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

Abstract

Summary Pressure-transient analysis (PTA) is commonly used in the 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 leakoff [mini-fracture analysis (MFA) and diagnostic-fracture-injection tests (DFITs) of stimulation hydraulic fractures] and high leakoff [injection-falloff (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 leakoff, in spite of many efforts to date. We present detailed numerical simulations of fracture closure as a result of shut-in and after-closure reservoir flow following hydraulic-fracture propagation at arbitrary rate and for arbitrary leakoff (from very low to very high). By use of the trends resulting from these simulations, we propose a novel, very-simple analytical PTA method for estimating fracture dimensions, skin, leakoff coefficient, and reservoir permeability. This method is applicable to fractured waterflood injectors and to mini-fractures in hydraulic-fracture stimulation. It presents a consistent approach to before-closure analysis (BCA) and after-closure analysis (ACA). We compared the numerical simulations with our novel analytical PTA method with more conventional MFA and IFO approaches, such as g-functions 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 leakoff (MFA) or high leakoff (IFO) of our proposed PTA method, and that our novel methodology provides superior results to the existing approaches, particularly in the intermediate-leakoff range. We also use our numerical methodology to demonstrate that more recently proposed analysis methods by use of generalized versions of the Agarwal/Bourdet et al. (1989) 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 in many cases, unique identification of fracture closure from PTA will be very difficult. Finally, we apply our PTA method to a few field cases, thereby demonstrating its simplicity of usage.