Permeability Estimation from Fracture Calibration Test Analysis in Shale and Tight Gas

Abstract

Summary URTeC 1569587 Fracture calibration test analysis using the log-log diagnostic plot standard for pressure transient analysis has been demonstrated. However, up to now examples have shown only normal leakoff behavior before fracture closure and radial flow after closure. In reality, many fracture calibration tests show abnormal leakoff behavior, and often radial flow is absent after closure because it may be impractical to extend the falloff long enough to reach the time required to reach radial or pseudo-radial flow. This paper shows before- and after0closure analysis for field examples with abnormal leakoff behavior and lacking after-closure radial flow. Using the standard G-function plot to identify abnormal leakoff behavior including pressure dependent leakoff, transverse storage, and multiple closure events, we show how these behaviors can be recognized on the log-log diagnostic plot. We also show how to diagnose before-closure wellbore fluid expansion analogous to wellbore storage and fracture linear flow. After-closure bilinear, linear, and radial flow are easily identified on the same log-log diagnostic plot, avoiding the need for a plethora of specialized plots with various different plot axes. We then use relationships between formation permeability and pressure can to estimate the least certain of the two depending on which flow regimes appear in the pressure falloff response. Four distinct field examples illustrate the proposed closure diagnostic approach and permeability estimation method on log-log plot for cases of normal leakoff, pressure-dependent leakoff, variable fracture storage, and multiple-closure events, respectively. The results show fracture closure stress values consistent with the widely recognized G-function approach and, further, that the relationship between reservoir permeability and reservoir pore pressure can be used effectively for reducing uncertainty in formation permeability and pressure estimated from after-closure bilinear, linear, or radial flow.