In-situ stress state and critically stressed fracture analysis from the Hassi D'Zabat field, Algeria – A study from the naturally fractured Cambro-Ordovician reservoirs

Abstract

This study presents a comprehensive geomechanical modeling of the naturally fractured Paleozoic reservoirs of the Hassi D'Zabat field, Algeria, to assess the in-situ stress state and critically stressed fractures. The Cambrian and Ordovician reservoirs exhibit vertical to sub-vertical open to partially open fractures, as interpreted from the cores as well as the image log. Routine core analysis indicates higher vertical permeability in the fractured reservoir samples. A cumulative of 41 m of ‘B-quality’ breakouts was interpreted from a 452 m acoustic image log indicating a mean SHMax azimuth of N118°E (standard deviation 8.95°). Shmin was calibrated with the closure pressure from the hydraulic fracture test. Based on the breakout occurrence, SHMax was constrained following the frictional faulting mechanism. The inferred in-situ stress magnitudes (SHMax > Sv > Shmin) indicate a strike-slip tectonic regime in the study area. The practical injection threshold has been inferred as 1850 psi to ensure caprock integrity. The onset of slip on the optimally oriented vertical fractures is estimated to occur at 1000 psi of fluid injection at the Ordovician reservoir level, while only 150 psi of injection can induce shear slippage on fractures within the Cambrian reservoirs. Within the practical injection window, the stress-based model indicates that 58 out of 215 steeply dipping fractures may become critically stressed and therefore, potentially experiencing shear slippage.