Injection-Induced Slip Potential of the Naturally Fractured Cambrian Reservoirs – A Case Study from the Hassi D'Zabat Field, Algeria

ABSTRACT: This study presents a comprehensive geomechanical modeling of the naturally fractured Cambrian reservoirs of the Hassi D'Zabat field, Algeria to assess the in-situ stress state and critically stressed fractures. The Cambrian reservoir exhibits vertical to sub-vertical open to partially open fractures, as interpreted from the cores as well as the image log. A cumulative of 41 m of ‘B-quality’ breakouts was interpreted from the acoustic image log indicating a mean SHMax azimuth of N118˚E. 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 150 psi of fluid injection in the Cambrian reservoirs. Within the practical injection window, the stress-based model indicates 41 out of 136 steeply dipping fractures becoming critically stressed and therefore experiencing shear slippage. 1. INTRODUCTION Geomechanics plays a critical role in optimizing the drilling trajectory, fluid injection, and production enhancement (Almalikee and Sen, 2021, 2022; Leila et al., 2021; Farouk et al., 2022; Sen et al., 2024a-c). It has critical implications for naturally fractured reservoirs. When the shear stress to effective normal stress ratio acting on a weak plane (fractures or faults) exceeds the sliding friction of that plane, the probability of shear failure occurrence becomes high, and the fracture is considered to be critically stressed. This study focuses on the prolific Cambro-Ordovician reservoir interval of the Hassi D’Zabat (HDZ), located in the Central Algerian Sahara. The reservoirs are naturally fractured. Waterflood (i.e., water injection) is commonly practiced in these reservoirs at the very early stage of production, to maintain aquifer pressure support. The primary objective of this study was to understand the interplay between natural fractures and in-situ stresses by geomechanical modeling and assess critically stressed fractures during any fluid injection.