New Before Closure Analysis Model for Unconventional Reservoirs

Abstract Minifrac/diagnostic fracture injection tests (DFITs) are an important and integral part of hydraulic fracture design and reservoir evaluation. Models for analysis of the before and after closure data have been developed. While several models exist and are being used to analyze the after closure data, before closure analysis (BCA) is not widely used, and only a handful of models have been developed. Additionally, the models are based on similar approaches and assumptions. This paper introduces a new BCA model based on rigorous treatment of the fluid flow equations in the fracture and the formation. In this new model, the assumptions of linear leakoff and short injection time for a minifrac test are made. In general, these assumptions are valid. In unconventional reservoirs, where permeability is expected to be very low, the assumption of linear leakoff is particularly valid and would last most of the duration of the before closure period. In most minifrac tests, the injection time is relatively small with respect to after shut-in time, which validates this new model assumption of short injection time. Fracture stiffness is a critical parameter when using this model, which can be easily calculated or measured from logging or experimental data. By using the before closure model presented here, formation permeability could be estimated by interpretation of the slope of straight line in the specialized Cartesian plot using before closure data. This paper details the development of the new model and compares it to existing models. In addition, several field examples from unconventional formations are presented and analyzed using various techniques, demonstrating the applicability of this new model. The results are also compared to the after closure analysis (ACA).