Comprehensive before-closure model and analysis for fracture calibration injection falloff test

Abstract

The fracture calibration injection falloff test, also known as mini-frac and diagnostic fracture injection test (DFIT), is widely used to quantify essential input parameters for the following main fracture treatment design. Over time, data presentations used to facilitate the analysis have revealed trends in the before-closure falloff transient data that have been labeled as abnormal closure. This paper provides new models for all known before-closure leakoff behaviors and a comprehensive model that enables a match with the entire before-closure transient response.The before-closure leakoff behaviors in the comprehensive model include wellbore storage (WBS), friction dissipation, tip extension, pressure dependent leakoff area (PDLA), multiple apparent closures, transferred storage (also known as transverse storage), height recession and variable fracture compliance. Besides the parameters such as closure pressure, leakoff coefficient, and the hydraulic fracture geometry that can be quantified by commonly known analysis techniques, the new model enables quantification of several new parameters.Two diagnostic plots are used to facilitate identification and quantification of the standard and newly modeled parameters: the Bourdet log-log plot of pressure change and derivative and the composite G-function plot. Tip extension is more easily identified on the G-function plot, and the model match with this behavior quantifies the tip extension distance after shut-in and the fracture minimum propagation pressure. So-called pressure dependent leakoff and transferred storage may be related to opening of secondary fractures, and may be seen as multiple closure events easily identified on either diagnostic plot. Such effects enable quantification of closure stress and surface areas for the secondary fracture sets. Alternatively, when the behavior that looks like transferred storage is explained as fracture height recession, the closure stress of overlying or underlying layers can be estimated along with the recessed fracture area. Variable fracture compliance has similar appearance with transferred storage as well in the diagnostic plots. When it occurs, the way to determine fracture closure stress and other parameters are provided.Matches with field data reflect highly enhanced understanding of the nature of hydraulic fracturing and natural fracture flow mechanisms that have not been possible with other models and that greatly refine well design strategies.