, and reservoir permeability. This method is applicable both to fractured waterflood injectors and mini-fracs in hydraulic fracture stimulation. It presents a consistent approach to before-closure analysis (BCA) and after-closure analysis (ACA).We compared the exact simulations with our novel analytical PTA method, and with more conventional MFA and IFO approaches, such as g-functions, square root time, and other existing BCA and ACA methods. For all these cases, it is demonstrated that the currently existing approaches present limiting cases for either low leak-off (MFA) or high leak-off (IFO) of our proposed PTA method, and that our novel methodology provides superior results to the existing approaches particularly in the intermediate leak-off range. We also use our numerical methodology to demonstrate that more recently proposed analysis methods using generalized versions of the Agarwal/Bourdet derivative do not necessarily result in a ‘correct’ interpretation, in particular with respect to identifying the point of fracture closure. With respect to the latter, we also demonstrate that often identification of fracture closure from PTA will be virtually impossible.Finally, we apply our PTA method to a few field cases, thereby demonstrating its simplicity of usage.