After-Closure Idiosyncrasies of Fracture Calibration Test Analysis in Shale Formations
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AbstractFracture Calibration Tests (FCT), are strategically used to estimate, amongst other important parameters, reservoir permeability and pore pressure during early appraisal stages of unconventional formations, where estimation of permeability and pore pressure by pressure transient buildup tests is impossible because no flow occurs without hydraulic fracture stimulation. This study investigates some of the idiosyncrasies that have been reported in a large number of FCTs, which make after-closure (AC) analysis ambiguous or inconsistent.We recently introduced a novel comprehensive model for FCT analysis, combining before-closure (BC) and AC behavior in a fully consistent way, facilitating the detection of abnormal BC events (i.e. multiple closure and non-linear leakoff) and improving the intrinsic weakness of standalone AC analysis methods. Extensive use of this model on a vast number of shale gas FCTs has revealed the existence of apparent AC anomalies. This paper provides a novel approach to identify and diagnose these apparent anomalies and reconcile them with unique aspects of the tested unconventional shale formations.In the AC anomaly that we firstly addressed in SPE-144028-MS, the logarithmic derivative shows only a minimal indication of formation linear flow, showing instead an abrupt transition to pseudo-radial flow. This unresolved anomaly inspired the work presented in this paper, where twenty FCT published case histories were used to study and reveal four distinct anomalies: AC logarithmic derivative exhibits unit slope, a symptom of horizontal fracture;Multiple closures followed by AC logarithmic derivative that exhibits a late-time "hook";AC logarithmic derivative exhibits abrupt transition to pseudo-radial flow with limited or no linear flow;AC logarithmic derivative exhibits an apparent dual porosity "dip".These AC idiosyncrasies have been rationalized and integrated into our established comprehensive model for FCT analysis, allowing a realistic characterization of the geological, tectonic, and wellbore -related geometric signatures that very commonly manifest themselves in all shale formations.
